SEXUAL REPRODUCTION

I. Sexual Reproduction: Meiosis Why does it take place? Each organism must inherit a single copy of every gene from each of its parent Gamete or sex cells are formed by a process that separates the two sets of genes so that each gamete ends up with just half the original number of genes

YOUR TURN ! Draw the resulting cell and explain why this would not be correct.

from mom from dad child too much! meiosis reduces genetic content Just right!

DNA molecules are packaged into chromosomes Number of chromosomes DNA molecules are packaged into chromosomes All organisms have different numbers of chromosomes Human somatic or body cells have 46 chromosomes Human gametes or sperm / eggs have 23 chromosomes They have a pair of chromosomes that are homologous

YOUR TURN ! In your own define homologous.

Chromosomes Continued Each chromosomes that comes from the male parent has a corresponding chromosome that comes from the female parent A cell that contains 2 sets of homologous chromosomes is said to be diploid Sometimes represented by the symbol 2n Sex cells contain only a single set of chromosomes, and therefore a single set of genes These cells are called haploid and represented by the symbol n (1n)

YOUR TURN ! If the original organism had 4 chromosomes, draw what a diploid cell and haploid cell would look like in this organism

Meiosis: cell division in two parts Sister chromatids separate Diploid Meiosis I (reduction division) Meiosis II Haploid Chromatids separate (equational division) 2n n Result: one copy of each chromosome in a gamete.

Meiosis I Stages: Prophase I - each chromosome pairs with its corresponding homologous chromosome to form a tetrad

Prophase I: Crossing over Portions of the homologous chromosomes break off and attach to adjacent chromatids on the homologous chromosomes in a process called crossing over, which produces a new combination of genes

Metaphase I – tetrads line up in the middle of the cell

Anaphase I – the centromeres pull the chromosomes toward the opposite ends of the cell

Telophase I and cytokinesis – cell separates into two cells that have half the number of chromosomes, but they contain two copies of that chromosome

YOUR TURN ! Draw a cell in each of the phases of Meiosis I Prophase I:

Meiosis II Stages: Prophase II – meiosis I resulted in two haploid (n) daughter cells, each with half the number of chromosomes as the original cell

Metaphase II – the chromosomes line up in the middle of the cell

Anaphase II – the sister chromatids separate and move toward opposite ends of the cell

Telophase II and cytokinesis – each cell is separated into two cells with half the number of chromosomes as the parent; at the end of meiosis II four haploid cells are formed

YOUR TURN ! Draw a cell in each of the phases of Meiosis II Prophase II:

Meiosis II : the equational division Prophase II (haploid) Metaphase II Anaphase II Telophase II Four Non-identical haploid daughter cells

Fertilization: Fertilization occurs when two haploid cells combine ( sperm and egg) The union of the sperm and egg forms a zygote The zygote has the same number of chromosomes as the original parent (2n)

YOUR TURN ! In your own words define zygote

“Putting It All Together” - Fertilization

Review 1. Copy the following tables in your notes and complete then in order to compare mitosis and meiosis

Mitosis Meiosis Number of divisions Number of daughter cells Genetically identical? Chromosome # What type of cell When would it occur Role in an organism

MITOSIS MEIOSIS Occurs in Produces Results in

Mitosis Meiosis Number of divisions 1 2 Number of daughter cells 4 Genetically identical? Yes No Chromosome # Same as parent Half of parent What type of cell Somatic cells Sex cells When would it occur Throughout life At sexual maturity Role in an organism Growth and repair Sexual reproduction

MITOSIS MEIOSIS Occurs in somatic (body) cells Occurs in gametes (sex cells) Produces cells for repair, maintenance, growth, asexual reproduction Only produces gametes (sex cells: egg/sperm) Results in identical diploid (2n) daughter cells Results in reduction division results in haploid (n) cells