8.1 why do cells divide? - gavilan...

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1.

Describe the structure of DNA. Be sure to include what forms the skeleton and how are the strands held together?

2.

Compare and contrast chromosomes, chromatids, genes, and alleles.3.

Compare and contrast prokaryotic and eukaryotic cell division.4.

Describe the process of asexual reproduction in eukaryotic cells. (DNA replication and mitosis)5.

Compare and contrast animal and plant cell asexual reproduction.

(mitosis)6.

Compare and contrast mitosis and meiosis.7.

Without genetic testing how could you determine if an organism is homozygous or heterozygous for a specific trait (ie

hair color)?8.

Describe three ways that genetic variability is increased.9.

Two fruitflies

are bred. One is true breeding for red eyes and one is true breeding for white eyes. Red eyes are dominant. What will the genotype and phenotype of the offspring be?

10.If two of the offspring of the above match are crossbred what will the genotype and phenotype of their offspring be?

11.In the above examples how would the genotypes and phenotypes be different if red eye color was partially dominant producing pink eyes when heterozygous?

12.In the example above (#9) the red-eyed fruitfly

has straight wings and the white-eyed fruitfly

has wrinkled wings. Straight wings are dominant. What would the genotype and phenotype of the offspring be?

13.What characteristics can make genetic disorders more likely to be passed from one generation to the next? (at least 3)

14.Describe the process of DNA replication. What is meant by semiconservative

replication? How are continuous synthesis and discontinuous synthesis involved in the

process?15.How common are mistakes in replication? What safeguards are in place to prevent mistakes?

What types of mistakes are relatively common?16.Compare and contrast DNA and RNA.17.Describe the process of transcription.18.Describe the process of translation.19.What are codons

and how do they function in protein synthesis?20.Describe the ways by which gene expression may be regulated.


8.1 Why Do Cells Divide?

Cells reproduce by cell division.• One cell gives rise to two or more cells, called

daughter cells.• Each daughter cell receives a complete set of

heredity information—identical to the information in the parent cell—and about half of the cytoplasm.

Cell division transmits hereditary information to each daughter cell.• The hereditary information in each cell is

deoxyribonucleic acid (DNA).• DNA is contained in chromosomes.• A molecule of DNA consists of smaller subunits called

nucleotides.


8.1 Nucleotide Structure

A nucleotide consists of a phosphate, a sugar (deoxyribose), and one of four bases.• Adenine (A) • Thymine (T)• Guanine (G)• Cytosine (C).

The nucleotides are held together by hydrogen bonding between the bases in two strands forming a double helix.



The structure of DNA

Fig. 8-1(a) (b)

nucleotidephosphatebasesugar

A single strand of DNA The double helix

A

C

A

T

A

A

A

T

T

T

GC

AT

C G

G

C

C

G



Genes, segments DNA, are the units of inheritance.

Each gene spells out the instructions for making one or more proteins.

When a cell divides, it first replicates its DNA, and a copy is transferred into each daughter cell.



Cell division is required for growth and development.• Cell division in which the daughter cells are

genetically identical to the parent cell is called mitotic cell division.

• After cell division, the daughter cells may grow and divide again, or may differentiate, becoming specialized for specific functions.

• The repeating pattern of division, growth, and differentiation followed again by division is called the cell cycle.



Most multicellular

organisms have three categories of cells.• Stem cells: retain the ability to divide and can

differentiate into a variety of cell types• Other cells capable of dividing: typically

differentiate only into one or two different cell types (progenitor cells)

• Permanently differentiated cells: differentiated cells that can never divide again



Cell division is required for sexual and asexual reproduction.• Sexual reproduction in eukaryotic organisms

occurs when offspring are produced by the fusion of gametes (sperm and eggs) from two adults.

• Gametes are produced by meiotic cell division, which results in daughter cells with exactly half of the genetic information of their parent cells.

• Fertilization of an egg by a sperm results in the restoration of the full complement of hereditary information in the offspring.



Reproduction in which offspring are formed from a single parent, without having a sperm fertilize an egg, is called asexual reproduction.• Asexual reproduction produces offspring that

are genetically identical to the parent.• Examples of asexual reproduction occur in

bacteria, single-celled eukaryotic organisms, multicellular

organisms such as Hydra, and

many trees, plants, and fungi.


8.2 What Occurs During The Prokaryotic Cell Cycle?

The prokaryotic cell cycle consists of a long period of growth, during which the cell duplicates its DNA.

The prokaryotic cell cycle

cell division by binary fission

cell growth and DNA replication


8.2 What Occurs During the Prokaryotic Cell Cycle?

Cell division in prokaryotes occurs by binary fission, which means “splitting in two.”

The prokaryotic chromosome is attached at one point to the plasma membrane of the cell.


8.2 The Prokaryotic Cell Cycle?

Fig. 8-3b(1)

cell wall

plasma membrane

circular DNA

attachment site

The circular DNA double helix is attached to the plasma membrane at one point.

The DNA replicates and the two DNA double helices attach to the plasma membrane at nearby points.

New plasma membrane is added between the attachment points, pushing them further apart.

The plasma membrane grows inward at the middle of the cell.


8.2 The Prokaryotic Cell Cycle

The prokaryotic cell cycle (continued)

Fig. 8-3b(5)

The parent cell divides into two daughter cells.


8.3 How Is The DNA In Eukaryotic Cells Organized?

Unlike prokaryotic chromosomes, eukaryotic chromosomes are separated from the cytoplasm by a membrane-bound nucleus.

Eukaryotic cells always have multiple chromosomes.

Eukaryotic chromosomes contain more DNA than prokaryotic chromosomes.

The eukaryotic chromosome consists of DNA bound to protein.



Duplicated chromosomes separate during cell division.• Prior to cell division, the DNA within each

chromosome is replicated.• The duplicated chromosomes then consist of two

DNA double helixes and associated proteins that are attached to each other at the centromere. Each of the duplicated chromosomes attached at the centromere

is called a sister chromatid.• During mitotic cell division, the sister chromatids

separate and each becomes a separate chromosome that is delivered to one of the two resulting daughter cells.


sister chromatids

centromere genes

duplicated chromosome (2 DNA double helices)

A replicated chromosome consists of two sister chromatids

Sister chromatids separate during cell division

(a)

(b)

independent daughter chromosomes, each with one identical DNAdouble helix

8.3 How Is the DNA In Eukaryotic Cells Organized?

Eukaryotic chromosomes during cell division

Fig. 8-5



Chromosomes with the same genes are called homologous chromosomes, or homologues.

Cells with pairs of homologous chromosomes are called diploid.

Homologous chromosomes are usually not identical.• The same genes on homologous

chromosomes may be different due to mutations, changes in the sequence of nucleotides in the DNA.



Not all cells have paired chromosomes.

The ovaries and testes undergo a special kind of cell division, called meiotic cell division, to produce gametes (eggs and sperm).• Gametes contain only one member of each pair of

autosomes, plus one of the two sex chromosomes.

• Cells with half the number of each type of chromosome are called haploid cells.

• Fusion of two haploid cells at fertilization produces a diploid cell with the full complement of chromosomes.



The number of different types of chromosomes in a species is called the haploid number and is designated n.• In humans, n = 23.

Diploid cells contain 2n chromosomes.• Humans body cells contain 2n = 46 (2 x 23)

chromosomes.


8.4 What Occurs During The Eukaryotic Cell Cycle?

The eukaryotic cell cycle is divided into two major phases: interphase

and cell division.

• During interphase, the cell acquires nutrients from its environment, grows, and duplicates its chromosomes.

• During cell division, one copy of each chromosome and half of the cytoplasm are parceled out into each of two daughter cells.


cell growth

synthesis of DNA; chromosomes are duplicated

cell growth and differentiation

telophaseand

cytoki nesi s

anaphase

metaphase

prophase

mitotic cell

division

interphase


The eukaryotic cell cycle

Fig. 8-7



There are two types of division in eukarytic

cells: mitotic cell division and meiotic cell division.• Mitotic cell division may be thought of as

ordinary cell division, such as occurs during development from a fertilized egg, during asexual reproduction, and in skin, liver, and the digestive tract every day.

• Meiotic cell division is a specialized type of cell division required for sexual reproduction.



Mitotic cell division• Mitotic cell division consists of nuclear division

(called mitosis) followed by cytoplasmic division (called cytokinesis) and the formation

of two daughter cells.



Meiotic cell division• Is a prerequisite for sexual reproduction in all

eukaryotic organisms.• Meiotic cell division involves a specialized

nuclear division called meiosis.• It involves two rounds of cytokinesis,

producing four daughter cells that can become gametes.


haploid

diploid

meiotic cell division in testes

meiotic cell division in ovaries

adults

eggfertilized egg

fusion of gametes

sperm

embryo

baby

mitotic cell division, differentiation, and growth



8.4 The Eukaryotic Cell Cycle

The life cycle of eukaryotic organisms include both mitotic and meiotic cell division.

Fig. 8-8

Flash


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?

Mitosis is divided into four phases.• Prophase• Metaphase• Anaphase• Telophase

Flash


nuclear envelope chromatin

nucleolus

centriole pairs

beginning of spindle formation

kinetochore

spindle pole

spindle polecondensing chromosomes

spindle microtubules

Late InterphaseDuplicated chromosomes are in the relaxed uncondensed state; duplicated centrioles remain clustered.

Early ProphaseChromosomes condense and shorten; spindle microtubules begin to form between separating centriole pairs.

Late Prophase Thenucleolus disappears; the nuclear envelope breaks down; spindle microtubules attach to the kinetochore of each sister chromatid.

MetaphaseKinetochores interact; spindle microtubules line up the chromosomes at the cell’s equator.

(a) (b) (c) (d)


Interphase, prophase, and metaphase

Fig. 8-9a–d


chromosomes extending nuclear envelope

re-forming

Anaphase Sisterchromatids separate and move to opposite poles of the cell; spindle microtubules that are not attached to the chromosomes push the poles apart.

Telophase One set ofchromosomes reaches each pole and relaxes into the extended state; nuclear envelopes start to form around each set; spindle microtubles begin to disappear.

CytokinesisThe cell divides in two; each daughter cell receives one nucleus and about half of the cytoplasm.

Interphase of daughter cells Spindlesdisappear, intact nuclear envelopes form, chromosomes extend completely, and the nucleolus reappears.

unattached spindle microtubules

(e) (f) (g) (h)


Anaphase, telophase, cytokinesis, and interphase

Fig. 8-9e–h



Three major events happen in prophase:• The duplicated chromosomes condense.• The spindle microtubules form and attach to the

kinetochore

of the chromatids.• The chromosomes migrate with the spindle poles to

opposite sides of the nucleus.

Fig. 8-9b–c



During metaphase, the chromosomes line up along the equator of the cell.• At this phase, the spindle apparatus lines up

the sister chromatids

at the equator, with one kinetochore

facing each cell pole.

Fig. 8-9d



During anaphase, sister chromatids

separate and

move to opposite poles of the cell.• Sister chromatids

separate, becoming independent daughter chromosomes.

• The kinetochores

pull the chromosomes poleward

along the

spindle microtubules.



During telophase, nuclear envelopes form around both groups of chromosomes.• Telophase

begins when

the chromosomes reach the poles.

• The spindle microtubules disintegrate and the nuclear envelop forms around each group of chromosomes.



During Telophase

cytokinesis

occurs splitting the

cytoplasm between the two daughter cells ending mitosis and beginning Interphase.

Fig. 8-10

Scanning electron micrograph of cytokinesis.

Microfilaments contract, pinching the cell in two

The microfilament ring contracts, pinching in the cell’s “waist.”

The waist completely pinches off, forming two daughter cells

Microfilaments form a ring around the cell’s equator.

(b)(a)



Cytokinesis

in plant cells has an additional

step.

Fig. 8-11


8.6 How Does Meiotic Cell Division Produce Haploid Cells?

Meiosis is the production of haploid cells with unpaired chromosomes derived from diploid parent cells with paired chromosomes.

Meiosis includes two nuclear divisions, known as meiosis I and meiosis II.• In meiosis I, homologous chromosomes pair

up, but sister chromatids

remain connected to each other.

• In meiosis II, chromosomes behave as they do in mitosis—sister chromatids

separate and are

pulled to opposite poles of the cell. Flash



Fig. 8-12a–d

paired homologous chromosomes

recombined chromatids

spindle microtubule

kinetochoreschiasma

(a) (b) (c) (d)Prophase I Duplicated chromosomes condense. Homologous chromosomes pair up and chiasmata occur as chromatids of homologues exchange parts by crossing over. The nuclear envelope disintegrates, and spindle microtubules form.

Metaphase IPaired homologous chromosomes line up along the equator of the cell. One homologue of each pair faces each pole of the cell and attaches to the spindle microtubules via the kinetochore (blue).

Anaphase IHomologues separate, one member of each pair going to each pole of the cell. Sister chromatids do not separate.

Telophase ISpindle microtubules disappear. Two clusters of chromosomes have formed, each containing one member of each pair of homologues. The daughter nuclei are therefore haploid. Cytokinesis commonly occurs at this stage. There is little or no interphase between meiosis I and meiosis II.


(e) (f) (g) (h) (i)Prophase IIIf the chromosomes have relaxed after telophase I, they recondense. Spindle microtubules re-form and attach to the sister chromatids.

Metaphase IIThe chromosomes line up along the equator, with sister chromatids of each chromosome attached to spindle microtubules that lead to opposite poles.

Anaphase IIThe chromatids separate into independent daughter chromosomes, one former chromatid moving toward each pole.

Telophase IIThe chromosomes finish moving to opposite poles. Nuclear envelopes re-form, and the chromosomes become extended again (not shown here).

Four haploid cellsCytokinesis results in four haploid cells, each containing one member of each pair of homologous chromosomes (shown here in the condensed state).


Fig. 8-12e–i



Flash


8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability?

Ways to produce genetic variability from meiotic cell division and sexual reproduction• Shuffling of homologues• Crossing over• Fusion of gametes


The four possible chromosome arrangements at metaphase of meiosis I

The eight possible sets of chromosomes after meiosis I

(a)

(b)

8.7 Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability

Shuffling of homologues creates novel combinations of chromosomes during meiosis producing genetic variability.

Fig. 8-13


8.7 Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability

Crossing over creates chromosomes with novel combinations of genetic material.

Fig. 8-14

pair of homologous duplicated chromosomes

sister chromatids of one duplicated homologue

chiasmata (sites of crossing over)

parts of chromosomes that have been exchanged between homologues


8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability?

Fusion of gametes creates genetically variable offspring.• Because every egg and sperm are genetically

unique, and it is random as to which sperm fertilizes which egg, every fertilized egg is also genetically unique.


9.1 What Is The Physical Basis Of Inheritance?

Inheritance occurs when genes are transmitted from parent to offspring.• The units of inheritance are genes.

Genes are segments of DNA at specific locations on chromosomes.• A gene’s physical location on a chromosome is

called its locus.• Each member of a pair of homologous

chromosomes carries the same genes, located at the same loci.

• Different versions of a gene at a given locus are called alleles.



The relationship among genes, alleles, and chromosomes

Fig. 9-1

Both chromosomes carry the same allele of the gene at this locus; the organism is homozygous at this locus

This locus contains another gene for which the organism is homozygous

Each chromosome carries a different allele of this gene, so the organism is heterozygous at this locus

a pair of homologous

chromosomes

gene loci

the chromosome from the male

parent

the chromosome from the female parent



Mutations are the source of alleles.• Differences in alleles at a given locus are due

to mutations at that gene.• If a mutation occurs in gametes, it can be

passed on from parent to offspring.

An organism’s two alleles may be the same or different.• A diploid organism has pairs of homologous

chromosomes with two copies of each gene at a given locus.



An organism’s two alleles may be the same or different (continued).• If both homologous chromosomes have the

same allele at a locus, the organism is said to be homozygous.

• If two homologous chromosomes have different alleles at a locus, the organism is heterozygous at that locus.

• The gametes of a homozygous individual are all the same at a particular locus, while gametes of a heterozygous individual would contain half one allele and half the other allele.


9.3 How Are Single Traits Inherited?

True-breeding traits of organisms, such as purple flower color, are always inherited by all of their offspring that result from self-

fertilization.• In one experiment, Mendel cross-fertilized

white-flowered plants with purple-flowered plants.

• When he grew the resulting seeds, he found all the first-generation offspring, or the F1

generation, produced purple flowers.• What happened to the white color?



The F2

generation • Next, Mendel allowed the F1

flowers to self- fertilize, collected the seeds, and grew the

second generation, called the F2

generation.• Flowers in the F2

generation were three- fourths purple and one-fourth white, in a ratio

of 3 purple to 1 white.• This showed that the gene for white flowers

was “hidden” in the F1

generation, but appeared again in the F2

generation.


First- generation offspring (F1 )

3/4 purple 1/4 white

Second- generationoffspring (F2 )

self-fertilize


Cross of F1

plants with purple flowers

Fig. 9-5



All the white-flowered plants in the F2

generation only produced additional white- flowered plants.

Purple-flowered plants were of two types:• About ⅔ were true-breeding for purple, while ⅔ produced both purple-

and white-flowered

offspring (ratio 3 purple/1 white).• Therefore, the F2

generation included ¼ true- breeding purple plants, ½ hybrid purple, and

¼ true-breeding white plants.



This allows us to develop a five-part hypothesis to explain the inheritance of single traits.

1.

Each trait is determined by pairs of distinct physical units called genes.

• There are two alleles for each gene, one on each homologous chromosome.

2.

When two different alleles are present in an organism, the dominant allele may mask the expression of the recessive allele; but the recessive allele is still present.

3.

The two alleles of a gene segregate (separate) from one another during meiosis (Mendel’s law of segregation).



4.

Which allele ends up in any given gamete is determined by chance.

5.

True-breeding (homozygous) organisms have two copies of the same allele for a given gene; hybrid (heterozygous) organisms have two different alleles for a given gene.


homozygous parent

Gametes produced by a homozygous parent

Gametes produced by a heterozygous parent

gametes

heterozygous parent gametes

A A AA

A a aA

(a)

(b)


The distribution of alleles in gametes

Fig. 9-6



Mendel’s hypothesis was that two plants may look alike (phenotype) but have a different allele composition (genotype).

Purple pea plants had PP or Pp genotypes, but their phenotype (purple color) was the same.

The F2

generation could be described as having three genotypes (¼ PP, ½ Pp, and ¼ pp) and two phenotypes (¾ purple and ¼ white).



The Punnett square method

Fig. 9-8

PP

pp

Pp

eggs

self-fertilizePp

12

12

12

414

4 4

12

pP

1

1 1

P p

P

p

sper

m



Mendel predicted the outcome of cross-fertilizing Pp plants with homozygous recessive plants (pp)—there should be equal numbers of Pp (purple) and pp (white) offspring.

Fig. 9-9

pollen

all sperm

pp

Pp

PP or Pp

if PP if Pp

eggs eggs

sperm unknown

12

12

12

12

all Pp

pp all eggs

sper

m

p

pp

p

PP


9.4 How Are Multiple Traits Inherited?

Mendel next crossed pea plants that differed in two traits, such as seed color (yellow or green) and seed shape (smooth or wrinkled).• He knew from previous crosses that smooth

and yellow were both dominant traits in peas.• His first cross was a true-breeding plant with

smooth, yellow seeds (SSYY) to a true- breeding plant with wrinkled, green seeds

(ssyy).



All the offspring of this cross (F1

generation) were SsYy and had smooth, yellow seeds (both dominant traits).

F1

plants were allowed to self-fertilize and produced F2

offspring in the phenotypic ratio 9:3:3:1.



Mendel concluded that multiple traits are inherited independently.• Mendel realized that these results could be

explained if the genes for seed color and seed shape were inherited independently.

• The independent inheritance of two or more distinct traits is called the law of independent assortment.

• Multiple traits are inherited independently because the alleles of one gene are distributed to gametes independently of the alleles of other genes.



Predicting genotypes and phenotypes

Fig. 9-11

14

14

14

14

14

14

14

116

116

116

116

116

116

116

116

116

116

116

116

116

116

116

116

SSYY SsYY

ssYY

ssyY

SsyY

SSYy SsYy

SsYy

ssYy

ssyy

SsyySSyy

sSyY sSyy

sSYY sSYy

SSyY

eggs

self-fertilize

14

sper

m

SY

SY

Sy

Sy

sY

sY

sy

sy



Independent assortment of alleles

Fig. 9-12

independent assortment produces four equally likely allele combinations during meiosis

pairs of alleles on homologous chromosomes in diploid cells

chromosomes replicate

replicated homologues pair during metaphase of meiosis I, orienting like this

or like this

meiosis II

meiosis I

SY sYsy Sy

S

S

S

S

S

S

SS

S

s

s

s

s s

s

s

s s

ss

Y

Y

Y

Y

Y

S

Y

Y

Y

Y

Y Y

y

y

y

y

y

y

y

y y y

S

y


9.5 How Are Genes Located on the Same Chromosome Inherited?

Genetic linkage is the inheritance of genes as a group because they are on the same chromosome.• Genes that are located on the same chromosome are

inherited together, rather than sorted independently.• In peas, the gene for flower color and the gene for

pollen shape occur on the same chromosome and are inherited together.

• Because the two genes are located on the same chromosomes, they tend to end up in gametes together, and are then expressed in the plants.


9.5 How Are Genes Located on the Same Chromosome Inherited?

Crossing over can create new combinations of linked alleles.• Genes on the same chromosome do not always stay

together.• During prophase I of meiosis, homologous

chromosomes sometimes exchange parts in the process, called crossing over.

• Crossing over produces a new allele combination on both homologous chromosomes.

• Therefore, the chromosomes of each haploid daughter cell receives different combinations of alleles from those of the parent cell.


9.6 How Is Sex Determined?

female offspring

eggs

male offspring

male parent

sper

m

X1

X2

Xm

Y

X1 X2

X1 X2

X1

Xm

XmX2

YY

Y

Xm

female parent

Offspring sex is determined sex chromosomes.• In mammals, females have two X

chromosomes and males have an X chromosome and a Y chromosome.

• Y chromosomes are much smaller than the X chromosomes.


9.7 How Are Sex-Linked Genes Inherited?

Genes that are found on one sex chromosome but not on the other are called sex-linked.• Because females have two X chromosomes,

they can be either homozygous or heterozygous for genes on the X chromosome.

• Males only have one X chromosome, and therefore express all the alleles they have on their X chromosome.


9.8 Do Mendelian Rules Of Inheritance Apply To All Traits?

When a heterozygous phenotype is intermediate between the two homozygous phenotypes, the pattern of inheritance is called incomplete dominance.• Human hair texture is influenced by a gene

with two incompletely dominant alleles, C1 and C2 .

• A person with two copies of the C1 allele has curly hair; two copies of the C2 allele produces straight hair; heterozygotes

with C1 C2

genotype have wavy hair.


Two wavy-haired people could have the following children: ¼ curly (C1 C1 ), ½ wavy (C1 C2 ), and ¼ straight (C2 C2 ).


Fig. 9-17

eggs

C1 C2

mother

father

C1 C2

C1 C2 C2 C2

C1 C1 C1 C2

sper

m

C1 C2

C1

C2



A single gene may have multiple alleles.• A single individual can have only two alleles

for any gene, one on each homologous chromosomes.

• However, within all the members of a species there could be dozens of alleles for every gene.



A single trait may be influenced by several genes.• Many physical traits are governed not by

single genes, but by interactions among two or more genes, a phenomenon called polygenic inheritance.

• The more genes that contribute to a single trait, the greater the number of phenotypes and the finer the distinctions among them.


9.10 How Are Single-Gene Disorders Inherited?

Some human genetic disorders are caused by recessive alleles.• Many genes encode information to synthesize

enzymes or structural proteins in cells, and a defective allele in such a gene may cause damaged or inactive protein.

• In some cases, a defective gene may be masked when one normal allele is also present and makes enough functional protein.


9.10 How Are Single-Gene Disorders Inherited?

Some human genetic disorders are caused by dominant alleles.• Many genetic diseases are caused by

dominant alleles, in which a single defective allele is enough to cause the disorder.

• For dominant diseases to be inherited, at least one parent must suffer from the disease but live long enough to have children.

• Some diseases, like Huntington disease, do not appear until after the affected person has reproduced.


10.1 What Is The Structure Of DNA?

Individual traits of an organism are transmitted from parent to offspring in discrete units of DNA called genes.

Genes are located on chromosomes found within the nucleus of cells.

What makes all organisms different from each other is the arrangement and molecular composition of its genes.



DNA is composed of four different subunits, called nucleotides.• Each nucleotide has three parts:

• A phosphate group• Deoxyribose, a 5 Carbon sugar• One of four different nitrogen-containing

bases• Thymine• Cytosine• Adenine• Guanine



A DNA molecule contains two nucleotide strands.• A DNA molecule consists of two DNA strands

of linked nucleotides.• Within each strand, the phosphate group of

one nucleotide binds to the sugar group of the next nucleotide.

• The sugar-phosphate bonding produces a sugar-phosphate backbone to the DNA molecule.



The Watson-Crick model of DNA structure

Fig. 10-2

free phosphate

phosphate

base (cytosine)

sugar

free sugar

Hydrogen bonds hold complementary base pairs together in DNA

Two DNA strands form a double helix

Four turns of a DNA double helix

nucleotide nucleotide

(a) (b) (c)



Nucleotide rungs only result in specific pair combinations.• Adenine only pairs with Thymine.• Guanine only pairs with Cytosine.• This A–T and G–C coupling is called

complementary base pairing.


10.3 How Is DNA Copied?

Cells reproduce themselves by making two daughter cells from each parental cell, each with a complete copy of all the parental cell’s genetic information.

During cell reproduction, the parental cell synthesizes two exact copies of its DNA through a process called DNA replication.

One copy goes into each daughter cell.



DNA replication produces two DNA double helices, each with one original strand and one new strand.• DNA replication requires three ingredients:

• The parental DNA strands• Free nucleotides that were synthesized in

the cytoplasm and then imported to the nucleus

• Enzymes that unwind the parental DNA double helix and synthesize the new DNA strands



DNA replication produces two DNA double helices, each with one original strand and one new strand (continued).• The first step involves enzymes called DNA helicases,

which pull apart the parental DNA double helix.• Next, enzymes called DNA polymerases move along

each separated parental DNA strand, matching each base on the strand with free nucleotides.

DNA replication keeps, or conserves, one parental DNA strand and produces one new daughter strand (semiconservative

replication).



The basic features of DNA replication

Fig. 10-3

free nucleotides

The parental DNA is unwound

New DNA strands are synthesized with bases complementary to the parental strands

Each new double helix is composed of one parental strand (blue) and one new strand (red)

Parental DNA double helix

1

2

4

3


10.4 What Are The Mechanisms Of DNA Replication?

DNA helicase

separates the parental DNA strands

by breaking the hydrogen bonds between complementary bases. • This activity separates the two strands and forms a

replication bubble where the parental strands are no longer paired.

• Replication then proceeds.• There is a replication fork on each end of the bubble,

where replication is taking place and the original DNA strand is unzipping.

• The unzipping and replication continues in both directions until the new strands are completely formed.



The mechanism of DNA replication, step (2)

Fig. 10-5(2)

replication forks

DNA helicase DNA helicase



DNA polymerase synthesizes new DNA strands.• At the replication forks, DNA polymerase

recognizes unpaired nucleotide bases in the parental strand and matches them up with free nucleotides.

• It then links up the phosphate of the incoming nucleotide with the sugar of the previously added nucleotide, thereby contributing to the growing molecule backbone.



DNA helicase

and DNA polymerase work

together to copy each strand of separated parental DNA.• Polymerase # 1 lands on one strand of DNA

and follows behind the helicase

toward the free phosphate end of the DNA, making a continuous new DNA strand.

• DNA polymerase # 2 on the other parental strand moves away from the helicase

and

makes only part of the new DNA strand.



As the helicase

continues to unwind more of

the double helix, additional DNA polymerase (# 3, # 4, etc.) must land on this strand to synthesize more pieces of DNA.

Therefore, DNA synthesis on the second parental strand is discontinuous.




Fig. 10-5(4)

DNA polymerase #1 continues along the parental DNA strand

DNA polymerase #3

DNA polymerase #2 leaves



Multiple DNA polymerases make many pieces of DNA of varying lengths that need to be tied together to form a single continuous DNA polymer.

DNA ligase

joins together the separate

segments of DNA.




Fig. 10-5(5)

DNA polymerase #4

DNA polymerase #3 leaves

DNA ligase joins the daughter DNA strands together



Proofreading produces almost error-free replication of DNA.• DNA polymerase is almost 100% perfect in

matching free nucleotides with those on the original parental strands.

• Once in every 10,000 base pairs, there is an error in replication.

• Some types of DNA polymerase recognize errors when they are made and correct them.

• This keeps the total errors in a complete DNA molecule to one mistake in every billion base pairs.

Mistakes that remain in the DNA nucleotide sequence are called mutations.


Types of Mutations

Point mutation -

individual nucleotide in the

DNA sequence is changed

Insertion mutation -

one or more nucleotide

pairs are inserted into the DNA double helix

Deletion mutation -

one or more nucleotide

pairs are removed from the double helix

Inversion -

piece of DNA is cut out of a

chromosome, turned around, and re-inserted into the gap

Translocation -

chunk of DNA (often very

large) is removed from one chromosome and attached to another


11.1 How Is The Information In DNA Used In A Cell?

Most genes contain information for the synthesis of a single protein.• A gene is a stretch of DNA encoding the

instructions for the synthesis of a single protein.

• Proteins form cellular structures and the enzymes that catalyze cellular chemical reactions.



Protein synthesis occurs in two steps, called transcription and translation.

Translation of the mRNA produces a protein molecule with an amino acid sequence determined by the nucleotide sequence in the mRNA

Transcription of the gene produces anmRNA with a nucleotide sequence complementary to one of the DNA strands

DNA

messenger RNA

protein

ribosome

gene

Transcription

Translation

(cytoplasm)(nucleus)

(a)

(b)



Transcription: the information contained in the DNA of a specific gene is copied into one of three types of RNA

• Messenger RNA (mRNA)• Transfer RNA (tRNA)• Ribosomal RNA (rRNA)

In eukaryotic cells, transcription occurs in the nucleus.

Flash



Translation: ribosomes

convert the base

sequence in mRNA to the amino acid sequence of a protein• In eukaryotic cells, translation occurs in the

cytoplasm.

Flash


11.2 What Are The Functions Of RNA?

Messenger RNA carries the code for a protein from the nucleus to the cytoplasm.• All RNA is produced by transcription from

DNA, but only mRNA carries the code for amino acid sequence of a protein.

• mRNA is synthesized in the nucleus and enters the cytoplasm through nuclear envelope pores.

• In the cytoplasm, mRNA binds to ribosomes, which synthesize a protein specified by the mRNA base sequence; DNA remains in the nucleus.



Ribosomal RNA and proteins form ribosomes.• Each ribosome consists of two subunits—one small

and one large.• The small subunit has binding sites for mRNA, a

“start” tRNA, and other proteins that cooperate to read mRNA to start protein synthesis.

• The large subunit has two binding sites for tRNA molecules, and one catalytic site where peptide

bonds join amino acids together into a protein.• During protein synthesis, the two subunits come

together, clasping an mRNA molecule between them.



Transfer RNA molecules carry amino acids to the ribosomes.• Each cell synthesizes many different kinds of

transfer RNA, one or more for each amino acid.

• Twenty different kinds of enzymes in the cytoplasm, one for each amino acid, recognize the rRNA

and attach the correct amino acid.

• These “loaded” tRNA

molecules deliver their amino acids to the ribosome, where they are incorporated into the growing protein chain.


11.3 What Is The Genetic Code?

The genetic code translates the sequence of bases in nucleic acids into the sequence of amino acids in proteins.• A sequence of three bases codes for an

amino acid; the triplet is called a codon.• There are 64 possible combinations of

codons, which is more than enough to code for the 20 amino acids in proteins.



How does a cell recognize where codons start and stop, and where the code for an

entire proteins starts and stops?• Most codons

specify a specific amino acid in a

protein sequence, but others are punctuation marks that indicate the end of one protein sequence and the start of another.

• All proteins begin with the start codon

AUG (methionine), and all end with UAG, UAA, or UGA, called stop codons.

• Almost all amino acids are coded for by more than one codon

(e.g., six codons

code for

leucine).


11.4 How Is The Information In A Gene Transcribed Into RNA?

Transcription copies the genetic information of DNA into RNA in the nucleus of eukaryotic cells.• Transcription is made up of three different

processes:• Initiation: the promotor

region at the

beginning of a gene starts transcription• Elongation: the main body of a gene is

where the RNA strand is elongated• Termination: the termination signal at end of

a gene is where RNA synthesis stops



Transcription begins when RNA polymerase binds to the promotor

of a gene.

• RNA polymerase catalyzes the transcription of DNA to RNA.

• RNA polymerase first finds the promoter region (a non-transcribed sequence of DNA bases) that marks the start of a gene, and then binds to it, opening up the DNA as it does.

• Transcription of the gene begins after the promoter is bound to RNA polymerase.



Elongation generates a growing strand of RNA.• RNA polymerase adds complementary bases

to those in the DNA template strand, to make a growing RNA strand that has uracil

rather

than thymine complementary to adenine.• The two strands of DNA re-form the original

double helix.• One end of the growing RNA strand drifts

away from the DNA molecule, while the other remains attached to the DNA template strand by the RNA polymerase.



Transcription stops when RNA polymerase reaches the termination signal.• RNA polymerase continues along the DNA

template strand until it comes to the termination signal (a specific sequence of DNA bases).

• At the termination signal, RNA polymerase drops off the DNA and releases the completed RNA molecule.

• The enzyme is ready to bind to another promoter, to start the process over.



Transcription is selective.• Some genes are transcribed in all cells

because they encode essential proteins, like the electron transport chain of mitochondria.

• Other genes are transcribed only in specific types of cells.

• Proteins bind to “control regions” near gene promotors

and block or enhance the binding of

RNA polymerase.• By this means, the amount of a specific

protein encoded by a specific gene in a cell can be controlled.


11.5 How Is The Information In Messenger RNA Translated Into Protein?

mRNA, with a specific base sequence, is used during translation to direct the synthesis of a protein with the amino acid sequence encoded by the mRNA.• Decoding the base sequence of mRNA is the

job of tRNA

and ribosomes

in the cytoplasm.• The ability of tRNA

to deliver the correct

amino acid to the ribosomes

depends on base pairing between each codon

of mRNA and a

set of three complementary bases in tRNA, called the anticodon.



Like transcription, translation has three steps:• Initiation of protein synthesis• Elongation of the protein chain• Termination of translation



Initiation

Fig. 11-5(1,2,3)

tRNA anticodonfirst tRNA binding site

catalytic sitesecond tRNA binding site

mRNA

large ribosomal subunit

start codon

small ribosomal subunit

methionine tRNA

amino acid

initiation complex

The large ribosomal subunit binds to the small subunit. The methionine tRNA binds to the first tRNA site on the large subunit.

The initiation complex binds to an mRNA molecule. The methionine (met) tRNA anticodon (UAC) base-pairs with the start codon (AUG) of the mRNA.

A tRNA with an attached methionine amino acid binds to a small ribosomal subunit, forming an initiation complex.

Initiation:

metmet

C

A ACC G GG U U U

A

A

ACC G GG U U U

CAU

CAUU

met



Elongation

Fig. 11-5(4,5,6)

peptide bond

initiator tRNA detaches

catalytic site

ribosome moves one codon to the rightThe second codon of mRNA

(GUU) base-pairs with the anticodon (CAA) of a second tRNA carrying the amino acid valine (val). This tRNA binds to the second tRNA site on the large subunit.

The “empty” tRNA is released and the ribosome moves down the mRNA, one codon to the right. The tRNA that is attached to the two amino acids is now in the first tRNA binding site and the second tRNA binding site is empty.

The catalytic site on the large subunit catalyzes the formation of a peptide bond linking the amino acids methionine and valine. The two amino acids are now attached to the tRNA in the second binding site.

Elongation:

AUA ACC G GG U U U A ACC G GG GU U U

C A A A ACAU

A ACC G GG U U U

C A ACAU C

metmet

valvalval

met



Elongation (continued)

Fig. 11-5(7,8)

The catalytic site forms a peptide bond between valine and histidine, leaving the peptide attached to the tRNA in the second binding site. The tRNA in the first site leaves, and the ribosome moves one codon over on the mRNA.

The third codon of mRNA (CAU) base-pairs with the anticodon (GUA) of a tRNA carrying the amino acid histidine (his). This tRNA enters the second tRNA binding site on the large subunit.

G AUA A

A

CC G G

G

GU U

U

U

C A A

AUA A

A

CC G G G

G

G U U

U

U

C A A

met

valval

met

hishis



Termination

Fig. 11-5(9)

stop codon

completed peptide

This process repeats until a stop codon is reached; the mRNA and the completed peptide are released from the ribosome, and the subunits separate.

Termination:

AUGAC A UG AA C Um

etva

l

hisarg

argile



Summing up: transcription and translation• With a few exceptions, each gene codes for a

single protein.• Transcription of a protein-coding gene

produces an mRNA that is complementary to the template strand of the DNA for the gene.

• Enzymes in the cytoplasm attach the appropriate amino acid to each tRNA.

• The mRNA moves from the nucleus to the cytoplasm.

• tRNAs

carry their attached amino acids to the ribosome.



Summing up: transcription and translation (continued)• At the ribosome, the bases in tRNA

anticodons

bind to the complementary bases in mRNA codons.

• The amino acids attached to the tRNAs

line up in the sequence specified by the codons.

• The ribosome joins the amino acids together with peptide bonds to form a protein.

• When a stop codon

is reached, the finished protein is released from the ribosome.



Complementary base pairing is critical to decoding genetic information.

Fig. 11-6

template DNA strand

complementary DNA strand

gene

codons

anticodons

amino acids

etc.

etc.

etc.

etc.methionine glycine valine

DNA

mRNA

tRNA

protein

etc.(a)

(b)

(c)

(d)

A U G G G A G U U

U A C C C U C A A

T A C C C T C A A

A T G G G A G T T


11.6 How Do Mutations Affect Gene Function?

Mutations are the raw material for evolution.• Mutations are the ultimate source of all genetic

differences among individuals.• Without mutations, individuals would share the same

DNA sequence.• Most mutations are harmful; some improve the

individual’s ability to survive and reproduce.• The mutation may be passed from generation to

generation and become more common over time.• This process is known as natural selection, and is the

major cause of evolutionary change.


11.7 Are All Genes Expressed?

All of the genes in the human genome are present in each body cell, but individual cells express only a small fraction of them.• The particular set of genes that is expressed

depends on the type of cell and the needs of the organism.

• This regulation of gene expression is crucial for proper functioning of individual cells and entire organisms.



Gene expression differs from cell to cell and over time.• The set of genes that are expressed depends

on the function of a particular cell.• Hair cells synthesize the protein keratin, while

muscle cells make the proteins actin

and myosin but do not make keratin.

• A human male does not express a casein gene, the protein in human milk, but will pass on the gene for casein synthesis to his daughter, who will express it if she bears children.



Environmental cues influence gene expression.• Changes in an organism’s environment help

determine which genes are transcribed.• Longer spring days stimulate the sex organs

of birds to enlarge and produce sex hormones.

• These hormones cause the birds to produce eggs and sperm, to mate, and to build nests.

• All these changes result directly or indirectly from alterations in gene expression.


11.8 How Is Gene Expression Regulated?

A cell may regulate gene expression in many different ways.• It may alter the rate of transcription of mRNA.• It may affect how long a given mRNA

molecule lasts before being broken down.• It may affect how fast the mRNA is translated

into protein.• It may affect how long the protein lasts, or how

fast a protein enzyme catalyzes a reaction.



Regulatory proteins that bind to promoters alter the transcription of genes.• Many steroid hormones act in this way.• In birds, estrogen enters cells of the female

reproductive system and binds to a receptor protein during the breeding season.

• The estrogen–receptor combination then binds to the DNA in a region near the promotor

of an albumen gene.



Regulatory proteins that bind to promoters alter the transcription of genes (continued).• This attachment makes it easier for RNA

polymerase to bind to the promotor

and to transcribe large amounts of albumen mRNA, which is translated into the albumin protein needed to make eggs.



Some regions of chromosomes are condensed and not normally transcribed.• Certain parts of eukaryotic chromosomes are

in a highly condensed, compact state in which most of the DNA is inaccessible to RNA polymerase.

• Some of these tightly condensed regions may contain genes that are not currently being transcribed, but when those genes are needed, the portion of the chromosome containing those genes becomes “decondensed” so that transcription can occur.



Entire chromosomes may be inactivated and not transcribed.• In some cases, almost an entire chromosome

may be condensed, making it largely inaccessible to RNA polymerase.

• In human females, one of their two X chromosomes may become inactivated by a special coating of RNA called Xist, which condenses the chromosome and prevents gene transcription.

8.1 why do cells divide? - gavilan...

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