MEIOSIS DR. A. TARAB DEPT. OF BIOCHEMISTRY HKMU

MEIOSIS DR. A. TARAB DEPT. OF BIOCHEMISTRY HKMU

Meiosis is a special type of cell division necessary for sexual reproduction in eukaryotes such as animals, plants and fungi The number of sets of chromosomes in the cell undergoing meiosis is reduced to half the original number, typically from two sets (diploid) to one set (haploid) The cells produced by meiosis are either gametes (the usual case in animals) or otherwise usually spores from which gametes are ultimately produced (the case in land plants)

In many organisms, including all animals and land plants (but not some other groups such as fungi), gametes are called sperm in male and egg cells or ova in females Since meiosis has halved the number of sets of chromosomes, when two gametes fuse during fertilization, the number of sets of chromosomes in the resulting zygote is restored to the original number

Meiotic division occurs in two stages, meiosis I and meiosis II, dividing the cells once at each stage The first stage starts with a diploid cell that has two copies of each type of chromosome, one from each the mother and father, called homologous chromosomes All homologous chromosomes pair up and may exchange genetic material with each other in a process called crossing over

Each pair then separates as two haploid cells are formed, each with one chromosome from every homologous pair In the second stage, each chromosome splits into two, with each half, called a sister chromatid, being separated into two new cells, which are still haploid This occurs in both of the haploid cells formed in meiosis I

Meiosis, showing chromosomal crossover

Therefore from each original cell, four genetically distinct haploid cells are produced These cells can mature into gametes

Gametic life cycle

Stages of meiosis Step 1 Prophase I The chromosomes condense, and the nuclear envelop breaks down Homologous chromosome pair along their length Crossing-over occurs when portions of a chromatid on one homologous chromosome are broken and exchanged with the corresponding chromatid portions of the other homologous chromosome

Step 2 Metaphase I The pairs of homologous chromosomes are moved by the spindle to the equator of the cell The homologous chromosomes remain together

Step 3 Anaphase I The homologous chromosome separate As in mitosis, the chromosomes of each pair are pulled to opposite poles of the cell by the spindle fibers But the chromatids do not separate all their centromeres each chromosome is still composed of two chromatids The genetic material, however, has recombined

Step 4 Telophase I Individual chromosomes gather at each of the poles In most organisms, the cytoplasm divides (cytokinesis), forming two new cells Both cells or poles contain one chromosome from each pair of homologous chromosomes Chromosomes do not replicate between meiosis I and meiosis II

Step 5 Prophase II A new spindle forms around the chromosomes Step 6 Metaphase II The chromosomes line up along the equator and are attached at their centromeres to spindle fibers

Step 7 Anaphase II The centromeres divide, and the chromatids (now called chromosomes) move to opposite poles of the cell Step 8 Telophase II A nuclear envelope forms around each set of chromosomes The spindle breaks down and the cell undergoes cytokinesis The result of meiosis is four haploid cells

Independent Assortment

Meiosis in male and female animals

Meiosis of male diploid cell results in four haploid sperm, while meiosis of a female diploid cell results in only one functional haploid egg cell

Differences between meiosis and mitosis Meiosis Definition: A type of cellular reproduction in which the number of chromosomes are reduced by half through the separation of homologous chromosomes, producing two haploid cells. Mitosis Definition: A process of asexual reproduction in which the cell divides in two producing a replica, with an equal number of chromosomes in each resulting diploid cell.

Function: sexual reproduction Type of reproduction: Sexual Occurs in: Humans, animals, plants, fungi Function: Cellular Reproduction and general growth and repair of the body Type of reproduction: Asexual Occurs in: All organisms

Genetically: Different Crossing over: Yes, mixing of chromosomes can occur Number of divisions: 2 Genetically: Identical Crossing over: No, crossing over cannot occur Number of divisions: 1

Number of daughter cells produced: 4 haploid cells Number of daughter cells produced: 2 diploid cells Chromosome number: Reduced by half Chromosome number: Remains the same

Steps: The steps of meiosis are Interphase, Prophase I, Metaphase I, Anaphase I, Telophase I, Prophase II, Metaphase II, Anaphase II and Telophase II. Steps: The steps of mitosis are Interphase, Prophase, Metaphase, Anaphase, Telophase and Cytokinesis

Produces: Sex cells only: Female egg cells or Male sperm cells Produces: Makes everything other than sex cells