Genetics: Topic 3.3 Meiosis Flashcards
Define meiosis
Meiosis is the process by which sex cells (gametes) are made in the reproductive organs
It involves the reduction division of a diploid germline cell into four genetically distinct haploid nuclei
The process of meiosis consists of two cellular divisions:
The first meiotic division separates pairs of homologous chromosomes to halve the chromosome number (diploid → haploid)
The second meiotic division separates sister chromatids (created by the replication of DNA during interphase)
What happens in interphase before meiosis begins?
Meiosis is preceded by interphase, in which DNA is replicated to produce chromosomes consisting of two sister chromatids
A second growth phase called interkinesis may occur between meiosis I and II, however no DNA replication occurs in this stage
Explain meiosis 1 briefly
The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated
P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell
A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells
Explain meiosis ii briefly
The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)
P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells
Explain what is bivalent and how does it form
At the start of meiosis, homologous chromosomes pair up with each other in prophase
As DNA replication has already occurred, each chromosome is made up of two sister chromatids
This means that a pair of homologous chromosomes is made up of four DNA molecules
A pair of homologous chromosomes is known as a bivalent
The pairing process resulting in the formation of a bivalent is known as synapsis
Explain crossing over
The homologous chromosomes are held together at points called chiasmata (singular: chiasma)
Crossing over of genetic material between non-sister chromatids can occur at these chiasmata
During crossing over, two non-sister chromatids (i.e. one chromatid from each of the homologous chromosomes) form a junction
At this junction, the two chromatids break and rejoin with each other
As these crossover events occur at exactly the same position on the two non-sister chromatids, this allows genes to exchange between the chromatids
Explain crossing over as source of genetic variation
Non-sister chromatids are homologous but are not genetically identical and this means that some of the alleles of the exchanged genes will be different
This process, therefore, produces chromatids with completely new combinations of alleles (that were not previously present in the DNA of the ‘parent’ cell)
As these chromatids will eventually be split up into different gametes, crossing over is of great importance because it is a significant source of genetic variation between gametes
This ensures there is genetic variation in populations of sexually-reproducing species, which is key to a species’ ability to evolve and adapt to changes in its environment over time
Explain random assortment and its signifiance
During metaphase I, homologous chromosomes line up at the equator as bivalents in one of two arrangements:
Maternal copy left / paternal copy right OR paternal copy left / maternal copy right
Spindle microtubules grow out from the poles of the cell and attach to the centromeres of the chromosomes
Each of the two homologous chromosomes in a bivalent is attached to a different pole
The orientation of the bivalents when they line up at the cell equator determines which pole each chromosome gets attached to (and eventually pulled towards)
The orientation of the bivalents is completely random
In addition, the bivalents also assort independently of one another (i.e. the orientation of one bivalent never affects the orientation of another)
As this random assortment will occur for each homologous pair, the number of possible gamete combinations are dependent on the number of homologous pairs
Gamete combinations = 2n (where n represents the haploid number)
Explain how meiosis I is reduction division
ring meiosis, the homologous chromosomes forming a bivalent separate in a process known as disjunction
The homologous chromosomes then move to opposite poles of the cell
As one chromosome of each type moves to each pole, the two separate nuclei formed by the first division of meiosis (meiosis I) now only contain one of each type of chromosome, making the two new cells haploid
Essentially, the chromosome number of the cells has been halved
