MEIOSIS

Pp275-278

What Mendel Knew…• Each organism must inherit a

single copy of every gene from each of its “parents”

• Each of the organisms gametes must contain just one set genes• When gametes are formed, there

must be a process that separates the 2 sets of genes so each gamete gets one set

Chromosome Number• Homologous chromosomes

• Chromosome that has a corresponding chromosome from the opposite-sex parent

• Fruit fly has 8 chromosomes• 4 from mom• 4 from dad

Diploid• Di= two sets• Cell that contains both sets of

homologus chromosomes• Cell contains

• 2 complete sets of chromosome• 2 complete sets of genes

• Number of chrms in diploid cell represented by 2N

• For Drosophilia (fruit fly) 2N=8• Mendel said:

• Each adult cell contains two copies of each gene

Haploid• Means “one set”• Refers to cells that contain only

one set of chromosomes• Gametes (sex cells)• Represented by N• Drosophilia fruit fly

• N=4

How are haploid (N) gametes made from diploid (2N) cells?

Meiosis• Process of reduction division in

which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell

Meiosis• 2 distinct stages• Meiosis I

• A diploid cell enters here• Meiosis II

• At the end of this, the diploid cell that entered meiosis has become 4 haploid cells

Meiosis I• Before meiosis 1, each chromosome is

replicate• Then they divide like in mitosis• What happened in mitosis?

• PMAT• Tetrad

• STRUCTURE MADE WHEN EACH CHROMOSOME PAIRS UP WITH ITS HOMOLOGOUS CHROMOSOME

• 4 CHROMATIDS IN A TETRAD

Prophase 1• Each chromosome pairs with its

homologous chromosome making a tetrad

• As they pair up in tetrads, chromosomes exchange portions of their chromatids in the process …. CROSSING OVER

Crossing Over

Metaphase1• Spindle fibers attach to

chromosomesAnaphase 1• The spindles pull homologous

chromosomes apart to opposite poles/ends

Telophase 1• Nuclear membranes form and cell

separates into two new cells

Now what do we have?• 2 new daughter cells• Are they identical to the parents?

• No• The parent has 4 chromosomes• Each daughter cell only has 2 chromosomes

(4 chromatids, but they are different because of crossing-over)

• Each daughter cell has a set of chromosomes and alleles different from each other and different from the parent diploid cell

Meiosis II• Unlike Mitosis, Neither cell goes

through a round of chromosome replication

• Each cell’s chromosome has 2 chromatids

Prophase II• Meiosis resulted in 2 haploid (N)

cells, each with ½ the number of chromosomes in the original cell

Metaphase 2• Chromosomes line up in middleAnaphase 2

• Sister chromatids separate and move to opposite poles

Telophase 2• Meiosis II results in 4 haploid (N) daughter cells• 4 daughter cells contain haploid number of

chromosomes, just 2 each

Gamete Formation• Male

• Haploid gametes produced by meiosis are called sperm

• Female• Haploid gamete produced by meiosis is

called an egg• Cell divisions at the end of meiosis one and

two are uneven so one cell gets most of the cytoplasm (the EGG) and the other three are called polar bodies (don’t participate in reproduction)

Mitosis vs. Meiosis• Mitosis

• Results in the production of two genetically identical DIPLOID cells

• Daughter cells have sets of chromosomes identical to each other and to parent cell

• MITOSIS allows body to grow and replace other cells

• Asexual reproduction• Meiosis

• Results in four genetically different HAPLOID cells

• MEIOSIS is how sexually reproducing organisms make gametes

Genes

Gene Linkage• Would genes on the same chromosome

be inherited together?• Thomas Hunt Morgan gave us the

answer• 1910 PRINCIPLE of LINKAGE• 50 Drosophilia genes• Seemed to contradict Principle of

Independent Assortment b/c certain genes were always inherited together

• He grouped the fly’s genes into linkage groups

• Linkage groups are made up of genes that seem to be inherited together

• Linkage groups assort independently but all genes in one group are inherited together

• Drosophilia• 4 linkage groups• 4 chromosomes

• What can be concluded?

Conclusions1. Each chromosome is actually a group

of linked genes2. Mendel’s Principle of Independent

Assortment holds true

• IT IS THE CHROMOSOMES THAT ASSORT INDEPENDENTLY, NOT THE INDIVIDUAL GENES

How Mendel missed it…• What 7 genes did he study?• 6 of the 7 genes were on different

chromosomes• Two genes were on the same

chromosome but they were so far apart on the same chromosome that they assorted independently

• If two genes are on the same chromosome, does that mean they are linked for ever?• No, chromosomes cross over during

meiosis so they can separate• Crossing over produces new

combinations of alleles• Important for genetic diversity

Lucky Student• Alfred Sturtevant 1911• Columbia University• Worked in Morgan’s Lab• Hypothesis

• If two genes are farther apart on a chromosome, the they are more likely to be separated during meiosis

• Experiment• Measured the rate at which linked genes were

separated and recombined to make a “map” of distances between genes

• Conclusion• Recombination rates could be used to make gene

maps• Gene maps showed the location of a gene on a

chromosome