meiosis the process in which the nucleus of a cell completes two successive divisions that produce...

Meiosis

• The process in which the nucleus of a cell completes two successive divisions that produce four nuclei, each with a chromosome number that has been reduced by half.

• Known as reduction division – A diploid cell is reduced to four haploid cells

through two divisions

Genetic Variety• Meiosis is a process to convert a diploid

cell to a haploid gamete, and cause a change in the genetic information to increase diversity in the offspring.

• Formation of sex cells (gametes)– Female- produces the egg– Male- produce the sperm

• new individual is formed by a combination of two haploid sex cells (gametes). 223 possibilities

• The different types of gametes produced is 2n, where n = the haploid number

• Haploid (n)-- one set of chromosomes • Diploid (2n)-- two sets of chromosomes• Gamete-haploid cell that fuses with another

during fertilization (the egg and sperm)

Meiosis I

• The first four of eight total phases of meiosis• These phases have the same names as

mitosis, but the events of each phase differ significantly.

Prophase I

• Chromosomes start to coil up and so become shorter and thicker

• Homologous chromosomes pair up• Crossing over occurs• Centrioles move to the poles in animal cells• Nucleoli break down• At the end of prophase I the nuclear

membrane breaks down

Metaphase I

• Chromosomes continue to shorten and thicken• Spindle microtubles attach to the centromeres• Homologous chromosomes line up on the equator• Chiasmata slide towards the ends of the

chromosomes, causing the shape of the pairs to change

• At the end of metaphase I the chromosomes start to move

Anaphase I

• The two chromosomes of each pair move to opposite poles. This halves the chromosome number. Each chromosome consists of two chromatids. Because of crossing over the two chromatids are not identical.

• At the end of anaphase I the chromosomes reach the poles

Telophase I

• Nuclear membranes form around the groups of chromosomes at each pole

• The cell divides to form two haploid cells• The chromosomes uncoil partially• At the end of telophase I the two cells either

enter a brief period of interphase or immediately proceed to the second division of meiosis. The DNA is NOT replicated

Meiosis II

• Meiosis II is identical to mitosis except that the chromosomes DO NOT replicate before they divide.

• The final product of Meiosis II are four haploid cells (or the egg and sperm)

Prophase II

• Chromosomes become shorter and thicken again by coiling

• Centrioles move to the poles in animal cells• At the end of prophase II the nuclear

membrane breaks down

Metaphase II

• Spindle microtubules attach to the centromeres

• Chromosomes line up on the equator• At the end of metaphase II the centromeres

divide

Anaphase II

• The two chromatids of each chromosome move to opposite poles

• At the end of anaphase II the chromatids reach the poles

Telophase II• Nuclear membranes form around the groups of

chromatids at each pole. Each chromatid is now considered to be a chromosome

• The two cells each divide to form four cells total• The chromosomes uncoil• Nucleoli appear• In most organisms the cells formed at the end of

telophase II develop into gametes

• Crossing-over-the exchange of genetic information by non-sister chromatids of paired chromosomes at the start of meiosis (prophase I)– Chiasma (ki’az-um) point where homologous

chromosomes are held together (forms the X)

• Homologous chromosome-the two copies of each chromosome (diploid) that have the same genes, in the same sequence, but not necessarily the same alleles

Recombination

• The reassortment of genes or characteristics into different combinations from those of the parents

Non-disjunction

• The failure of chromosomes to separate during either meiosis I or meiosis II

• Results in gametes with either one too many or too few chromosomes

• Gametes with one too few usually die quickly• Gametes with one too many sometimes

survive

• Meiosis Video