Ch 10 Review & Ch 11 Meiosis Intro

Ch 10: Mitosis Review

Chromosomes Def – the genetic information passed down from parent to offspring Each/every human cell has 46 chromosomes 44 = non-sex chromosomes (22 pairs) 2 = sex chromosomes  X or Y (1 pair) All body cells (except sex cells) go through mitosis Mitosis produces cells that are: Clones/genetically identical to parent

Chromosomes, cont. Before cell division, each chromosome/DNA strand must replicate/copy itself Occurs during S phase of Interphase Creates two identical sister parts Called sister chromatids Chromatids are attached at a point called centromere

BEFORE chromosome replication AFTER chromosome replication

Cell growth and division A cell goes through 3 distinct life stages: 1) Interphase Growth, preparation 2) Mitosis Nuclear division 3) Cytokinesis Cytoplasmic division M phase

The Cell Cycle Two phases of cell cycle: 1) Interphase (G1, S, G2) Cells prepare for division Copy/replicate DNA Serves as a checkpoint for M phase Cell will NOT go on to M phase unless p53 gene has proofread and detects NO problems

Cell cycle, cont. 2) M phase Made of two distinct phases: A) Mitosis (nuclear division) B) Cytokinesis (cytoplasmic division) Four phases within mitosis: Prophase, metaphase, anaphase, telophase At the end, two cells produced are genetically identical to parent cells Serves as a source of new cells as organisms grows, develops and repairs old/injured cells

Mitosis animation

Daughter cells vs. parent cells Initial Comparison Mitosis Meiosis # of cells produced 2 4 Daughter cells vs. parent cells Identical Not identical (Why? crossing over) # of chromosomes Same (46  46 in humans) Cut in ½ (46  23 in humans) Purpose To produce new cells (growth, repair old/damaged cells) To produce gametes -egg and sperm (for sexually reproducing organisms)

Ch 11: Meiosis Intro.

Introduction Occurs in the testes and ovaries Meiosis forms gametes Gametes are sex cells Gametogenesis is the production of gametes Ex: sperm (male), egg (female) Occurs in the testes and ovaries Males: continually make sperm Females: are born with all eggs (eggs form through meiosis during development) Mitosis occurs in the REST of the animal body

An offspring must inherit a single copy of every gene/chromosome from EACH parent Why? If you inherited more, you would end up with more than 2 copies of genes/chromosomes Three copies = genetic disorders (Down syndrome) If you inherited less, you would end up with less than 2 copies One copy = normally, fetus death

When your body forms gametes, the two original sets of genes (one from mom, one from dad) must separate Why? Each sperm/egg will receive ONE copy of the genes Why only ONE copy? When gametes combine (during fertilization), the offspring receives two copies One from mom, one from dad

Chromosome Number All animals have the same number of chromosomes in their body cells Humans: Body cells: 46 Sex cells: 23 Fruit fly: Body cells: 8 Sex cells: 4 23 from MOM; 23 from DAD 4 from MOM; 4 from DAD

Chromosome #, cont. In body cells, chromosomes are found in pairs These have a special term to describe them… Homologous chromosomes: A partner or corresponding opposite sex chromosome (with same gene in the same location) ONLY found in body cells NOT found in sex cells Why? Remember – sex cells only have ONE copy of the chromosomes

Diploid and Haploid Two types of cells in body: 1) Diploid Contain two full sets of chromosomes Humans = 46; Fruit fly = 8 One set from mom; one set from dad Represented by “2N” N = one set of chromosomes Cell type: all body cells (muscle cell, red blood cell, heart cell, etc)

Diploid and Haploid,cont. Contain only ONE full set of chromosomes Humans – 23; Fruit fly – 4 Represented by “N” Cell type: gametes (egg and sperm) When egg and sperm combine during fertilization, the offspring (zygote) will have TWO sets of chromosomes After fertilization…the zygote is diploid!

Meiosis Intro Process of reduction division in which the number of chromosomes per cell is cut in half Separates homologous chromosomes in a diploid cell End result: 4 daughter cells with ½ # of chromosomes Humans: 1 parent cell (w/ 46 chromo)becomes 4 daughter cells (w/ 23 chromo) Diploid cell becomes haploid Occurs in sexual organs of animals and plants Forms gametes (sex cells)

Meiosis I Meiosis has two distinct phases: Meiosis I: Meiosis I Meiosis II Meiosis I: Before Meiosis I, chromosomes are duplicated (in the S-phase of Interphase) Just like mitosis!

Meiosis I, cont. Steps are similar to mitosis Two differences: 1) Metaphase Mitosis: Chromosomes line-up using centromere Meiosis: Homologous chromosomes line-up 2) Crossing over A process that results in the exchange of alleles between homologous chromosomes Produces new combinations of alleles This explains why you aren’t EXACTLY like mom/dad Mitosis: does NOT occur Meiosis: occurs during prophase I

Homologous chromosome pair

Meiosis I, cont. Crossing over produces: Recombinant DNA Def – DNA that is randomly changed into a different combination of genetic material Uses homologous chromosomes Sorts/shifts DNA through the switching of sections of chromosomes

Meiosis I

Meiosis II Two cells produced in Meiosis I now enter into a second division (called Meiosis II) UNLIKE Meiosis I, cells DO NOT replicate DNA before dividing Meiosis II is identical to mitosis Only two differences: No Interphase before Meiosis II division End result?? Daughter cells with ½ # of chromos Daughter cells contain haploid set (started with diploid set)

Gamete Formation In animals: Male gamete  sperm Female gamete  egg/ova/ovum Usually only ONE gamete (per sex) is involved in fertilization Exceptions: twins, triplets, etc.

Gamete formation, cont. Male gamete formation: All 4 daughter cells become working sperm Males produce new sperm every day

Gamete formation, cont. Female gamete formation: Only 1 of 4 daughter cells become an egg Cytoplasm (from other 3) is “stolen” by this ONE egg Forms 3 polar bodies and 1 egg/ovum Why? ONE really healthy egg is formed every time! Females produce eggs BEFORE baby is born