Meiosis Introduction Unit 08

Chromosomes  What is a chromosome?  A piece of coiled DNA  How many chromosomes do human cells have?  46 (in non-sex cells)  Do ALL humans have the same number of chromosomes in their cells?  YES! (Except people with genetic disorders causing extra chromosomes or missing chromosomes)  Where did the 46 chromosomes come from?

Turn to a friend and discuss…  If we get our chromosomes from our parents, why do we not have a total of 92, as in 46 from mom and 46 from dad?  How does each parent manage to only give 23 chromosomes to their child?

Two basic types of Cells  Diploid Cells contain a 2 sets of chromosomes (2 of each type).  Haploid Cells only contain one set of chromosomes, or half of the chromosomes of a diploid cell. (1 of each type)

So…  YOU were made from one cell from your mom and one cell from your dad. So, how is it the same? Shouldn’t you have double the number of chromosomes?  No, the number is the same, because the egg and sperm cells only have half the number of chromosomes, so when they combine they have the full 46.  SO, sex cells have how many chromosomes?  23 chromosomes

Cell Cycle & Mitosis

Mitosis — Quick Review  Four Phases :  Interphase (not really a phase)  Prophase  Metaphase  Anaphase  Telophase  DNA replication happens before mitosis during the synthesis phase of the cell cycle.

Sex Cells -- Gametes  How are gametes (sex cells) formed, if they only have 23 chromosomes?  Through a process called MEIOSIS  Meiosis is similar to Mitosis but has some differences.

Meiosis - Simplified  Meiosis I:  Starts with a single cell with a full set of chromosomes.  Double your chromosomes (DNA).  Chromosomes ”Cross Over”  Cells begin to divide  Meiosis II:  Divided cells each have a complete set of chromosomes.  Cells divide again, leaving each new cell with only a half set of chromosomes (half the DNA). Meiosis I Meiosis II

“Crossing Over”  “Crossing Over“ is also called “recombination”. During this process, two chromosomes pair up and exchange pieces of their chromosomes. “Crossing Over” is important because it results in new combinations of genes that are different from either parent, and this contributes to genetic diversity.

Mitosis vs. Meiosis Primary Difference  Mitosis results in… (somatic cells)  TWO identical daughter cells—no genetic diversity.  Meiosis results in… (gametic cells)  FOUR genetically distinct (different) daughter cells that are different from parent cells, resulting in genetic diversity

Meiosis  Meiosis Animation #1 Meiosis Animation #1