Meiosis
Meiosis definition biology
Meiosis is the type of cell division involved in the formation of sex cells, or gametes.
What is the purpose of meiosis?
In humans, this takes place in the testes and ovaries. Meiosis involves two stages of cell
division that have some similarities to the phases in mitosis.
In mitosis, the chromosome number of the daughter cells is the same as in the parent cell.
In meiosis, the chromosome number of the daughter cells is half that of the parent cell.
A human cell containing 46 chromosomes will undergo meiosis and produce gametes that have 23 chromosomes. Each gamete will contain both the same number and the same kind of chromosomes.
The number of chromosomes in a gamete is called the haploid chromosome number, or n ; the number of chromosomes in all other cells having a nucleus is twice the haploid number and is called the diploid number, or 2n.
In humans,the haploid chromosome number is 23 and the diploid chromosome number is 46.
Offspring carry genetic information from each of the parents. This explains why you might have your father’s eyes but your mother’s hair. Although you may look more like one parent than another, you receive genetic information from each parent.
For example, your father gives you a chromosome with genes that code for eye colour, but so does your mother.
Each of the 23 chromosomes that you receive from your biological father is matched by 23 chromosomes from your biological mother, so that each parent gives you half of your genetic information.
The paired chromosomes are called homologous chromosomes because they are similar in shape, size, and gene arrangement . The genes in homologous chromosomes deal with the same traits. Each cell in your body, except the sex cells, contains 23 pairs of homologous chromosomes, or 46 chromosomes in total.
The 23rd pair of chromosomes, which determine sex in mammals, are called the X and Y chromosomes and are only partially homologous.
Males receive an X and a Y chromosome and females receive two X chromosomes.
You will learn more about these chromosomes later in this chapter.
During fertilization, a haploid (n = 23) sperm cell unites with a haploid
(n = 23) egg cell to produce a diploid (2n = 46) zygote.
The fusion of male and female
gametes restores the diploid chromosome number in the zygote. The zygote will begin
dividing by mitosis and will eventually become a multicellular human baby.
phases of meiosis
Meiosis I is often called reduction division because the diploid, or 2n, chromosome number is reduced to the haploid, or n, chromosome number.
The second phase, meiosis II, is marked by a separation of the two chromatids.The phases used to describe the events of mitosis can also be used to describe meiosis. As with mitosis, DNA synthesis occurs prior to the cell division phase
meiosis 1
meiosis prophase 1
During prophase I, the nuclear membrane begins to dissolve, the centriole splits and its parts move to opposite poles within the cell, and spindle fibres are formed. The chromosomes come together in homologous pairs. Each chromosome of the pair is a homologue and is composed of a pair of sister chromatids. The whole structure is then referred to as a tetrad because each pair is composed of four chromatids.
This process is referred to as synapsis. As the chromosomes synapse, the chromatids
often intertwine.
Sometimes the intertwined chromatids from different homologues break
and exchange segments in a process called crossing over.
Crossing over permits the exchange of genetic material between homologous pairs of chromosomes.
meiosis metaphase 1
Metaphase I follows prophase I. The homologous chromosomes attach themselves to the spindle fibres and line up along the equatorial plate.
meiodis anaphase 1
During anaphase I, the homologous chromosomes move toward opposite poles. The process is known as segregation.
At this point of meiosis, reduction division occurs. One member of each homologous pair will be found in each of the new cells. Each chromosome will be consists of two sister chromatids.
meiosis telophase 1
During telophase I, a membrane begins to form around each nucleus.
However,unlike in mitosis, the chromosomes in the two nuclei are not identical because each of the daughter nuclei contains one member of the homologous chromosome pair.
Although homologous chromosomes are similar, they are not identical. They carry
genes for the same traits (for example, eye colour), but those genes may differ (for
example, coding for brown eyes or coding for blue eyes). The cells are now ready to
begin the second stage of meiosis.
Summary of phases of meiosis
meiosis prophase 1- >
meiosis metaphase 1 ->
Chromosomes line up at the equatorial plate.
meiosis anaphase 1->
Each chromosome separates from its homologue. They move to opposite poles of the cell.
meiosis telophase 1 ->
The nucleus completes its division. The chromosomes are still composed of sister
chromatids. The cytoplasm divides after telophase.
meiosis 2
Meiosis II occurs at approximately the same time in each of the haploid daughter cells.
However, for simplicity, consider the events in only one of the cells.
During meiosis II, pairs of chromatids will separate and move to opposite poles. Note that, unlike with mitosis and meiosis I, there is no replication of chromosomes prior to meiosis II.
meiosis prophase 2
Prophase II signals the beginning of the second meiotic division. During this stage, the nuclear membrane dissolves and the spindle fibres begin to form.
meiosis metaphase 2
Metaphase II follows prophase II. It is signalled by the arrangement of the chromosomes, each with two chromatids, along the equatorial plate. The chromatids remain pinned together by the centromere.
meiosis anaphase 2
Anaphase II can be identified by the breaking of the attachment between the two chromatids and by their movement to the opposite poles. This stage ends when the nuclear membrane begins to form around the chromatids, now referred to as chromosomes.
meiosis telophase 2
The cell then enters its final stage of meiosis: telophase II. During this stage, the second nuclear division is completed and then the second division of cytoplasm occurs. Four haploid daughter cells are produced from each meiotic division.
Summary Of phases of meiosis 2:
meiosis prophase 2->
The centrioles in the two new cells move to opposite poles and new spindle fibres form.The chromosomes become attached to the spindle.
meiosis metaphase 2->
Chromosomes line up at the equatorial plate.
meiosis anaphase 2->
Sister chromatids of each chromosome separate and move to opposite poles.
meiosis telophase 2->
The cytoplasm separates, leaving four haploid daughter cells. The chromosome number has been reduced by half. These
cells may become gametes.
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