Intro to Meiosis December 4th/5th, 2008

Meiosis Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half (resulting in a haploid cell) through the separation of homologous chromosomes in a diploid cell. Meiosis creates sex cells.

Sex Cells Also called “Gametes”. Gametes are haploid. When they combine, they form a diploid zygote. Female gamete = Egg Male gamete = Sperm

Chromosomes Chromosomes are made up of DNA, which carries the genetic information that is passed on from one generation of cells to the next.

Haploid Refers to the number of chromosomes in a cell. Gametes contain a single set of chromosomes, and therefore a single set of genes. Haploid = “one set” and is usually represented by the letter n. Just like in math, n can stand for any number. In humans, n =23. n = 4

Diploid Refers to the number of chromosomes in a cell. When the gametes of the mother and father combine, they form a diploid zygote. Diploid = “two sets” and is usually written as 2n. In humans, 2n =46. 2n = 8

Homologous Chromosomes Homologous refers to two chromosomes – one from the mother and one from the father – that have the same genes at the same place.

Karyotype

Reflection Take 3 minutes, on your own and without your notes, to try and answer the reflection questions on the back of your notes. After three minutes it up, I will tell you that you can use your notes to help. You will have 2 more minutes. Then you will have 3 minutes to pair/share with the people at your table before we share out at a class.

Crossing Over In Meiosis I, homologous chromosomes pair up to form a structure called a tetrad. The homologous chromosomes exchange portions of their chromatids in a process called crossing-over. The indefinite combinations of genes created by crossing-over is what makes you unique from your siblings.

Phases of Meiosis How are haploid (N) gamete cells produced from diploid (2N) cells? That's where meiosis (my-OH-sis) comes in. Meiosis usually involves two distinct divisions, called meiosis I and meiosis II. By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells. Haploid Diploid Algae 74 Cow 60 Elephant 28 Who can complete the table to the right?

Meiosis I  Prior to meiosis I, each chromosome is replicated. The cells then begin to divide in a way that looks similar to mitosis. In mitosis, the 4 chromosomes line up individually in the center of the cell. The 2 chromatids that make up each chromosome then separate from each other. Who can draw the replicated version of each of these chromosomes. Single Replicated Replicated Single

In prophase of meiosis I, however, each chromosome pairs with its corresponding homologous chromosome to form a structure called a tetrad. There are 4 chromatids in a tetrad. This pairing of homologous chromosomes is the key to understanding meiosis. Who can point to the tetrads in this picture?

As homologous chromosomes pair up and form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing-over. Crossing-over, shown in the figure at right, results in the exchange of alleles between homologous chromosomes and produces new combinations of alleles.

PROPHASE Mitosis Meiosis I

What happens next? The homologous chromosomes separate, and two new cells are formed. Although each cell now has 4 chromatids (as it would after mitosis), something is different. Because each pair of homologous chromosomes was separated, neither of the daughter cells has the two complete sets of chromosomes that it would have in a diploid cell. Those two sets have been shuffled and sorted almost like a deck of cards. The two cells produced by meiosis I have sets of chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.

TELOPHASE and CYTOKINESIS Mitosis Meiosis I

Meiosis II  The two cells produced by meiosis I now enter a second meiotic division. Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II. Each of the cell's chromosomes has 2 chromatids. During metaphase II of meiosis, chromosomes line up in the center of each cell. In anaphase II, the paired chromatids separate. In this example, each of the four daughter cells produced in meiosis II receives 2 chromatids. Those four daughter cells now contain the haploid number (N)—just 2 chromosomes each.

TELOPHASE II and CYTOKINESIS Meiosis TELOPHASE II and CYTOKINESIS

Meiosis

End Result (haploid or diploid) Mitosis vs. Meiosis Mitosis Meiosis Number of Divisions Type of Cell End Result (haploid or diploid) Creates…

Gamete Formation Meiosis produces four genetically different haploid cells. In males, meiosis results in four equal-sized gametes called sperm. In females, the cell divisions at the end of meiosis I and meiosis II are uneven, so that a single cell, which becomes an egg, receives most of the cytoplasm, as shown in the figure at right. The other three cells produced in the female during meiosis are known as polar bodies and usually do not participate in reproduction.