Cell Division and Mitosis

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Cell Division and Mitosis

• Chapter 9

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Understanding Cell Division

• What instructions are necessary for inheritance?
• How are those instructions duplicated for
  distribution into daughter cells?
• By what mechanisms are instructions parceled out
  to daughter cells?

3
Reproduction

• Parents produce a new generation of cells or
  multicelled individuals like themselves
• Parents must provide daughter cells with
  hereditary instructions, encoded in DNA, and
  enough metabolic machinery to start up their own
  operation

4
Division Mechanisms

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

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Roles of Mitosis

• Multicelled organisms
• Growth
• Cell replacement
• Some protistans, fungi, plants, animals
• Asexual reproduction

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Chromosome

• A DNA molecule attached proteins
• Duplicated in preparation for mitosis

one chromosome (unduplicated)
one chromosome (duplicated)
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Chromosome Number

• Sum total of chromosomes in a cell
• Somatic cells
• Chromosome number is diploid (2n)
• Two of each type of chromosome
• Gametes
• Chromosome number is haploid (n)
• One of each chromosome type

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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 chromosomes--two
  of each type

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Organization of Chromosomes
DNA
one nucleosome
DNA and proteins arranged as cylindrical fiber
histone
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The Cell Cycle
interphase
G1
S
telophase
anaphase
Mitosis
G2
metaphase
prophase
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Interphase

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

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Mitosis

• Period of nuclear division
• Usually followed by cytoplasmic division
• Four stages
• Prophase
• Metaphase
• Anaphase
• Telophase

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Control of the Cycle

• Once S begins, the cycle automatically 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

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Stopping the Cycle

• Some cells normally stop in interphase
• Neurons in human brain
• Arrested cells do not divide
• Adverse conditions can stop cycle
• Nutrient-deprived amoebas get stuck in interphase

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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
• Moves chromosomes during mitosis

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Spindle Apparatus
one spindle pole
one of the condensed chromosomes
spindle equator
microtubules organized as a spindle apparatus
one spindle pole
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Maintaining Chromosome Number

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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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Results of Mitosis

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

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Cytoplasmic Division

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

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Cell Plate Formation
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Animal Cell Division
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HeLa Cells

• Line of human cancer cells that can be grown in
  culture
• Descendents of tumor cells from a woman named
  Henrietta Lacks
• Lacks died at 31, but her cells continue to live
  and divide in labs around the world

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Meiosis

• Chapter 10

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Sexual Reproduction

• Chromosomes are duplicated in germ cells
• Germ cells undergo meiosis and cytoplasmic
  division
• Cellular descendents of germ cells become gametes
• Gametes meet at fertilization

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Asexual Reproduction

• Single parent produces offspring
• All offspring are genetically identical to one
  another and to parent

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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
• One chromosome in each pair from mother, other
  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 leads to
  variations in traits
• This 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

ovaries
anther
ovary
testes
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Chromosome Number

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

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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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Meiosis I
Each homologue in the cell pairs with its
partner,
then the partners separate
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Meiosis II

• The two sister chromatids of each duplicated
  chromosome are separated from each other

two chromosomes (unduplicated)
one chromosome (duplicated)
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Meiosis I - Stages
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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 pushed and pulled into the middle
  of cell
• The spindle is fully formed

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Anaphase I

• Homologous chromosomes segregate
• The sister chromatids remain attached

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Telophase I

• The chromosomes arrive at opposite poles
• Usually followed by cytoplasmic division

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Prophase II

• Microtubules attach to the kinetochores of the
  duplicated chromosomes

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Metaphase II

• Duplicated chromosomes line up at the spindle
  equator, midway between the poles

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Anaphase II

• Sister chromatids separate to become independent
  chromosomes

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Telophase II

• The chromosomes arrive at opposite ends of the
  cell
• A nuclear envelope forms around each set of
  chromosomes
• Four haploid cells

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Crossing Over

• Each chromosome becomes zippered to its homologue
• All four chromatids are closely aligned
• Nonsister chromosomes exchange segments

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Effect of Crossing Over

• After crossing over, each chromosome contains
  both maternal and paternal segments
• Creates new allele combinations in offspring

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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

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Random Alignment

• Either the maternal or paternal member of a
  homologous pair can end up at either pole
• The chromosomes in a gamete are a mix of
  chromosomes from the two parents

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Possible Chromosome Combinations

• As a result of random alignment, the number of
  possible combinations of chromosomes in a gamete
  is
• 2n
• (n is number of chromosome types)

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Possible ChromosomeCombinations
1
2
3
or
or
or
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Plant Life Cycle
multicelled sporophyte
mitosis
zygote
Diploid
meiosis
fertilization
Haploid
spores
gametes
multicelled gametophytes
mitosis
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Animal Life Cycle
multicelled body
mitosis
zygote
Diploid
meiosis
fertilization
Haploid
gametes
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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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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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Fertilization

• Male and female gametes unite and nuclei fuse
• Fusion of two haploid nuclei produces diploid
  nucleus in the zygote
• Which two gametes unite is random
• Adds to variation among offspring

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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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Mitosis Meiosis Compared

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

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

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Prophase vs. Prophase I

• Prophase (Mitosis)
• Homologous pairs do not interact with each other
• Prophase I (Meiosis)
• Homologous pairs become zippered together and
  crossing over occurs

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Anaphase, Anaphase I, and Anaphase II

• Anaphase I (Meiosis)
• Homologous chromosomes separate from each other
• Anaphase/Anaphase II (Mitosis/Meiosis)
• Sister chromatids of a chromosome separate from
  each other

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Results of Mitosis and Meiosis

• Mitosis
• Two diploid cells produced
• Each identical to parent
• Meiosis
• Four haploid cells produced
• Differ from parent and one another