Meiosis

Think, Pair, Share Time Refresh my memory on Mitosis Turn to your neighbor and discuss. Be prepared to share your responses. What is the end product of Mitosis? Answer: 2 identical daughter cells

Think, Pair, Share Continued How many phases are there in Mitosis? Answer: 4! How many pairs of chromosomes do human beings have? Answer: 23 pairs (or 46 individual) Why are our chromosomes arranged in pairs? Answer: because 1 comes from mom & 1 from dad

Recall Mitosis Results in 2 daughter cells Process has 4 stages prophase, metaphase, anaphase, telophase and cytokinesis Humans have 23 PAIRS of chromosomes

Remember: IPMAT!

Can that be right!?!?! If the end result of mitosis is two identical daughter cells, then when a male gamete(sperm) and a female gamete(egg) come together during fertilization there must be 92 chromosomes!!! Mom (23 pairs) + Dad (23 pairs) = Baby with 46 pairs!!!!

Meiosis! Meiosis is a second round of cell division that separates sister chromatids in sex cells allowing the offspring to have the same number of chromosome pairs as their parents! Males: Sperm 23 chromosomes Females: Egg 23 chromosomes

Half from Dad, half from Mom combine in offspring for 23 PAIRS!

More on Meiosis Meiosis allows offspring to have the same number of chromosomes as the parents (not doubled!) Meiosis produces gametes. Male gametes = sperm Female gametes = egg Gametes have ½ the number of chromosomes as the parents, so when they come together in fertilization, the offspring has 23 pairs of chromosomes. Meiosis has 2 stages of division, not just 1 like in mitosis. Meiosis does NOT produce identical cells

Chromosome Details Chromosomes are in pairs: 1 from mom & 1 from dad. The picture below shows 2 pairs of homologous chromosomes. This term means that the pair has genes for the same traits (in the same order)

Talking like a scientist…scientific vocabulary. Haploid: cell containing one of each kind of chromosome (n) Diploid: cell with 2 of each kind of chromosome (2n)

Turn notes over! No peeking! Quick! How many individual chromosomes do humans have? 46 A chimpanzee has a diploid number of 48, what is its haploid number? 24

Alleles Our traits, (hair color, eye color, height, etc.), are controlled by genes which are located on our chromosomes. Genes exist in alternate forms – We call these different gene forms alleles. Allele Example: short/tall, brown/blonde, etc.

Crossing-over Crossing over results in the exchange of genetic material between homologous chromosomes. Thus increasing the genetic variation in the offspring or babies.

Crossing over results in the exchange of genetic material.

Independent Assortment The random distribution of chromosomes during meiosis results in millions of different gametes. This independent assortment produces millions upon millions of possible combinations of gametes for fertilization between sperm and egg.

Foundational Facts Recall from mitosis: IPMAT =interphase, prophase, metaphase,anaphase, telophase Chromosome: tightly wound DNA Sister Chromatids: two halves of the doubled chromosome structure

Meiosis Overview http://youtu.be/D1_-mQS_FZ0

Mitosis vs. Meiosis The first five stages of meiosis look identical to mitosis…however it is the further separation of chromosomes that result in haploid gametes.

Mitosis vs. Meiosis Create which cells: Somatic (Body) Cells Sex Cells (Gametes) Result in # of Chromosomes 46 Chromosomes 23 Chromosomes Haploid or Diploid? Diploid Haploid 2 identical daughter cells (46 chromosomes each) Final Result: 4 Gametes (23 chromosomes each)

Problems in Meiosis Nondisjunction: when homologous chromosomes do not separate normally Can produce a gamete with an extra chromosome Can produce a gamete that is missing a chromosome Example: Down Syndrome (47 chromosomes)

Polyploidy                                                                                  Organisms with more than the usual number of chromosome sets. Very rare – usually results in death in animals Common in agriculture!