MEIOSIS

Meiosis Division of sex cells, called gametes Meiosis is sexual reproduction

Humans have 46 chromosomes

Numbers of Chromosomes Diploid= 2 sets (2n) Haploid= 1 set (n)

Meiosis vs Mitosis Meiosis is the division of gametes (sex cells) Mitosis the division of somatic cells There are two cell divisions during meiosis and only for mitosis!

Gametes Sperm and egg

Fertilization When sperm (n) and egg (n) fuse to make a zygote (2n) A zygote is the product of the fusion of an ovum (egg) and a sperm. It contains two copies of each chromosome, one from each parent. Egg and sperms cells, on the other hand, each contain only one copy of each chromosome. The zygote develops into an embryo. Fertilization When sperm (n) and egg (n) fuse to make a zygote (2n)

Sister Chromatids Exact copies of DNA

Homologous Chromosomes Pair of chromosomes have 2 chromatids each, with exact same genes on each Homologous pairs (tetrads) are 2 chromosomes paired together

Homologous Chromosomes Each locus, where alleles or traits are located, are in the same position on all chromosomes

Homologous Chromosomes Pair of chromosomes (maternal and paternal) that are similar in shape and size. Homologous pairs (tetrads) carry genes controlling the same inherited traits. Each locus (position of a gene) is in the same position on homologues. Humans have 23 pairs of homologous chromosomes. a. 22 pairs of autosomes b. 01 pair of sex chromosomes

Phases of Meiosis

Meiosis I (four phases) Cell division that reduces the chromosome number by one-half. four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I

Interphase Chromosomes replicate during the S phase of the cell cycle. Each duplicated chromosome consists of 2 sister chromatids atttached at their centromere.

Interphase Centrioles duplicate Chromosomes are visible

Interphase Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus

Prophase 1 Chromosomes condense. Synapsis occurs when 2 homologous chromosomes are paired (Which is a tetrad)

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad

Prophase I centrioles spindle fiber aster fibers

Prophase 1 Crossing Over may occur between chromatids of different chromosomes in a tetrad.

Crossing Over - variation Tetrad nonsister chromatids chiasmata: site of crossing over variation

Metaphase I Tetrads align in the middle of the cell. INDEPENDENT ASSORTMENT OCCURS: 1. Mixes genetic info from maternal and paternal chromosomes 2. Leads to different combinations of chromosomes

Metaphase I metaphase plate OR metaphase plate

Independent Assortment

Anaphase I Homologous chromosomes separate and move towards the poles. Sister chromatids remain attached at their centromeres.

Anaphase I

Telophase I Each pole now has haploid set of chromosomes. Cytokinesis occurs and two haploid daughter cells are formed.

Telophase I

Meiosis II No interphase II (or very short - no more DNA replication) Remember: Meiosis II is similar to mitosis

Prophase II

Metaphase II metaphase plate

Anaphase II sister chromatids separate, daughter chromosomes move to different ends of the cell

Telophase II Nuclei form. Cytokinesis occurs. Four haploid daughter cells produced. gametes = sperm or egg

Telophase II

Variation Important to population as the raw material for natural selection. Question: What are the three sexual sources of genetic variation?

Answer: Remember: variation is good! 1. crossing over (prophase I) 2. independent assortment (metaphase I) 3. random fertilization Remember: variation is good!

Question: A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

Answer: 10 chromosomes (haploid)

Karyotype A method of organizing the chromosomes of a cell in relation to number, size, and type.

Karyotype

Karyotype Diploid= 2 copies Triploid= 3 copies Polyploid= more than 2 copies