cell division - napa valley college

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Chapter 13 – Meiosis and Sexual Life Cycles

Overview

I. Cell Types

II. Meiosis

I. Meiosis I

II. Meiosis II

III. Genetic Variation

IV. Reproduction

Overview: Variations on a Theme

Living organisms are distinguished by their

ability to reproduce their own kind

Genetics is the scientific study of heredity and

variation

Heredity is the transmission of traits from one

generation to the next

Variation is demonstrated by the differences in

appearance that offspring show from parents

and siblings

© 2011 Pearson Education, Inc.

Figure 13.1

Offspring acquire genes from parents by

inheriting chromosomes

In a literal sense, children do not inherit

particular physical traits from their parents

It is genes that are actually inherited


Inheritance of Genes

Genes are the units of heredity, and are made up of segments of DNA

Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs)

Each gene has a specific location called a locus on a certain chromosome

Most DNA is packaged into chromosomes


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Comparison of Asexual and Sexual Reproduction

In asexual reproduction, a single individual

passes genes to its offspring without the fusion

of gametes

A clone is a group of genetically identical

individuals from the same parent

In sexual reproduction, two parents give rise

to offspring that have unique combinations of

genes inherited from the two parents


Figure 13.2

(a) Hydra (b) Redwoods

Bud

Parent

0.5 mm

Fertilization and meiosis alternate in sexual

life cycles

A life cycle is the generation-to-generation

sequence of stages in the reproductive

history of an organism


Reproduction

Asexual reproduction: Parent cell divides into

two daughter cells (Mitosis). The end result is

a two daughter cells identical to parent cell

Sexual reproduction: The union of two

gametes (sex cells) to form a single zygote

Eggs and Sperm are gametes

Fertilized egg is zygote

Zygote is different from gametes

Fertilization

Fertilization is the union between the sperm

and the egg.

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Cell Types

Mitosis occurs in all the body’s cells except the

cells that are responsible for reproduction

Gametes: are the cells that are responsible for

reproduction

All the rest of the body’s cells are somatic cells

Gametes = reproductive cells

Sperm and eggs are reproductive cells –

gametes

The cells that divide to produce gametes

undergo meiosis

Remember that we have 23 pairs of

chromosomes = 46 chromosomes

If gametes (sperm and egg) combined with

all these chromosomes then the offspring

will have 92 chromosomes

How do gametes overcome this problem?

Before the gametes come together they

need to reduce their number of

chromosomes in half.

So instead of 23 pairs (46 chromosomes)

they need to have 23 chromosomes total.

The answer to their problem is meiosis –

halving their number of chromosomes

Sets of Chromosomes in Human Cells

Human somatic cells (any cell other than a

gamete) have 23 pairs of chromosomes

A karyotype is an ordered display of the

pairs of chromosomes from a cell

The two chromosomes in each pair are

called homologous chromosomes, or

homologs

Chromosomes in a homologous pair are the

same length and shape and carry genes

controlling the same inherited characters


Figure 13.3

Pair of homologous duplicated chromosomes

Centromere

Sister chromatids

Metaphase chromosome

5 m

APPLICATION

TECHNIQUE

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The sex chromosomes, which determine

the sex of the individual, are called X and Y

Human females have a homologous pair of X

chromosomes (XX)

Human males have one X and one Y

chromosome

The remaining 22 pairs of chromosomes are

called autosomes


Sex Chromosomes in Human Cells

Each pair of homologous chromosomes

includes one chromosome from each parent

The 46 chromosomes in a human somatic cell

are two sets of 23: one from the mother and

one from the father

A diploid cell (2n) has two sets of

chromosomes

For humans, the diploid number is 46 (2n = 46)


Sets of Chromosomes in Human Cells Terminology

Diploid = Cells that contain two sets of chromosomes. In humans, cells that have 46 chromosomes or 23 pairs; all somatic cells are diploid (2n)

Haploid = Cells that have one set of chromosomes. In humans, cells that have 23 chromosomes; gametes are haploid (1n)

Polyploidy = three sets of chromosomes; rare in animals, common in plants

Meiosis is when a diploid cell divides to produce haploid reproductive cells

Meiosis

First the chromosomes (DNA) are

duplicated during Interphase

Then there are two cell divisions

Remember that mitosis had chromosome

(DNA) duplication followed by one cell

division

DNA Replication

In a cell in which DNA synthesis has

occurred, each chromosome is replicated

Each replicated chromosome consists of

two identical sister chromatids


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Remember that there are pairs

chromosomes, each chromosome has two

chromatids just after DNA replication

Figure 13.4

Sister chromatids of one duplicated chromosome

Key

Maternal set of chromosomes (n 3)

Paternal set of chromosomes (n 3)

Key

2n 6

Centromere

Two nonsister chromatids in a homologous pair

Pair of homologous chromosomes (one from each set)

Meiosis

The DNA has already replicated during interphase – the chromosomes have become duplicated

In Meiosis the chromosome homologous pairs separate and the cell divides = 1st cell division

Then the chromatids separate and cell divide = 2cd cell division

The figures are going to show only one pair of chromosomes – but there are 23 pairs at the start

Figure 13.7-1

Pair of homologous chromosomes in diploid parent cell

Duplicated pair of homologous chromosomes

Chromosomes duplicate

Sister chromatids

Diploid cell with duplicated chromosomes

Interphase

Figure 13.7-2




Sister chromatids


Homologous chromosomes separate

Haploid cells with duplicated chromosomes

Meiosis I

1

Interphase Figure 13.7-3




Sister chromatids



Haploid cells with duplicated chromosomes

Sister chromatids separate

Haploid cells with unduplicated chromosomes

Interphase

Meiosis I

Meiosis II

2

1

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Remember Meiosis happens to form gametes

– the reproductive cells (sperm and eggs)

The cells that produce the gametes start out

diploid before meiosis, and will end up haploid

There are two stages of Meiosis: Meiosis I

and II

Each Stage of Meiosis has Prophase,

Metaphase, Anaphase, and Telophase

Meiosis Overview

1. DNA replicates – chromosomes become

duplicated (two chromatids), the cell is diploid

(2n). This happens in Interphase.

2. Meiosis 1: homologous chromosomes separate

and the cell divides resulting in two haploid

cells (1n)

3. Meiosis 2: The chromatids separate and then

the cell divides resulting in four haploid cells

(1n)


BioFlix: Meiosis

Interphase

Meiosis I is preceded by interphase, when

the chromosomes are duplicated to form

sister chromatids

The sister chromatids are genetically

identical and joined at the centromere

The single centrosome replicates, forming

two centrosomes

Homologous Chromosomes in Prophase I

During Prophase I the homologous

chromosomes are attracted to each other and

become associated with each other forming a

tetrad.

The process of homologous chromosomes

pairing up during prophase I is called

synapsis.

A tetrad contains two chromosomes, both are

duplicated so there are four chromatids.

Crossing Over

Prophase I: Duplicated homologous

chromosomes condense and intertwine – this

produces genetic variation

Crossing over: genetic material is exchanged

between the homologous chromosomes

The sites of crossing over are called

chiasmata (singular, chiasma)

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Crossing Over

37

Prophase I

38

Metaphase I

39

Independent Assortment

During Metaphase I homologous pairs of

chromosomes line up the at the center of the

cell (the equator)

The tetrads arrange themselves randomly – this

also gives genetic variation = independent

assortment (alignment)

Metaphase I

In metaphase I, tetrads line up at the metaphase

plate, with one chromosome facing each pole

Microtubules from one pole are attached to the

kinetochore of one chromosome of each tetrad

Microtubules from the other pole are attached to

the kinetochore of the other chromosome


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Independent Assortment Anaphase I

44

Anaphase I

In anaphase I, pairs of homologous

chromosomes separate

One chromosome moves toward each pole,

guided by the spindle apparatus

Sister chromatids remain attached at the

centromere and move as one unit toward the

pole


Telophase I

46

Telophase I and Cytokinesis

In the beginning of telophase I, each half of

the cell has a haploid set of chromosomes;

each chromosome still consists of two sister

chromatids

Cytokinesis usually occurs simultaneously,

forming two haploid daughter cells


End of Telophase I - Cytokinesis

We now have two haploid cells (1n) which

means there are 23 chromosomes total in each

cell

The chromosomes are still in the duplicated

form – two chromatids

Note: not all species have cytokinesis after

telophase I

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Interphase

Interphase between Meiosis I and II is brief.

The S phase does not take place

Preparation for Meiosis II: centrosome

replicates

Meiosis II

Prophase II: The 23 chromosomes are already

condensed. The Nuclear membrane dissolves.

Metaphase II: Chromosomes line up at the

equator

Anaphase II: Chromatids separate

Telophase II and cytokinesis: Cells separate

Now there are four haploid cells: each has 23

chromosomes (not in the duplicated state)

Figure 13.8b

Prophase II Metaphase II Anaphase II Telophase II and

Cytokinesis


Haploid daughter cells forming

During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.

Prophase II

52

Metaphase II

53

Anaphase II

54

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Telophase II

55

Figure 13.8

MEIOSIS I: Separates homologous chromosomes

Prophase I Metaphase I Anaphase I Telophase I and

Cytokinesis

Centrosome (with centriole pair)

Sister chromatids

Chiasmata

Spindle

Homologous chromosomes

Fragments of nuclear envelope

Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n 6 in this example.

Centromere (with kinetochore)

Metaphase plate

Microtubule attached to kinetochore

Chromosomes line up by homologous pairs.

Sister chromatids remain attached


Each pair of homologous chromosomes separates.

Cleavage furrow

Two haploid cells form; each chromosome still consists of two sister chromatids.

MEIOSIS I: Separates sister chromatids

Prophase II Metaphase II Anaphase II Telophase II and

Cytokinesis


Haploid daughter cells forming

During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.

Figure 13.8a

Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis

Centrosome (with centriole pair)

Sister chromatids

Chiasmata

Spindle

Homologous chromosomes

Fragments of nuclear envelope

Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n 6 in this example.

Centromere (with kinetochore)

Metaphase plate

Microtubule attached to kinetochore

Chromosomes line up by homologous pairs.

Sister chromatids remain attached


Each pair of homologous chromosomes separates.

Cleavage furrow

Two haploid cells form; each chromosome still consists of two sister chromatids.

At the end of Meiosis I how many cells are there?

1. One

2. Two

3. Three

4. Four

One

Tw

o

Thre

e

Four

25% 25%25%25%

At the end of Meiosis I are these cells haploid or diploid?

1. Haploid

2. Diploid

Hap

loid

Dip

loid

50%50%

At the end of Meiosis I, how many chromosomes

are there in each cell?

1. 23 chromosomes

2. 46 chromosomes

23

chro

mos

omes

46

chro

mos

omes

50%50%

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At the end of Meiosis I, are the chomosomes

in the duplicated state?

1. Yes

2. No

Yes N

o

50%50%

At the end of Meiosis II how many cells are

there?

1. One

2. Two

3. Three

4. Four

One

Tw

o

Thre

e

Four

25% 25%25%25%

At the end of Meiosis II are these cells

haploid or diploid?

1. Haploid

2. Diploid

Hap

loid

Dip

loid

50%50%

At the end of Meiosis II, how many

chromosomes are there in each cell?

1. 23 chromosomes

2. 46 chromosomes

23

chro

mos

omes

46

chro

mos

omes

50%50%

At the end of Meiosis II, are the

chromosomes in the duplicated state?

1. Yes

2. No

Yes N

o

50%50%

Three events are unique to meiosis, and all three occur in meiosis l

Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information

At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes

At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate


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Genetic diversity through meiosis

There are three places in this process that

contribute to the genetic diversity of the

offspring.

Prophase I: The pairs of chromosomes

crossing over.

Metaphase I: The way the chromosomes

line up on the equator is random =

independent assortment

Random fertilization

Independent Assortment

68

Figure 13.10-1

Possibility 1 Possibility 2

Two equally probable arrangements of chromosomes at

metaphase I

Figure 13.10-2



metaphase I

Metaphase II

Figure 13.10-3



metaphase I

Metaphase II

Daughter cells

Combination 1 Combination 2 Combination 3 Combination 4

The number of combinations possible when

chromosomes assort independently into

gametes is 2n, where n is the haploid number

For humans (n = 23), there are more than 8

million (223) possible combinations of

chromosomes


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Figure 13.11-1 Prophase I of meiosis

Nonsister chromatids held together during synapsis

Pair of homologs



Pair of homologs

Chiasma

Centromere

TEM



Pair of homologs

Chiasma

Centromere

TEM

Anaphase I



Pair of homologs

Chiasma

Centromere

TEM

Anaphase I

Anaphase II

Figure 13.11a

Chiasma

Centromere

TEM

Random Fertilization

Random fertilization adds to genetic variation

because any sperm can fuse with any ovum

(unfertilized egg)

The fusion of two gametes (each with 8.4

million possible chromosome combinations

from independent assortment) produces a

zygote with any of about 70 trillion diploid

combinations


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Animation: Genetic Variation Right-

click slide / select “Play”

Meoisis and Gender

The gametes now contain 23 chromosomes,

haploid, and are not in the duplicated form

One of these chromosomes will be a sex

chromosome

Eggs will contain a X chromosome

Sperms will contain either a X or a Y

chromosome

X and Y are non-homologous chromosomes

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Spermatogenesis

In the male testes sperm are produced. One

cell produces 4 sperm.

Each sperm has 23 chromosomes, they are

not in the duplicated form

The sperm can have either an X or a Y sex

chromosome

The sperm have a small head and a long tail

= flagellum for locomotion

The sperm need to contain the genetic

material and deliver it to the egg.

The heads contain the chromosomes and

lots of mitochondria to power the flagella

About 400 million sperm are produced

each day

Egg Formation cont

All of the cells that produce the eggs are

made before the female mother is even born.

So when a girl is born, her ovaries contain all

the cells that produce her eggs

Each month one of these cells will leave the

ovary and go on to mature – and produce the

egg and polar bodies

Egg Formation

The ovaries in females produce eggs

One cell will produce one egg and three non-functioning “polar bodies”

The one egg gets most of the cytoplasm, leaving the other three cell not able to survive

The one egg has 23 chromosomes, with a X sex chromosome

The one egg is large enough to support the embryo

Fertilization

Fertilization is the union between the

sperm and the egg. Results in a diploid

zygote.

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Reproduction in Plants

Mosses and Ferns – use spores and egg

and sperm, have alternation of generation

Angiosperms – flowing plants use sperm

and egg

Reproduction using spores

Gametophytes produce gametes (eggs

and sperm)

Sporophyte:

Produce spores

Are dependent on the gametophytes

(they grow out of the gametophytes)

Fig. 11.2c

Sexual Reproduction in Angiosperms

Sperm – in pollen produced in anther of stamen

Egg – in ovary of carpel

Both these cells are haploid

Fruit – mature ovary

Pollination – transfer of pollen to stigma of female carpel

Fertilization – when the pollen grain fuses with the egg – producing the diploid zygote

Embryo develops inside seed

Seed germinates into plant

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Review of Mitosis vs Meiosis

Mitosis and Meiosis both start with a diploid

cell (46 chromosomes, 23 pairs)

Before both Mitosis and Meiosis the DNA

replicates during interphase, forming

duplicated chromosomes, each containing

two chromatids

Mitosis occurs in somatic cells (cells other

than those that produce the gametes),

Meiosis produces gametes

Mitosis

During Mitosis:

The chromatids are separated to

produce two cells, each with 46

chromosomes, 23 pairs of non

duplicated chromosomes

These cells are diploid (2n) cells

There is no exchanging of genetic

material

Meiosis – Two stages

Meiosis I: the pairs of chromosomes line up

and the chromosomes are separated,

resulting in 2 cells, each with 23

chromosomes, in the duplicated state =

haploid cells

Meiosis II: The chromatids are separated

producing two haploid cells that contain 23

non duplicated chromosomes.

One original cell produces four haploid

cells

101

Important concepts

Know all the vocabulary presented in the lecture

Know which cells undergo mitosis vs meiosis

How is genetic diversity introduced into

meiosis? What events contribute to genetic

diversity and when (what stage of meiosis) do

these events take place

How is the gender of the offspring determined.

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Important Concepts

How many functioning sperm are produced

from one spermatocyte. What sex

chromosomes can a sperm have? How many

functioning eggs are produced from one

oocyte? What sex chromosomes do eggs

have?

Know what pollen and eggs are in plants,

know the reproductive parts of plants, know

what pollination and fertilization are in plants,

what is the seed

Important Concepts for Lab Exam

For Meiosis: Know each stage, the order of

the stages, and what happens in each stage.

Know what the end result is of meiosis I and II

Know what state the cell and the

chromosomes are in at the beginning and end

of mitosis, meiosis I and at the end of meiosis

II. For example: Are the cells haploid or

diploid? Are the chromosomes duplicated, or

not duplicated? How many chromosomes are

there in the cell? Are they in pairs?

cell division - napa valley college

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