Chapter 9: Meiosis Pages

Sexual Reproduction the union of 2 specialized sex cells (gametes) to form a single cell called a zygote Gametes usually come from 2 different parents Results in genetic variation among offspring

How is Chromosome Number Kept the Same with Sexual Reproduction? Homologous Chromosomes - each chromosome in a somatic (body) cell has a partner chromosome Similar in size, shape, and position of centromeres

The sex chromosomes are called X and Y Human females have a homologous pair of X chromosomes (XX) Human males have one X and one Y chromosome The 22 pairs of chromosomes that do not determine sex are called autosomes

Each pair of homologous chromosomes includes one chromosome from each parent The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father A diploid cell (2n) has two sets of chromosomes For humans, the diploid number is 46 (2n = 46)

A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n) For humans, the haploid number is 23 (n = 23) Each set of 23 consists of 22 autosomes and a single sex chromosome In an unfertilized egg (ovum), the sex chromosome is X In a sperm cell, the sex chromosome may be either X or Y

Gametes are Haploid When egg and sperm join at fertilization, each gamete is haploid The diploid number is restored in the fertilized egg (zygote). When the zygote divides by mitosis to form the first 2 cells of the embryo, each daughter cell receives the diploid number of chromosomes

Polyploid 3 or more sets of chromosomes Rare in animals Common among plants Important factor in plant evolution Larger and hardier than diploid Wheat, cotton

Meiosis Diploid cells -> Haploid Cells Meiosis: 1 cell undergoes 2 divisions – Meiosis I and II Produces 4 haploid cells Similar to mitosis, except: 2 divisions Only 1 DNA duplication (first division) Genetic information is shuffled

Meiosis

In a cell in which DNA synthesis has occurred, each chromosome is replicated Each replicated chromosome consists of two identical sister chromatids

Prophase I Remember – the chromosomes are duplicated during the S phase (before meiosis) Homologous chromosomes synapse – line up side by side Form tetrads Crossing over occurs – genetic material is exchanged between homologous chromatids Produces a new combination of genes Increases the genetic variation among offspring

Metaphase I Kinetochores are attached by spindle fibers to the poles Both sister chromosomes are attached to the same pole, the other sister chromosomes to the other pole Tetrads (4 chromatids) line up on the equatorial plane of the cell Tetrads are held together at chiasmata – sites of prior crossing-over

Anaphase I Paired homologous chromosomes separate and move toward opposite poles Each pole receives a random mixture of maternal and paternal chromosomes Only one member of each homologous pair is present at each pole. Sister chromatids are united at their centromere regions.

Telophase I One of each pair of homologous chromosomes is at each pole. Each new nucleus contains the haploid number of chromosomes, but each chromosomes is a duplicated chromosome. Cytokinesis occurs: Often called interkinesis Video

Meiosis II

Prophase II Metaphase II Chromosomes condense again NO crossing over occurs Chromosomes line up on equatorial planes of their cells Chromatids are in groups of two

Anaphase II Telophase II Sister chromatids separate chromosomes move to opposite poles Nuclei form at opposite poles of each cell Cytokinesis occurs

Results of Meiosis Four n cells with unduplicated chromosomes Animals – production of gametes “gametogenesis” Spermatogenesis or oogenesis Plants – production of spores Tutorial

Oogenesis Human females have about 700,000 primary oocytes at birth Suspended in Prophase I until puberty Usually 1 egg is ovulated per cycle ~ 28 days Oocyte fully matures by finishing meiosis Human female ovulates ~ 400 times/lifetime Meiosis II only finishes if fertilization occurs