Meiosis

Homero Martínez #10 3°B

Meiosis

Gene: The fundamental, physical, and functional unit of heredity.

Homologous chromosome: a chromosome with the same gene sequence as another

Gamete: A reproductive cell or sex cell that contains the haploid set of chromosomes, e.g. spermatozoon or sperm cell (male reproductive cell) and egg cell or ovum (female reproductive cell).

Haploid: Thenumber of chromosomes in a gamete of an organism, symbolized by n.

Fertilization: A process in sexual reproduction that involves the union of male (sperm) and female (ovum) gametes(each with a single, haploid set of chromosomes) to produce a diploid zygote.

Diploid: A cell or an organism consisting of two sets of chromosomes: usually, one set from the mother and another set from the father. In adiploid state the haploid number is doubled, thus, this condition is also known as 2n.

Meiosis: A form of cell division happening in sexually reproducing organisms by which two consecutive nuclear divisions (meiosis I and meiosis II) occur without the chromosomal replication in between, leading to the production of four haploid gametes (sex cells), each containing one of every pairof homologous chromosomes (that is, with the maternal and paternal chromosomes being distributed randomly between the cells).

Crossing Over: A process occurring during meiosis wherein two chromosomes pair up and exchange segments of their genetic material.

Meiosis 1
Meiosis I separates homologous chromosomes, producing two haploid cells (N chromosomes, 23 in humans), so meiosis I is referred to asa reductional division. A regular diploid human cell contains 46 chromosomes and is considered 2N because it contains 23 pairs of homologous chromosomes. However, after meiosis I, although the cell contains 46 chromatids, it is only considered as being N, with 23 chromosomes. This is because later, in Anaphase I, the sister chromatids will remain together as the spindle fibers pull the pair toward thepole of the new cell. In meiosis II, an equational division similar to mitosis will occur whereby the sister chromatids are finally split, creating a total of 4 haploid cells (23 chromosomes, N) - two from each daughter cell from the first division.
Prophase I
It is the longest phase of meiosis. During prophase I, DNA is exchanged between homologous chromosomes in a process called homologousrecombination. This often results in chromosomal crossover. The new combinations of DNA created during crossover are a significant source of genetic variation, and may result in beneficial new combinations of alleles. The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. The process ofpairing the homologous chromosomes is called synapsis. At this stage, non-sister chromatids may cross-over at points called chiasmata (plural; singular chiasma).
Leptotene
The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads".[4]:27In this stage of prophase I, individual chromosomes—each consisting of two sister chromatids—change fromthe diffuse state they exist in during the cell's period of growth and gene expression, and condense into visible strands within the nucleus. However the two sister chromatids are still so tightly bound that they are indistinguishable from one another. During leptotene, lateral elements of the synaptonemal complex assemble. Leptotene is of very short duration and progressive condensation and coilingof chromosome fibers takes place.
Zygotene
The zygotene stage, also known as zygonema, from Greek words meaning "paired threads", occurs as the chromosomes approximately line up with each other into homologous chromosome pairs. This is called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. At this stage, the synapsis (pairing/coming together) of...