Meiosis, genetic variation and mutations

In a nutshell

Meiosis is a form of cell division. Meiosis is crucial for the production of gametes, commonly known as sperm cells in males and egg cells in females. During this process, chromatids also re-organise themselves to promote genetic variation. Division through meiosis begins with diploid cells and results in haploid cells.

The process of meiosis

Meiosis is a type of cell division which produces gametes or sex cells. Meiosis occurs in the reproductive organs. There are three primary phases: interphase, meiosis I and meiosis II.

Interphase

1.	DNA is uncoiled to make access to the nucleotides easier.
2.	Replication takes place.
3.	Chromosomes double to create sister chromatids.

Meiosis I

After DNA replication and interphase the cell divides itself into two diploid daughter cells

Prophase I	DNA begins to condense. Centromeres form in the middle of two sister chromatids. Chromosomes arrange into homologous pairs. Crossing over occurs. Sister chromatids create an X-shape.
Metaphase I	Homologous chromosomes line up at the equator of the cell. Chromosomes are prepared to be separated. The equator of the cell is known as the metaphase plate.
Anaphase I	Homologues are separated and each sister chromatid remains attached.
Telophase I	Homologues arrive at opposite sides of the cell. The nuclear membrane begins to form and each daughter cell is separated. This results in two diploid cells. Cytokinesis takes place.

Meiosis II

After the first division, the two diploid daughter cells divide into four haploid daughter cells.

Prophase II	DNA is not replicated after meiosis I. DNA condenses. The nuclear envelope begins to break.
Metaphase II	The two sister chromatids of each chromosome align at the metaphase plate.
Anaphase II	The sister chromatids are separated to opposite sides of the cell.
Telophase II	Nuclear membrane form to separate each chromatid into a cell. Cytokinesis splits these cells into two haploid cells. The end of meiosis II results in four haploid cell.

Genetic variation in meiosis

Genetic variation is a factor that separates meiosis from mitosis. Variation is a result of the law of segregation and crossing over.

Law of segregation

The law of segregation occurs prior to the first division in meiosis I. Homologous chromosomes, both paternal and maternal are paired up randomly. This randomises the order of homologous chromosomes passed down to the daughter cells and allows for different alleles to be paired.

Biology; Cells, viruses and reproduction of living things; KS5 Year 12; Meiosis, genetic variation and mutations

Crossing over

Crossing over occurs before independent segregation and it involves the exchange of genetic material between homologues. As a result, each chromatid will have different allele combinations.

Biology; Cells, viruses and reproduction of living things; KS5 Year 12; Meiosis, genetic variation and mutations — 1. Paternal chromosome, 2. Maternal chromosome, 3. DNA replication, 4. Crossing over, 5. Meiosis I.

Differences between mitosis and meiosis

In this table, the process of meiosis is compared with mitosis.

Mitosis	Meiosis
Consists of one cell division.	Consists of two cell divisions.
Results in two diploid cells.	Results in four haploid cells.
The two diploid cells are genetically identical.	The four haploid cells have genetic variation.
Occurs in somatic cells.	Occurs in sex cells.

Chromosome mutations

Chromosome mutations are changes or alterations to whole chromosomes and sets of chromosomes rather than specific nucleotides or sequences. These mutations are caused by genetic errors. This can lead to a variety of adverse effects on an organism's functioning. Some of the common chromosome mutations are deletion, translocation and duplication.

Non-disjunction

Non-disjunction is an example of a chromosome mutation. This occurs when chromosomes fail to be separated correctly during division. Non-disjunction mutation results in the incorrect segregation of homologous chromosomes or sister chromatids. Due to this, haploid cells may be missing chromosomes or have multiples of a chromosome. Both of these scenarios can adversely affect an organism's life.

Example

In humans, non-disjunction of chromosome $21$ gives rise to a condition called Down's syndrome.