Download Overview Cell division in eukaryotes

Overview
• DNA replication is prerequisite to cell division.
• DNA is replicated by using each strand as template
for synthesis of the complementary strand.
– one strand is synthesized continuously, the other
discontinuously
– all nucleotides are added at the 3’ end of the growing strand
• Replicated chromosomes are partitioned to nuclei of
daughter cells in mitosis.
• Meiosis distributes replicated chromosomes to haploid
daughter cells in two nuclear and cell divisions.
Cell division in eukaryotes
• Mitosis
–
–
–
–
an asexual division
equal division of replicated chromosomes
occurs with any number of chromosomes
part of cell cycle
• Cell cycle
–
–
–
–
G1: gap (growth)
S: DNA replication, forms sister chromatids
G2: gap (growth)
M: mitosis (nuclear division) and cytokinesis
1
Mitosis
• Nuclear division, usually with cell division
• Divided into four continuous stages
– prophase
• chromosomes condense and attach to spindle via kinetochore
• nucleolus and nuclear membrane disappear
– metaphase: chromosomes align on equatorial plane of
cell
– anaphase: sister chromatids separate and migrate to
opposite poles
– telophase: cell division and reformation of nucleus and
nucleolus
2
Mitotic machinery
• Sister chromatids (replicated chromosomes)
attached by centromere
– identical in DNA sequence by rules of DNA replication
(base pairing)
– centromere acts as binding site for kinetochore, a site
for attachment of microtubules
• Spindle apparatus
– microtubules assembled from tubulin
– motor proteins
– provides force for separation of chromatids
Meiosis
• Specialized cell division to halve chromosome
number
• Compensates for doubling of chromosome number
by fertilization
• Occurs in meiocyte at fixed point in life cycle
– premeiotic S phase to replicate chromosomes
– meiocyte divides twice to yield tetrad of four haploid
cells
– meiocyte effectively 4n prior to division, yielding four
1n cells
Meiotic divisions
• Replicated homologous chromosomes pair along
their length (synapsis)
– form tetrad of four chromatids
– paired nonsister chromatids may undergo crossing-over
• At first meiotic division, chromosomes segregate
to opposite poles
– spindle attaches only one side of centromere
• At second meiotic division, sister chromatids
segregate to opposite poles
Consequences of meiosis
• Formation of four haploid cells, each with one
complete copy of genome
• Recombination
– crossing-over in prophase I
– independent assortment of nonhomologous
chromosomes
– results in genetic diversity
• Meiosis occurs at some point in life cycle of all
sexually reproducing organisms
3
Chapter 5
The Inheritance of SingleGene Differences
Alleles at single locus
Overview
• In matings, precise phenotypic ratios are produced in descendants
as a result of chromosome segregation.
• In heterozygotes, alleles segregate equally into meiotic products.
• Progeny ratios can be predicted from known genotypes of
parents.
• Parental genotypes can be inferred from phenotypes of progeny.
• In many organisms, sex chromosomes determine sex.
• X-linked genes can show different phenotypic ratios in male and
female progeny.
• In humans, single-gene traits can be studied in pedigrees.
• Organelle genes are inherited maternally.
Meiotic chromosome segregation
• In meiosis, each of the four haploid
products receives one of each kind of
chromosome
– A/A homozygotes → all get A chromosomes
– A/a heterozygotes → half get A chromosomes
half get a chromosomes
• As a consequence of chromosome
segregation, alleles of heterozygotes
segregate equally
4
Equal segregation
• First observed by Mendel in crosses with
peas
• Readily observed using some fungi and
protists in which all four haploid products
of meiocyte (tetrad) can be analyzed
A
a
A
a
Aa
Crosses
• Controlled mating of two individuals
– obtain desired genotype
– deduce genotypes of parents
• Selfing: individual with both reproductive organs
reproduces (crosses) to itself
• × symbol is used to indicate a cross
– a haploid mating: A × a
– a diploid mating: A/a × A/a
• A/– : – represents either A or a
Diploid crosses (1)
• Three possible diploid genotypes
A/A
a/a
A/a
• Six possible diploid crosses
Cross
A/A × A/A
a/a × a/a
A/A × a/a
A/a × A/A
A/a × a/a
A/a × A/a
Genotypic ratio
A/A
a/a
A/a
1A/A:1A/a
1A/a:1a/a
1A/A:2A/a:1a/a
Phenotypic ratio
all A
all a
all A
all A
1A:1a
3A:1a
5
Diploid crosses (2)
• Crosses between individuals heterozygous for the
same single gene are also called monohybrid
crosses
• A heterozygote for unexpressed recessive allele is
sometimes called a carrier, particularly in humans
• A cross between an unknown genotype (e.g., A/–)
and the homozygous recessive genotype (a/a) is
called a testcross
6