Meiosis PAP Biology

Our bodies have 2 types of cells: Somatic cells = Body Cells These are diploid (2n) Ex. Blood cells, lung cells, muscle cells, heart cells, etc. Gametes = Sex Cells These are haploid (n) Ex. Egg and Sperm

Our cells have 2 types of chromosomes: Autosomes Carry traits that make you who you are Ex. Chromosomes 1-44 Sex chromosomes Carry traits that make you who you are AND determine your gender Ex. Chromosomes 45-46 (X or Y) Karyotype: a diagram that shows the number and appearance of chromosomes in a cell.

Homologous Chromosomes Homologous chromosomes = chromosome pairs that have the same types of genes One from mom and one from dad Different from… Sister chromatids = 2 identical copies of the same chromosome

During sexual reproduction, the number of cells is important Diploid Cells = 2 full sets of chromosomes (2n) A set from mom and a set from dad Ex. Human somatic cells 2n = 46 Haploid Cells = 1 full set of chromosomes (n) Only one set that is a combination of chromosomes from mom and dad Ex. Gametes = Egg and Sperm n = 23

During sexual reproduction, the number of cells is important MEIOSIS Duplicated chromosomes 2n Meiosis I Duplicated chromosomes Duplicated chromosomes n n Meiosis II Unduplicated chromosomes Unduplicated chromosomes n n n n

Sexual Reproduction Sexual reproduction –fusing the genetic information (gametes) from two parents to produce offspring that are a genetic mixture of both parents. Fertilization – the actual fusion of egg and sperm to form a zygote

Purpose of Meiosis Meiosis is the process of creating gametes – sex cells that have HALF the normal number of chromosomes (only 1 set) We call these cells haploid because they only have one of each chromosomes. These cells are used ONLY for sexual reproduction (sperm in males or eggs in females) Each parent only passes on HALF of their chromosomes (23) – one from each pair – so the offspring ends up with a full set (46)

Before Meiosis I… Interphase = growth phase of cell. Remember it is divided into 3 phases: G1 (gap 1) phase - the cell grows and makes proteins. S (synthesis) phase - chromosomes replicate G2 (gap 2) phase - cells continue to grow and make proteins **At the end of interphase the cell has 2 full sets of duplicated chromosomes**

Meiosis I Meiosis II A separation of homologous chromosomes A reduction from diploid duplicated chromosomes to haploid duplicated chromosomes A separation of sister chromatids Duplicated haploid daughter cells from Meiosis I divide into individual chromosomes

Prophase I Nuclear membrane breaks down. Centrioles separate and make spindle fibers Homologous chromosomes pair up & become visible. Tetrad - cluster of 4 chromatids. Crossing over may occur between homologous chromosomes

Crossing Over During Prophase 1 homologous chromosomes are lined up together Sometimes chromosomes can cross over each other and get “tangled” When this happens, they swap pieces of DNA This process creates new combinations of genes – chromosomes that are “part mom/part dad” Image: © AndreaLaurel 2012 https://flic.kr/p/cnYgVy

Metaphase I Homologous chromosomes are lined up in the middle of the cell in pairs

Anaphase I Homologous chromosomes pairs separate, one chromosome (2 sister chromatids) to each side of the cell Sister chromatids remain attached!

Telophase I (and Cytokinesis) Chromosomes gather at poles Nuclear membrane MAY form Cytoplasm divides (cytokinesis) End Result: Results in 2 haploid daughter cells with duplicated chromosomes that are different from the sets in the original diploid cell

Prophase II Spindle fibers form & attach to the centromeres of the sister chromatids. Nuclear membrane breaks down.

Metaphase II Sister chromatids line up in the middle of the cell single file

Anaphase II Sister chromatids are pulled apart One chromatid from each pair travels to each pole

Telophase II (and Cytokinesis) Nuclear membrane forms around each set of chromosomes. Spindle fibers dissolve Cytoplasm divides End Result: 4 genetically unique, haploid daughter cells, each containing one set of chromosomes

Mitosis vs. Meiosis Creation of diploid somatic cells What When Where Why How Result Type of Reproduction Creation of diploid somatic cells Creation of haploid sex cells Females: before you are born; Males: throughout life Throughout your life Throughout body In ovaries and testes Resources I love: Crash Course Mitosis: https://www.youtube.com/watch?v=L0k-enzoeOM Crash Course Meiosis: https://www.youtube.com/watch?v=qCLmR9-YY7o For growth and repair To make babies PMAT once PMAT twice 2 identical diploid somatic cells 4 unique haploid gametes Asexual Sexual (c) It's Not Rocket Science