Cell Reproduction

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Cell Reproduction

• Starr, Chapter 7

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Roles of Cell Division

• Unicellular organisms
• reproduce by cell division
• Includes some protists, fungi, plants, animals
• Multicellular organisms
• depend on it for development from a fertilized
  egg, growth, and repair

3
Human Genome

• All of a cells genetic information, DNA, is
  called its genome
• When not dividing, DNA and proteins is a tangled
  mass of thin threads called chromatin
• Chromatin condenses when cell divides and forms
  compact structures of chromosomes

4
Chromosome

• A DNA molecule and attached proteins
• Duplicated in preparation for cell division

one chromosome (unduplicated)
one chromosome (duplicated)
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Organization of Chromosomes
supercoiling of coiled loops of DNA
centromere
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Organization of Chromosomes
DNA
one nucleosome
DNA and proteins arranged as cylindrical fiber
histone
7
Division Mechanisms

• Eukaryotic organisms
• Mitosis
• Meiosis
• Prokaryotic organisms
• Prokaryotic fission

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Eukaryotic Cell Cycle
INTERPHASE
G1 Interval of cell growth, before DNA
replication (chromosomes unduplicated)
S Interval of cell growth, when DNA
replication is completed (chromosomes duplicated)
Each daughter cell starts interphase
Cytoplasmic division
Telophase
Anaphase
Interphase ends for parent cell
G2 Interval following DNA
replication cell prepares to divide
Metaphase
Prophase
MITOSIS
Fig. 7-2, p.96
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Interphase

• Usually the longest part of cycle
• Cell increases in mass
• Number of cytoplasmic components doubles
• DNA is duplicated

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Control of the Cycle

• Once S begins, the cycle usually runs through G2
  and mitosis
• Cycle has a built-in molecular brake in G1
• Cancer involves a loss of control over the cycle,
  malfunction of brakes

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Chromosome Number

• Total number of chromosomes in a cell
• Somatic cells
• Chromosome number is diploid (2n)
• Paired chromosome, one from each parent
• Gametes (sex cells)
• Chromosome number is haploid (n)
• One chromosome of each type

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Human Chromosome Number
13
Maintaining Chromosome Number
chromosomes (unduplicated) in parent cell at
interphase
14
Spindles
spindle microtubules

• Consists of two distinct sets of microtubules
• Extend from cell poles
• Overlap at spindle equator
• Moves chromosomes during mitosis

chromosomes at spindle equator, midway between
spindle poles
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Stages of Mitosis

• Prophase
• Metaphase
• Anaphase
• Telophase

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Early Prophase Mitosis Begins

• Duplicated chromosomes begin to condense

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Late Prophase

• New microtubules are assembled
• One centriole pair is moved toward opposite pole
  of spindle
• Nuclear envelope starts to break up

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Transition to Metaphase

• Spindle forms
• Spindle microtubules become attached to the two
  sister chromatids of each chromosome

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Metaphase

• All chromosomes are lined up at the spindle
  equator
• Chromosomes are maximally condensed

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Anaphase

• Sister chromatids of each chromosome are pulled
  apart
• Once separated, each chromatid is a chromosome

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Telophase

• Chromosomes decondense
• Two nuclear membranes form, one around each set
  of unduplicated chromosomes

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Cytokinesis

• Usually occurs between late anaphase and end of
  telophase
• Two mechanisms
• Cleavage (animals)
• Cell plate formation (plants)

animal cell pinching in two
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Animal Cell Division
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Cell Plate Formation
cell wall
former spindle equator
cell plate
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Interphase

• Two daughter cells
• Each with same chromosome number as parent cell
• Chromosomes are in unduplicated form

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Types of Reproduction

• Asexual Reproduction
• Single parent produces offspring
• All offspring are genetically identical
• Sexual Reproduction
• Involves
• Meiosis
• Gamete production
• Fertilization
• Produces genetic variation among offspring

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Homologous Chromosomes Carry Different Alleles

• Cell has two of each chromosome
• Chromosome pairs one from mother, one from
  father
• Paternal and maternal chromosomes carry different
  alleles

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Sexual Reproduction Shuffles Alleles

• Through sexual reproduction, offspring inherit
  new combinations of alleles, which lead to
  variations in traits
• Variation in traits is the basis for evolutionary
  change

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Gamete Formation

• Gametes are sex cells (sperm, eggs)
• Arise from germ cells in reproductive organs

ovaries
testes
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Chromosome Number

• Total number of chromosomes in cell
• Germ cells are diploid (2n)
• Gametes are haploid (n)
• Meiosis halves chromosome number

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Diploid To Haploid
centromere
one chromatid
its sister chromatid
one chromosome in the duplicated state
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(No Transcript)
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Meiosis Two Divisions

• Two consecutive nuclear divisions
• Meiosis I
• Meiosis II
• DNA is not duplicated between divisions
• Four haploid nuclei form

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p.102
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Prophase I

• Each duplicated chromosome pairs with homologue
• Homologues swap segments
• Each chromosome becomes attached to spindle

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Metaphase I

• Chromosomes are moved to middle of cell
• Spindle is fully formed

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Anaphase I

• Homologous chromosomes separate
• Sister chromatids remain attached

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Telophase I

• Chromosomes arrive at opposite poles
• Usually followed by cytoplasmic division

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Prophase II

• Microtubules attach to duplicated chromosomes

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Metaphase II

• Duplicated chromosomes line up midway between
  spindle poles

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Anaphase II

• Sister chromatids separate to become independent
  chromosomes

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Telophase II

• Chromosomes arrive at opposite ends of cell
• Nuclear envelopes form around chromosome sets
• Four haploid cells

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Sexual Reproduction and Genetic Variation

• Two functions of meiosis provide variation in
  traits
• crossing over
• random alignment

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Variation byCrossing Over

• Each chromosome attaches to its homologue
• All four chromatids are closely aligned
• Nonsister chromosomes exchange segments
• Creates new allele combinations in offspring

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Variation by Random Alignment

• Between prophase I and metaphase I, chromosome
  pairs align randomly at metaphase plate
• Initial contact between microtubule and either
  maternal or paternal chromosome is random

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Possible ChromosomeCombinations
1
2
3
or

• Maternal and paternal chromosomes are randomly
  aligned

or
or
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Factors Contributing to Variation among Offspring

• Crossing over during prophase I
• Random alignment of chromosomes at metaphase I
• Random combination of gametes at fertilization

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Spermatogenesis
Spermatogenesis
primary spermatocyte (diploid)
spermato-gonium (diploid )
sperm (mature, haploid male gametes)
secondary spermatocytes (haploid)
spermatids (haploid)
Meiosis I, Cytoplasmic Division
Meiosis II, Cytoplasmic Division
Growth
cell differentiation, sperm formation
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Oogenesis
three polar bodies (haploid)
first polar body (haploid)
primary oocyte (diploid)
oogonium (diploid)
secondary oocyte (haploid)
ovum (haploid)
Meiosis I, Cytoplasmic Division
Meiosis II, Cytoplasmic Division
Growth
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Fertilization
Fig. 7-14, p.107
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Mitosis Meiosis Compared

• Mitosis
• Functions
• Asexual reproduction
• Growth, repair
• Occurs insomatic cells
• Produces clones

• Meiosis
• Function
• Sexual reproduction
• Occurs in germ cells
• Produces variable offspring

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Mitosis
TELOPHASE
PROPHASE
ANAPHASE
METAPHASE
Chromosomes align at spindle equator.
Sister chromatids of chromosomes separate.
two nuclei (2n)
Fig. 7-18, p.110
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Meiosis I
TELOPHASE I
PROPHASE I
ANAPHASE I
METAPHASE I
Crossing over occurs between homologues.
Homologous pairs align randomly.
typically two nuclei (n)
Homologues separate from their partner.
Fig. 7-18, p.110
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Meiosis II
no interphase between nuclear divisions
PROPHASE II
TELOPHASE II
ANAPHASE II
METAPHASE II
four nuclei (n)
Chromosomes align at spindle equator.
Sister chromatids of chromosomes separate.
Fig. 7-18, p.110