10.1 – Meiosis
10.1.1 – Describe the behaviour of the chromosomes in the phases of meiosis
Meiosis consists of two nuclear divisions but only one replication of the chromosomes
The chromosomes condense and become visible and centrioles duplicate. The homologous will chromosomes pair up, forming bivalents. Chiasmata, where crossing over has occurred, become visible. The centrioles move to the poles of the cell and the nuclear membrane begins to break down.
The chromosomes become shorter and thicker and the spindle microtubules attach to the centromeres. The bivalents lines up along the equator. Those with chiasmata temporarily have unusual shapes. The spindles attach to the chromosomes and start to pull them apart.
The spindle fibres begin to shorten, separating the homologous chromosomes. They are pulled to opposites poles, thereby halving the chromosome number. Each of the chromosomes is made up of two chromatids; however, these might not be identical due to crossing over.
The nuclear membranes begin to reform around the chromosomes. The cell membrane closes, dividing into two cells. The chromosomes begin to uncoil partially. The new cells may enter a short interphase, but there will be no further replication of the DNA.
The chromosomes recoil, becoming short and thick once again. The centrioles replicate and move to opposite poles. The nuclear membranes will break down.
The spindle fibres begin to form and attach to the chromosomes by the centromere. The chromosomes line up along the equator. The centromeres divide to allow the chromosomes to separate.
The sister chromatids are separated, pulled apart by the spindle fibres to the opposite poles.
The chromatids decondense and are now known as chromosomes. The nuclear membrane and nucleoli reform around the DNA. The cells divide, forming a total of four haploid cells.
Breakages of the chromatids occur frequently during the coiling and shortening process, but this is immediately repaired. However, since the homologous chromosomes are paired closely together, these fragments may swap between non-sister chromatids. The place where the crossing over occurs is called the chiasma (plural: chiasmata). This results in new combinations of genes.
10.1.3 – Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I
This is when fragments of DNA are swapped between homologous chromosomes, forming new combinations of linked genes. The new genotypes are not due to random assortment. There are virtually unlimited possibilities of recombinations.
During anaphase I, the bivalents line up randomly along the equator. The homologous pairs and their alleles are separated. However, the chromosomes move independently of each other, so there is no relationship between the chromosome and which pole it moves to. A total of 223 combinations can be formed in humans.
10.1.4 – State Mendel’s law of independent assortment
Allele pairs separate independently during the formation of gametes. If two genes are unlinked, then the pairs of alleles will be segregated randomly during meiosis. As a result, traits are transmitted to offspring independently of one another
10.1.5 – Explain the relationship between Mendel’s law of independent assortment and meiosis
Independent assortment is the result of random orientation of the homologous chromosomes during metaphase I. Mendel’s discoveries were based on physical characteristics, or phenotypes, and not the actual alleles or chromosomes.