Genetics Part 2 – Meiosis
Assessment Statement
4.2.1 State that meiosis is a reduction division of a diploid nucleus to form haploid
nuclei
4.2.2 Define homologous chromosomes
4.2.3 Outline the process of meiosis, including pairing of homologous
chromosomes and crossing over, followed by two divisions, which results in
four haploid cells
10.1.1 Describe the behaviour of the chromosomes in the phases of meiosis
10.1.2 Outline the formation of chiasmata in the process of crossing over
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
Meiosis –
The purpose of meiosis is to produce gametes. It only occurs in diploid cells and reduces
the number of chromosomes per cell.
The gametes contain half the number of chromosomes than the original cell. This means
it goes from diploid to haploid. When the two gametes meet in the process of
fertilization, the original number of chromosomes is restored.
Terms to know:
diploid –
homologous chromosomes – also called a homologous pair. They are two chromosomes
that look the same. They are the same size and show the same banding pattern in a
karyotype. They carry the same genes but not necessary the same alleles. For example
both chromosomes will carry the gene for eye colour, but one may have the allele for
brown and the other may have the allele for blue.
haploid –
gametes – sex cells with a haploid number of chromosomes.
zygote – the result of a fusion of gametes
Stages of Meiosis
Meiosis consists of two stages – Meiosis I and Meiosis II
Meiosis I
Interphase
 the DNA replicates
Prophase I
 Chromosomes condense
 Nucleolus becomes visible and the spindle forms
 Synapsis – when the homologous chromosomes are side by side ( now called a
bivalent) and they cross over at a point called a chiasmata)
 Nuclear membrane disappears
Metaphase I
 Homologous pairs (tetrads or bivalents) move to the middle of the cell
Anaphase I
 Homologous pairs split up
 One chromosome goes to each pole
Telophase I
 Chromosomes arrive at poles
 Spindle diappears
Cell / Egg rests.
Meiosis II
Prophase II
 New spindle forms at right angles to the previous spindle
Metaphase II
 Chromosomes move to the equator
Anaphase II
 Chromosomes separate, chromatids move to opposite poles
Telophase II
 Chromatids arrive at poles
 Spindle disappears, nuclear membrane reappears
 Nucleolus reappears, chromosomes become chromatin
Summary – It takes one cell that is diploid and creates 4 haploid cells – this is a
reduction division.
SEE MEIOSIS AND CROSS OVER WORKSHEETS.
Meiosis Continued
In what two ways does meiosis increase genetic variability?
The only way to get genes that are not identical is to have a mix of genetic material. This
is done by crossing over, or synapsis.
Cross Over
During Prophase I, the chromatids of the bivalent are close together. During the coiling
and shortening process, breakages of the chromatids occur frequently. Breakages are
common in non-sister chromatids.
Process of Cross Over
Broken ends rejoin more or less immediately, but where it rejoins are between non-sister
chromatids, swapping pieces of the chromatids. This is why it is called crossing over.
The point where the pieces join is called a chiasma (plural – chiasmata).
Every pair of homologous chromosomes forms at least one chiasma, and sometimes
having two or more in the same bivalent is very common.
The new combinations are called recombinants.
Recombination is the re-assortment of genes or characters into different combinations
from those of the parents.
As a result, meiosis results in an infinite number of variations in gametes during Prophase
I and randon orientation in Metaphase I.
The number of different types of gametes produced by random orientation is represented
by 2n, where n is the haploid number in a gamete. Throw in the affect of crossing over,
and you have a lot of genetic variety.
For example, the human has 23 chromosomes. Therefore, the possible combinations of
genetic material, would be 223. This works out to
A pea plant has 4 genes, so the possible combinations would be
In order for a gamete to have exactly the same genetic make-up as the parents would
occur every 246 or 7 x 10 13.
When the chromosomes separate during anaphase, this is called disjunction (junction –
joined). If the chromosomes do not separate properly, there can be a problem with the
amounts of chromosomes in the gametes, and as a result, more or less than the required
46 chromosomes in a human. This is called non-disjunction.
During each of the cell divisions, the chromosomes are pulled to opposite ends of the
cells. Sometimes the chromosomes do not separate properly, leading to a condition
known as aneuploidy.
Aneuploidy is either an extra or missing chromosome. This only happens in 9 cases,
with Down’s Syndrome being the most common. In some cases, total non-disjunction
takes place, which is called polyploidy.
HANDOUTS – MUTATIONS / DOWNS SYNDROME