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cell division: meiosis
biology 1
• Offspring acquire genes from parents by
inheriting chromosomes
• Two general strategies
– Sexual reproduction
– Asexual reproduction
• Fertilization and meiosis alternate in a
sexual lifecycle
• Meiosis reduces chromosome number from
diploid to haploid
• Sexual reproduction produces genetic
variation, a vital component of evolutionary
adaptation
A glossary...
• DNA is a nucleic acid composed of four different kinds of
nucleotide in different sequences
• Specific sequences of nucleotides that correspond to
synthesis of a specific protein are called genes
• Genes are joined together in strands called chromosomes
• The lineal location of a gene on a chromosome is called a
locus (plural: loci)
• Different expressions of a gene at a particular locus are
possible - these expressions are called alleles
• Inheritance is possible because DNA is precisely
replicated, with gene copies being passed onto offspring
• Each species has a characteristic chromosome number humans have 46
Asexual vs. sexual reproduction
Asexual reproduction
Sexual reproduction
Single individual is the sole parent
Two parents give rise to offspring
Single parent passes on all its genes to
its offspring
Each parent passes on half its genes to
its offspring
Offspring are genetically identical to the
parent
Offspring ha ve a unique combination of
genes inherited from both parents
Results in a clone, or genetically identical
individual. Rarely, genetic differences
may occur as a result of mutation
Results in greater genetic variation:
offspring vary genetically from their
siblings and parents
Genetic variation in sexual reproduction is a result of meiosis
The sexual life cycle
• Human somatic cells contain 46 chromosomes (as
determined by karyotyping)
• Closer examination reveals that these 46 can be assigned
into 23 pairs
– 22 pairs are homologous pairs (ie, per pair, same set of loci).
These are known as autosomes
– 1 pair carries different loci - these are sex chromosomes
• Each homologue from a pair is inherited from a specific
parent
• Thus, a human somatic cell consists of two sets of 23
chromosomes, each set inherited by a specific parent
• A cell that possesses both sets is said to be diploid (2n)
• A cell that has only one set is said to be haploid (n)
• In a sexual life cycle, meiosis halves the
chromosome number from diploid to
haploid to create gametes
• In fertilization, gametes fuse to become
a single celled zygote which restores
the diploid condition
• Depending on species (and Kingdom),
different periods of time spent in haploid
and diploid phases
• Occasionally, some organisms remain
in either haploid or diploid state
(although most organisms cycle)
Meiosis
• Steps to meiosis in some ways mirror
those in mitosis. However, meiosis
consists of two divisions (Meiosis I and
Meiosis II)
– Produces 4 daughter cells
– Each daughter cell is haploid
– Meiosis plays a key role in generating
variation
• As in mitosis, replication of DNA occurs
unseen while genetic material is
uncoiled
Interphase I
• Chromosomes replicate as in mitosis
• Each duplicated chromosome consists
of 2 sister chromatids attached at a
centromere
• BUT remember that in a diploid cell
each chromosome (pair of chromatids)
has a homologue
• Therefore, following duplication, for any
one gene, there will be two pairs of two
alleles
Prophase I
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Chromosomes condense and are visible
Homologues associate as a tetrad in the process of synapsis
During synapsis homologues may join at specific loci termed chiasma
At a chiasma, homologues may exchange a length of DNA (set of
genes). This process is known as crossing over. The joint between the
two homologues is known as a synaptonemal complex
– In humans, 2-3 chiasmata per chromosone pair
• Cell prepares for first division
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Migration of centrosomes
Dispersion of nuclear membrane
Formation of meiotic spindle
Chromosomes begin to migrate
• Prophase I accounts for 90% of the time spent in meiosis
Metaphase I
• Tetrads align along metaphase plate
• Centromeres of homologues point
towards opposite poles of cell
• Kinetochore microtubules connect to
kinetochore sites in centromere of each
homologue
Anaphase I
• Homologues separate and move towards
separate poles of the cell, pulled by
depolymerization of kinetochore
microtubules at kinetochore end
• Sister chromatids remain intact and travel
together to either pole
• Homologue separation is not necessarily
by parental assignment
Telophase I and Cytokinesis
• Each pole now has a haploid set of
chromosomes (homologues are at other
pole), each homologue consisting of a
chromatid pair
• Cytokinesis separates cell into two
daughter cells
• In some cases nuclear membranes
reform (interkinesis)
Prophase II
• The goal of meiosis II is to separate
sister chromatids
• In prophase II, nuclear envelope
disperses (if it reformed)
• Spindle apparatus reforms and
chromosomes start to move towards
metaphase plate
Metaphase II
• Chromosomes align on the metaphase
plate, with each sister chromatid
pointing towards a different pole of the
cell
• Each sister chromatid is joined to a
kinetochore microtubule at the
kinetochore
Anaphase II
• Centromeres of sister chromatids
separate
• Sister chromatids are pulled apart and
move towards separate poles of the cell
Telophase II and cytokinesis
• Nuclei form at opposite poles of the cell
• Cytokinesis occurs producing 4 haploid
daughter cells
Sexual life cycles produce variation
• Genetic variation is essential for evolutionary
adaptation
• In meiosis, variation occurs by
– Independent assortment
• Homologues do not assign to different poles of the cell
necessarily according to parental designation: 2n
possible combinations
– Crossing over
• Alleles associated previously with other alleles on the
same chromosome may now associate with different
alleles from other homologue
– Random fusion of gametes (2n x 2n)
• Beyond meiosis, variation can also occur
through mutation