how cells reproduce chapter 7. types of cell division eukaryotic organisms –mitosis –meiosis...
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How Cells ReproduceChapter 7
Types of Cell Division
Eukaryotic organisms
– Mitosis
– Meiosis
Prokaryotic organisms
– Prokaryotic fission
Roles of Mitosis
• Multicelled organisms
– Growth
– Cell replacement
• Some protistans, fungi, plants, animals
– Asexual reproduction
Chromosome
• A DNA molecule and attached proteins
• Duplicated in preparation for cell division
Organization of Chromosomes
DNA wound tightly around histones before replication
The Cell Cycle
Interphase
• Usually longest part of the cycle (varies by cell type)
• G1 phase normal cell function and growth
• S phase DNA is replicated
• G2 phase proteins made for division
Prophase Metaphase Anaphase Telophase
PMAT
Mitosis
• Period of nuclear division
• Usually followed by cytoplasmic division
Control of the Cycle
• Once S begins, the cycle usually runs through
G2 and mitosis
• The cycle has a built-in molecular brake in G1
• Cancer involves a loss of control over the
cycle, malfunction of the “brakes”
Chromosome Number • Sum total of chromosomes in a cell
• Somatic cells– Chromosome number is diploid (2n)– Two of each type of chromosome– One from mom and one from dad
• Gametes– Chromosome number is haploid (n)– One of each chromosome type
Why are haploid cells needed for gametes?
Human Chromosome Number
• Diploid chromosome number (n) = 46
• Two sets of 23 chromosomes each– One set from father– One set from mother
• Mitosis produces cells with 46 (diploid) chromosomes: two of each type
The Spindle Apparatus• Consists of two distinct sets of microtubules
– Each set extends from one of the cell poles– Two sets overlap at spindle equator– Begins in centrosome (has centrioles in animals)
• Moves chromosomes during mitosis
Maintaining Chromosome
Number
chromosomes (unduplicated) in parent cell at interphase
Stages of Mitosis
Prophase (early, late)
Metaphase
Anaphase
Telophase
Early Prophase: Mitosis Begins
Duplicated chromosomes begin to condense
Late Prophase
• New microtubules are assembled
• One centriole pair is moved toward opposite pole of spindle
• Nuclear envelope starts to break up
Transition to Metaphase
• Spindle forms
• Spindle microtubules become attached to the kinetochore on the two sister chromatids of each chromosome
Metaphase
• All chromosomes are lined up at the spindle equator
• Chromosomes are maximally condensed
Anaphase
• Sister chromatids of each chromosome are pulled apart
• Once separated, each chromatid is a chromosome
Telophase
• Chromosomes decondense
• Two nuclear membranes form, one around each set of unduplicated chromosomes
Results of Mitosis
• Two daughter nuclei
• Each with same chromosome number as parent cell
• Chromosomes are in unduplicated form
Cytoplasmic Division
• Usually occurs between late anaphase and end of telophase
• Two mechanisms – Cell plate formation (plants)– Cleavage (animals)
Cell Plate Formation
Animal Cell Division
Actin and myosin filaments pull in membrane and pinch off
Asexual Reproduction
• Single parent produces offspring
• All offspring are genetically identical to one another and to parent
• Parthenogenesis
Sexual Reproduction
• Involves
– Meiosis
– Gamete production
– Fertilization
• Produces genetic variation among offspring
Homologous Chromosomes Carry Different Alleles
• Cell has two of each chromosome
• One chromosome in each pair from
mother, other from father
• Paternal and maternal chromosomes
carry different alleles
Chromosome From Dad
Gene for eye color
Genes and alleles
Chromosome From Mom
An allele is a different expression of the same gene
Karyotype
Sexual Reproduction Shuffles Alleles
• Through sexual reproduction, offspring inherit new combinations of alleles, which lead to variations in traits
• This variation in traits is the basis for evolutionary change
• “Better” traits should lead to more offspring which over time will change the gene pool
Gamete Formation
• Gametes are sex cells (sperm, eggs)
• Arise from germ cells in reproductive organs
testes
ovaries
Chromosome Number• Sum total of chromosomes in a cell
• Germ cells are diploid (2n) (both sets
of chromosomes)
• Gametes are haploid (n) (only one
set of chromosomes)
• Meiosis halves chromosome number
Meiosis: Two Divisions
• Two consecutive nuclear divisions – Meiosis I
– Meiosis II
• DNA is not duplicated between divisions
• Four haploid nuclei form from one diploid
cell
Prophase I
• Each duplicated chromosome pairs with homologue forming a tetrad
• Crossing over --Homologues swap segments
• Each chromosome becomes attached to spindle
Crossing Over• Each chromosome
becomes zippered to its homologue
• All four chromatids are closely aligned
• Nonsister chromosomes exchange segments
Effect of Crossing Over
• After crossing over, each chromosome
contains both maternal and paternal
segments
• Creates new allele combinations in
offspring
Random Alignment
• During transition between prophase I and metaphase I, microtubules from spindle poles attach to kinetochores of chromosomes
• Initial contacts between microtubules and chromosomes are random
Possible Chromosome
Combinations
Y chromosomes rarely cross over with X
You can trace a lineage of males quite readily
East Asian men:1.5 million descendents today
16 million descendents today
Giocangga
Genghis Khan
Metaphase I
• Chromosomes are pushed and pulled into the middle of cell
• The spindle is fully formed
Anaphase I
• Homologous chromosomes segregate
• The sister chromatids remain attached
Telophase I
• The chromosomes arrive at opposite poles
• Usually followed by cytoplasmic division
• Are these cells haploid or diploid?
Prophase II
• Microtubules attach to the kinetochores of the duplicated chromosomes
Metaphase II
• Duplicated chromosomes line up at the spindle equator, midway between the poles
Anaphase II
• Sister chromatids separate to become independent chromosomes
Telophase II
• The chromosomes arrive at opposite ends of the cell
• A nuclear envelope forms around each set of chromosomes
• Four haploid cells, genetically different
Spermatogenesis
GrowthMeiosis I,
Cytoplasmic DivisionMeiosis II,
Cytoplasmic Division
cell differentiation, sperm formation
spermatids (haploid)
secondary spermatocytes
(haploid)
primary spermatocyte
(diploid)
spermato-gonium (diploid )
sperm (mature, haploid male
gametes)
Spermatogenesis
Spermatogenesis
Develop from the spermatogonium germ cell (2n)
Meiosis happens as they migrate through the testes
Produces four viable sperm
OogenesisGrowth
Meiosis I,Cytoplasmic Division
Meiosis II,Cytoplasmic Division
ovum (haploid)
primary oocyte (diploid)
oogonium (diploid)
secondary oocyte
(haploid)
first polar body
(haploid)
three polar bodies
(haploid)
OogenesisOogonium give rise to primary oocyte during embryonic development
Primary oocyte remains in prophase I until activated (one at a time) during puberty by hormones
Hormones start meiosis until metaphase II
Meiosis only complete when contact with sperm is made
Oogenesis
When primary oocyte divides one secondary oocyte receives most of the cytoplasm
Similarly, when the secondary oocyte divides the ovum gets most cytoplasm
Polar bodies degenerate
Oogenesis vs spermatogenesis
1. One gamete (egg) vs four gametes (sperm)
2. At birth female has a set number of primary oocytes
3. Oogenesis is interrupted with resting stages while spermatogenesis is continuous
Fertilization
• Male and female gametes unite
and nuclei fuse
• Fusion of two haploid nuclei produces diploid
nucleus in the zygote
• Which of the two gametes unite is random– Adds to variation among offspring
Factors Contributing to Variation among Offspring
• Crossing over during prophase I
• Random alignment of chromosomes at metaphase I
• Random combination of gametes at fertilization
Mitosis
• Functions– Asexual reproduction– Growth, repair
• Occurs insomatic cells
• Produces clones
Mitosis & Meiosis Compared
Meiosis
• Function – Sexual reproduction
• Occurs in germ cells
• Produces variable offspring
Animal Life Cycle
Usually pretty rapid from meiosis to fertilization
Plant Life Cycle
Not as rapid, from spores
Spores not a fully developed gamete
Can last for many years
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