1. Why do you share some but not all characters of each parent? Remember to guess Bellringer Monday

Reminders and Review Chromosomes = condensed chromatin (DNA) made up of 2 sister chromatids held together in the center by the centromere

Asexual reproduction is reproduction that requires only one parent. Most single-celled organisms, like bacteria and protozoans, reproduce this way.

Cell division (mitosis) is a type of asexual reproduction. Your body cells reproduce this way. In asexual reproduction, the DNA and internal structures are copied. Then the parent cell divides, forming two cells that are exact copies of the original.

What Meiosis is About Meiosis allows the creation of unique individuals through sexual reproduction.

Meiosis - reduces the number of chromosomes Why? It is necessary to reduce the number of chromosomes because in sexual reproduction the offspring gets1/2 the chromosomes from the mother and 1/2 the father!

MEIOSIS: A type of cell division that results in 4 daughter cells with half the number of chromosomes as the parent cell

Meiosis is the process of cell division in which gametes are formed Gametes -sex cells (eggs and sperm)

Gametes are haploid – (n) -half the number of chromosomes as the parent Body cells are diploid - 2(n) – cells with two of each kind of chromosome n = number of chromosomes from each parent

Gametes (sex cells)- haploid n = number of chromosomes from each parent n Body cells diploid 2n

Keys to Understanding Meiosis Chromosomes are paired with one from each parent and they contain genes Genes are sections of a chromosome responsible for a trait – like eye color, height, etc. The gene forms on a pair of chromosomes may be identical.. Brown eyes Tall Brown eyes Blue eyes Tall Short or different.

Chromosomes Humans: 46 (23 pairs) 23 from our mother and 23 from our father Mosquito: 6 (3 pairs) Toad: 22 (11 pairs) Potato: 48 (24 pairs) Dog: 78 (39 pairs)

Homologous chromosomes - paired chromosomes -(one from each parent) with genes for the same traits arranged in the same order When we talk about them we number them by size

Diploid vs. Haploid OrganismGamete (n)Body Cell (2n) Humans2346 Mosquito3 Toad22 Potato48 Dog39

Stages of Meiosis Meiosis I and Meiosis II

Stages of Meiosis: Meiosis I Meiosis I: reduces chromosome number

Stages of Meiosis: Meiosis II Meiosis II: separates chromatids

ÄProphase I: chromatin condenses, homologous pairs (with copies) align Metaphase I: homologous pairs (and copies) align at metaphase plate Meiosis I Prophase I: chromatin condenses (plump), homologous chromosomes pair up and crossing over occurs Metaphase I: homologous pairs (and copies) align at the middle

Meiosis I Anaphase I: homologous chromosomes (each with two chromatids) move to opposite ends Telophase I: two nuclei form, original cell divides

Meiosis II Prophase II: chromosomes condense (plump) again Metaphase II chromosomes line up (meet) in the middle

Meiosis II Anaphase II: chromatids separate to opposite poles Telophase II: two nuclei form -cells divide

Results of meiosis 4 sex cells that are haploid (n) - half as many chromosomes as parent cell (2n)

Meiosis – PMAT I and II Parent cell 4 gametes 1 st division 2 nd division

In males meiosis results in 4 sex cells Men are busy – meiosis produces roughly 250,000,000 sperm per day.

In females meiosis results in one large egg cell and 3 polar bodies Woman are less busy in meiosis than men – meiosis produces only a few hundred mature eggs over a lifetime.

The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis Look at the way the chromosomes pair and line up Mitosis The first (and distinguishing) division of meiosis

Crossing-Over Crossing-over -exchange of genetic material between homologous pairs during prophase 1

Humans have 23 pairs of chromosomes Autosomes - Chromosomes 1 to 22 Sex chromosome - Chromosome #23

Sex chromosomes X chromosome Y chromosome

Fertilization = female egg (n) + male sperm (n) Zygote – fertilized egg – diploid (2n)

A zygote = fertilized egg with diploid set of chromosomes. After fertilization, the zygote divides by mitosis and becomes an embryo. Later, the embryo becomes a fetus Zygote>Embryo>Fetus

Boy or Girl? The Y Chromosome “Decides”

Meiosis error - fertilization Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’. In humans this often occurs with the 21 st pair – producing a child with Downs Syndrome

Meiosis – division error Chromosome pair

21 trisomy – Downs Syndrome Can you see the extra 21 st chromosome? Is this person male or female?