Cell Reproduction
Chapter 19
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?
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
Eukaryotic Cell Division
• Mitosis, division of cytoplasm
– Body growth and tissue repair
• Meiosis, division of cytoplasm
– Formation of gametes, sexual reproduction
Chromosome
• A DNA molecule & attached proteins
• Duplicated in preparation for mitosis
one chromosome (unduplicated)
one chromosome (duplicated)
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
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
Cell Cycle
• Cycle starts when a new cell forms
• During cycle, cell increases in mass
and duplicates its chromosomes
• Cycle ends when the new cell divides
Interphase
• Usually longest part of the cycle
• Cell increases in mass
• Number of cytoplasmic components
doubles
• DNA is duplicated
Stages of Interphase
• G1
– Interval or gap after cell division
• S
– Time of DNA synthesis (replication)
• G2
– Interval or gap after DNA replication
HeLa Cells
• Line of human cancer cells that can be
grown in culture
• Descendants 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
Mitosis
• Period of nuclear division
• Usually followed by cytoplasmic division
• Four stages:
Prophase
Metaphase
Anaphase
Telophase
Stages of Mitosis
Prophase
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 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 in
unduplicated form
Cytoplasmic Division
• Usually occurs between late anaphase
and end of telophase
Effects of Irradiation
• Ionizing radiation
– May break apart chromosomes, alter
genes
• Large doses destroy cells
• Small doses over long time less
damaging
• Medical X rays, radiation therapy
Irradiated Food
• Foods irradiated to kill microorganisms,
prolong shelf life
• Not radioactive
• No evidence it is a health risk
• Critics worry about selecting for
radiation-resistant strains, changes in
food composition
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”
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
Organization of Chromosomes
DNA
DNA and proteins
arranged as cylindrical fiber
one nucleosome
histone
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
Sexual Reproduction
• Chromosomes are duplicated in
germ cells
• Germ cells undergo meiosis and
cytoplasmic division
• Cellular descendants of germ cells
become gametes
• Gametes meet at fertilization
Gamete Formation
• Gametes are sex cells (sperm, eggs)
• Arise from germ cells
ovaries
testes
Meiosis: Two Divisions
• Two consecutive nuclear divisions
– Meiosis I
– Meiosis II
• DNA is not duplicated between divisions
• Four haploid nuclei form
Meiosis I
Each homologue in the
cell pairs with its partner,
then the partners
separate
Meiosis II
• The two sister chromatids of each
duplicated chromosome are separated
from each other
two chromosomes
(unduplicated)
one chromosome
(duplicated)
Spermatogenesis
secondary
spermatocytes
(haploid)
spermatogonium
(diploid malereproductive
cell)
primary
spermatocyte
(diploid)
spermatids (haploid)
Growth
Mitosis I,
Cytoplasmic division
Meiosis II,
Cytoplasmic division
Oogenesis
first polar
body
(haploid)
oogonium
(diploid
reproductive
cell)
Growth
three polar
bodies
(haploid)
primary oocyte
(diploid)
secondary
oocyte
(haploid)
Mitosis I,
Cytoplasmic division
ovum
(haploid)
Meiosis II,
Cytoplasmic division
Stages of Meiosis
Meiosis I
Meiosis II
• Prophase I
• Prophase II
• Metaphase I
• Metaphase II
• Anaphase I
• Anaphase II
• Telophase I
• Telophase II
Meiosis I - Stages
Prophase I
Metaphase I Anaphase I
Telophase I
Meiosis II - Stages
Prophase II
Metaphase II Anaphase II Telophase II
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 parental
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
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
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)
1
Possible
Chromosome
Combinations
or
or
or
2
3
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
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 & 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
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
Anaphase, Anaphase I, and
Anaphase II
• Anaphase I (Meiosis)
– Homologous chromosomes are separated from
each other
• Anaphase/Anaphase II (Mitosis/Meiosis)
– Sister chromatids of a chromosome are separated
from each other
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