Cell biology/Meiosis

Asexual vs. Sexual Reproduction

In the first stage of sexual reproduction, "meiosis", the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n). During "fertilization", haploid gametes come together to form a diploid zygote and the original number of chromosomes is restored.
Copied from WP: w:Sexual reproduction

Asexual Reproduction

Aseuxal Reproduction: New individuals are created when a parent makes an exact copy of itself. Examples of this is in Mitosis, Binary Fission, Budding, etc.

Sexual Reproduction

Sexual Reproduction: An individual is created when two haploid gametes (sex cells) join together during fertilization. Example: Meiosis.

Karyotype - Display of chromosomes pairs of a cell arranged by size and shape. Homologous chromosomes are a pair of chromosomes with the same length, centromere position, and staining pattern. They both contain genes of same characteristics. Karyotypes can be used to see defect chromosomes and if there are too many/little chromosomes, leading to a genetic disorder.

Sex chromosomes - X and Y chromsomes (not homologous).

Autosomes - Chromosomes that are not sex chromosomes. "See diploidchromosomes.PNG"

Fertilization - Union of haploid sex cells (gametes) --> diploid (zygote) [fertilized egg].

Reproductive Cells

Explanation of homologous chromosomes.

Somatic Cells: Body cells → Chromosome # in humans (2n) = 46

• Somatic cells are diploid in number meaning they have a full set of chromosomes. Abbreviated as 2N.

Gametes/Germ Cells: Sex cells → Chromosome # in humans (n) = 23

• Gametes are haploid in number meaning they only have half a set of chromosomes.
• Sperm (n) → Chromosome # = 23
• Ova = egg (n) → Chromosome # = 23
• Abbreviated as N

Homologous chromosomes: This is a pair of chromosomes in which each parent donates 1 chromosome to the pair.

• Locus - A specific area on the chromosome where a gene is located.

Meiosis

Meiosis is when a full set of chromosomes in a cell is cut in half through the separation of homologous chromosomes. This occurs in two parts: Meisois I and Meisois II.

• You could call Meiosis the "reduction division".

Spermatogenisis: The creation of four sperm cells through the process of Meiosis.
Oogenesis: Creation of an ova (egg) and three polar bodies through Meiosis.

Tetrads and Crossing Over

• When each pair of chromosome pairs with its homologous chromosome they form a structure called a tetrad.
• Crossing Over - When tetrads are formed, homologous chromosomes may exchange portions of their chromatids (DNA). Results in a production of different sperm and egg (every sperm/egg is unique). Leads to better living conditions and survival of the species.

Stages of Meiosis

Meisois I

Stage name	Description
Prophase I	Chromosomes become visible. Synapsis occurs: Coupling of homologous chromosomes Tetrads form. Crossing Over occurs
Metaphase I	Homologous chromosomes attach to spindle fibers and line up in the middle of the cell. Independent Assortment - One allele can end up in one gamete as another allele can end up in any random gamete. Tetrads are aligned right next to each other.
Anaphase I	The fibers pull the homologous chromosomes toward opposite ends (polarized).
Telophase I & Cytokenisis	Number of cells produced: 2 Chromosome # in cells at this point: 46

Meisois II

Stage name	Description
Prophase II	Stage begins with 2 daughter cells, each with 46 chromosomes. Similar to prophase in Mitosis.
Metaphase II	The chromosomes line up in the middle similar to the metaphase stage of mitosis.
Anaphase II	The sister chromatids separate and move towards opposite ends of the cell.
Telophase II & Cytokinesis	Number of cells produced: 4 Chromosome # in cells at this point: 23 (haploid cells)

Overall Diagram

Fertilization

When male and female sex cells, or gametes, (n=23) combine to create a diploid zygote (2n=46).

Diversity

• Crossing over in Prophase I allows the genes to interact with each other.
• Random Fertilization - Allows any random sperm to fertilize with an egg.
• Random Alignment of Homologous Chromosomes - Allows the chromosomes to separate, giving away to IA. Gives other hereditary characteritstics a fair chance to take place.

Comparison

Description	Mitosis	Meiosis
Parent cell goes through how many divisions?	1	2
# of Daughter cells produced?	2	4
Type of Cells produced?	Body cells	Sex cells
Diploid (2N) or Haploid (N) cells?	Diploid (2N)	Haploid (N)
# of chromosomes in Humans	46	23
Daughter cells indentical or different from parent cell?	Identical	Different
Synonyms	Synonyms for body cells: Somatic Cells	Synonyms for sex cell: Gamete, Sperm, Egg

Why is mitosis necessary?

• To replace dead, worn out or damaged cells.
• So that an organism can grow and develop.

Why is meiosis necessary?

• To create gametes (sex cells) necessary for fertilization.
• Increase genetic diversity in a population.

Diagram