Lecture 15: Meiosis Flashcards
Why do gametes need to be produced via meiosis
If gametes (i.e. sperm and egg) were produced by mitosis, they would be 2n, and resulting embryos would be 4n.
Therefore, need a process of cell division that HALVES no. of chromosomes going into gametes (n), so that diploid number (2n)
is retained in zygote.
What is Meiosis
Cell division in sexually reproducing
organisms.
2 rounds of cell division but one round of DNA replication
Results in 4 haploid cells that are genetically distinct
What does haploid mean
A single set of chromosomes
What happens at G2 of interphase in Meiosis
Nuclear envelope intact
Nucleolus visible, contains one
or more nucleoli
Centrosomes formed
Chromosomes duplicated during S phase but not yet condensed
What are the four phases of Meiosis 1
Prophase I, Metaphase I, Anaphase I, Telophase I and Cytokenisis
What happens during prophase 1
Nuclear envelope breaks
down, chromosomes condense & spindle forms
Crossing over (exhange of DNA) between non-sister chromatids occurs at chiasmata
Each chromatid is now a mix of DNA from each homologous chromosome
What happens during metaphase 1
Chromosomes are
attached to the kinetochore microtubules, at each centromere.
Each pair is lined up independently
Paired homologous
chromosomes have moved
to the metaphase plate
Chiasmata line up on
metaphase plate.
*Not the centromeres,
as in Mitosis
What happens during anaphase 1
Recombined homologous chromosomes seperate/disjoin
Sister chromatids remain
attached to each other
*In anaphase of mitosis, sister chromatids separate
Cell elongates, nonkinetochore
microtubules extend.
each duplicated chromosome moves to the poles
What happens during telophase 1
Duplicated chromosomes (the pairs of sister chromatids) reach the poles at opposite ends of the cell
Spindle dissapears and nuclear envelope reforms
What happens during Cytokinesis 1
In animal cells, cytokinesis
involves the formation of
a cleavage furrow, which
pinches the cell into two.
Cytoplasm divides, resulting
in two haploid cells. Haploid
because only half the
genetic information is in
each new cell.
Cells and sister chromatids are genetically different due to crossing over
What happens in Meiosis II
Sister chromatids are separated
What happens in Prophase II
Spindle forms as the
centrosomes duplicate and
move to opposite poles
Kinetochore microtubules
attach to each duplicated
chromosome at the
centromere (via the
kinetochore proteins)
Each duplicated
chromosome is still
composed of two chromatids
attached at centromeres
What happens in Metaphase II
Duplicated
chromosomes
align at the
metaphase plate
Centromeres lie on
the metaphase
plate
What happens in Anaphase II
Sister chromatids disjoin at the
centromeres. Each chromatid becomes an independent daughter chromosome.
Daughter chromosomes move towards opposite poles as their kinetochore microtubules shorten
The nonkinetochore
microtubules lengthen, and
the cell elongates.
What happens in Telophase II & Cytokinesis
Two daughter nuclei (with
nuclear envelope) form in
the cell.
The meiotic division of one
parent cell produces four
daughter cells, each with a haploid set of (unduplicated) chromosomes
The four daughter cells
are genetical distinct from
one and other and from
the parent cell.
Why is sexual reproduction evolutionarily beneficial
leads to greater genetic diversity, better for evolution
independent assortment of chromosomes
crossing over and
random fertilization of gametes
