Genetics Part 2 Meiosis

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