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Topic 10 Genetics and Evolution
10.1 Meiosis
Vocab Review
• Diploid (2n) – cells that have 2 of each chromosomes
(SOMATIC or body cells)  chromosome number = 46
(humans)
• Homologous Chromosomes – pairs of chromosomes that occur in
diploid cells – similar in shape, structure, and size and have the
same genes, but not necessarily the same alleles.
• Haploid (n) – cells that have 1 of each chromosome
(GAMETES or sex cells)  chromosome number = 23
(humans)
• Meiosis – reduction division that creates haploid cells from a
diploid cell so organisms can sexually reproduce.
Interphase
Chromosomes replicate in interphase before meiosis.
Prophase I
• Homologous chromosomes pair up
• Spindle fibers form
• Nuclear envelope disappears
Prophase I – Behavior of
chromosomes
• Chromosomes condense and become visible.
• Paired homologous chromosomes are called a bivalent and the
process of their formation is called synapsis.
• Crossing over occurs between non-sister chromatids.
Crossing Over - Prophase I
• Crossing over is the exchange of DNA material between non-sister homologous
chromatids.
• The place where 2 homologous chromosomes are attached is called the
chiasma (plural chiasmata)
• Chismata formation between non-sister chromatids in a bivalent can result in an
exchange of alleles.
• Crossing over produces new combinations of alleles on the chromosomes of
the haploid cells.
Metaphase I
• Chromosome pairs line up on the equator of the cell (middle)
• Spindle fibers attach to chromosomes at the centromere, making
sure to extract one from each homologous pair to ensure that each
pair is represented in each new cell.
Behavior of chromosomes –
Metaphase I
• The bivalents line up randomly along the equator of the cell
(random orientation)
• Crossing over is terminated and the exchange of DNA is complete
resulting in chromatids from the same chromosome which are
usually no longer identical
• Nuclear membrane completely gone.
Anaphase I
• Chromosome pairs line up on the equator of the cell (middle)
• Spindle fibers attach to chromosomes at the centromere, making
sure to extract one from each homologous pair to ensure that each
pair is represented in each new cell.
• Homologous chromosomes separate
in meiosis I.
Telophase I
• Two new nuclear envelopes form.
• Spindle fibers disintegrate.
• Chromosomes uncoil.
Prophase II
• DNA condenses into visible chromosomes again.
• New spindle fibers are formed.
• Nuclear envelopes start to disappear.
• No more bivalents.
Metaphase II
• Individual chromosomes line up along the equator randomly.
• Spindle fibers attach to each of the sister chromatids at the
centromere.
Anaphase II
• Centromeres split (each chromatid is now an individual
chromosome).
• Spindle fibers pull individual chromatids to opposite ends of the cell.
• Because of random orientation, chromatids could be pulled to
either new daughter cell.
• Sister chromatids separate in meiosis II.
Telophase II
• Chromosomes unwind their strands of DNA.
• New nuclear envelopes form.
https://www.youtube.com/watch?v=jjEcHra3484
Independent Assortment
• Random Orientation – During the first division of meiosis (metaphase
I), each homologous pair of chromosomes lines up at the equator in
random order (remember that homologous pairs can have different
alleles for a certain gene).
• Spindle microtubules attach to whichever chromosome is closest.
• Each pole is equally likely to receive either chromosome.
• In humans, there are 23 chromosomes in each sex cell, therefore
there are millions of possible combinations of genes.
• Movements of chromosome in meiosis generate much genetic
variety.
Genetic Variation
• Recombination – occurs with the re-assortment of genes into
different combinations from the parents.
Ex: If the 2 parents are TT and Tt, the offspring will be Tt. (Tt is called the
recombinant).
• Unlinked Genes – genes that are not on the same chromosome.
Recombinants in the offspring happen as a result of random
orientation of the chromosomes (tetrads) during metaphase I.
• Linked Genes – genes found on the same chromosome and thus
normally travel together.
Usually do not follow Mendel’s Law of Segregation so no recombinants for
these particular traits will be created. However if crossing over occurs, then
recombinants can appear.
Mendel’s Law of Independent
Assortment
• Allele pairs separate independently during the formation of
gametes (sperm and egg). Mendel’s law of segregation.
• Therefore traits are transmitted to offspring independently of one
another (unless they are linked).
• Ex: In peas, the gene for height and the gene for pod color (Tt and Gg) are
packed into gametes separate from one another and can lead to multiple
combinations of genes in each gamete (TG, Tg, tG, tg)
• Mendel’s Law of Independent Assortment states that random
alignment of tetrad in Meiosis results in an almost limitless amount of
different offspring (2n where n is the haploid number)