Meiosis

Meiosis is a type of cell division often referred to as the reduction division. during meiosis the daughter cells will have half of the original chromosome number of the parent cell. daughter cells are NOT GENETICALLY IDENTICAL.

To ensure that the cells of organisms formed by sexual reproduction have the correct number of chromosomes

diploid is 2n and haploid is n

Somatic cells are diploid and Sex cells are haploid.

Autosomes are chromosomes that are not the sex chromosome (Chromosome pair 1 - 22) Gonosomes are the sex chromosomes (Pair 23)

In animals, Testes: In male Ovaries: In female. In plants, Ovaries: In pistillate flowers Anthers: In staminate flowers

DNA replication takes place chromosomes which are single threads become double. each chromosome contains 2 chromatids joined by a centromere. DNA replication helps to double the genetic material so that it can be shared by new cells arising from cell division.

• Chromosomes shorten and become visible as two chromatids joined by a centromere. • Homologous pairs of chromosomes are now visible. • The nuclear membrane and nucleolus disappear. • The spindle starts to form. • Chromatids from each homologous pair touch. The point where they touch is called a chiasma. • DNA is crossed over (swopped) at the chiasma. • The spindle continues to form.

• The spindle extends across the whole cell. • The homologous chromosome pairs line up along the equator of the spindle in their homologous pairs. • One chromosome of each pair lies on either side of the equator. • The centromere of each chromosome attaches to the spindle fibres. random arrangement of chromosomes at the equator allows for genetic variation in the cells

• The spindle fibres shorten and pull each chromosome of each homologous chromosome pair to opposite poles of the cell.

• The chromosomes reach the poles of the cell. • Each pole has half the number of chromosomes present in the original cell. • The cell membrane constricts and divides the cytoplasm in half to form two cells.

• Each cell formed during meiosis I now divides again. * A spindle forms in each of the new cells.

• Individual chromosomes line up at the equator of each cell, with the centromeres attached to the spindle fibres.

• The spindle fibres start to contract. * The centromeres split and CHROMATIDS are pulled to the opposite poles of each cell.

• The SINGLE STRANDED UNREPLICATED CHROMOSOMES reach the poles and a new nucleus forms. • The cell membrane of each cell constricts and the cytoplasm divides into two cells. • Four haploid cells are formed. • Each new cell has half the number of chromosomes of the original cell. • The new cells are genetically different from each other.

the process of meiosis reduces the number of chromosomes by half the halving effect ensures that the number of chromosomes in the species stays the same over generations. meiosis is important to introduce genetic variation

crossing over-prophase 1 random arrangement-metaphase 1 (chromosomes arrange at the equator) mutations during prophase 1 and anaphase 1

Down syndrome is a abnormal disorder that is happens because of a meiosis division error where pair 21 has 3 chromosomes instead of 2. this results in a total of 47 chromosomes instead of 46.

Anphase 1-the 2 chromosomes of a homologous pair do not separate and one gamete has one chromosome too many (3 in total) and the other has 4 too many.

change in chromosome number

A type of cell division that forms daughter cells from a parent cell takes place in the cell nucleus stages are prophase, metaphase, anaphase and telophase

mitosis- takes place in somatic cells meiosis- takes place in sex cells mitosis- the cell nucleus divides once meiosis- the cell nucleus divides twice mitosis- Two daughter cells are formed from one parent cell meiosis- four daughter cells are formed from one parent cell mitosis- chromosomes number remains constant meiosis- chromosome number is halved mitosis- exchange of genetic material/crossing over does not occur meiosis- exchange of genetic material/crossing over does occur

Crossing over brings about an exchange of genetic material during the process of gamete formation which results in the formation of new genetic combinations. This results in formation of gametes that will give rise to individuals that are genetically different from their parents and siblings.