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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