lecture 2: mitosis and meiosis - department of molecular...
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Lecture 2: Mitosis and meiosis
1. Chromosomes2. Diploid life cycle3. Cell cycle4. Mitosis5. Meiosis6. Parallel behavior of genes
and chromosomes
telomere
telomere
centromere
short arm (p)
long arm (q)
Basic morphology of chromosomes
Discovery of ‘chromosomes’ – stained bodies (in Greek)
End of the 19th century: cytology – studies of cells at the light microscopy level
Chromosome number is a constant feature within a species (normally)
Different species often can be distinguished by their chromosome numbers (e.g. human and chimp)
A full set of human male chromosomes
as seen in metaphase of mitosis, after staining with a certain dye
46 chromosomes(23 pairs of homologs):male = 44 + XYfemale = 44 + XX
One half of the set (23 chromosomes) come from father, and the other half from mother
Diploid life cycle
Development
It takes 250 mitoses to make an adult out of a zygote
Zygote formation
Two types of cell division in the diploid life cycle
Mitosis:
- in many types of cells- produces identicalcells
- in haploid and diploidcells
- one cell division
Meiosis:
- in germ line cells toproduce gametes
- reduces ploidy: 2n -> n
- only in diploid cells- two cell divisions
Imagine a cell with just one pair of homologs (2n = 2)
In G1 there is only one DNA molecule (one chromatid) per chromosome, then DNA replicates during S, and in G2 there are already two chromatids per each chromosome
MitosisInterphase
Late prophase
Metaphase
Early anaphase
Telophase
Mitosis is a continuous process with stage boundaries somewhat blurred
Snapshots of mitosisin a cell with 2n = 2
G2 (interphase)
Prophase
Metaphase
AnaphaseTelophase A metaphase chromosome
Twochromosomes andtwo chromatids per cell
centromere
G1 (interphase)
S (interphase)DNA replicates
Two chromosomes but four chromatids per cell
Notice in passing: cross-overs happen in Prophase I
Prophase I
Snapshots of mitosis in a cell with two chromosomes (2n = 2)
Duplication of the chromatids in S phase
Pairing (synapsis) of homologous chromosomes
In human females, oocytes remain in Pro I since the time when the fetus is just 7 months old, and they remain paired until puberty.
Interphase
Prophase I Metaphase I Anaphase I Telophase I
Prophase II Metaphase II Anaphase II Telophase II
Snapshots of mitosis in a cell with two chromosomes (2n = 2)
Duplication of chromatids in S phase
Pairing (synapsis) of homologous chromosomes
Lining up of the paired homologs in the equatorial plane
Separation (disjoining) of the homologs
Peparation for Meiosis II
Individual homologs line up in the equator
Separation (disjoining) of the sister chromatids
Completion of Meiosis I
Completion of Meiosis II
Interphase
Prophase I Metaphase I Anaphase I Telophase I
Prophase II Metaphase II Anaphase II Telophase II
The genetic outcome of meiosis is …
Duplication of chromatids in S phase
Pairing (synapsis) of homologous chromosomes
Lining up of the paired homologs in the equatorial plane
Separation (disjoining) of the homologs
Peparation for Meiosis II
Individual homologs line up in the equator
Separation (disjoining) of the sister chromatids
Completion of Meiosis I
Completion of Meiosis II
Interphase
…production of four haploid gamets (4 x n) out of one diploid (2n) meiocyte
2n
n n
n n
Reduction of chromosome number from 2n to noccurs during the first division of meiosis (Meiosis I)
A a
Using meiosis to explain Mendel’s lawsConsider the cross
P: A/A x a/a F1: A/a
How can we explain formation of two gametic types with equal frequency (½ A, ½ a) in such F1 heterozygote?
A/a
Law I: equal segregation
Using meiosis to explain Mendel’s lawsConsider the cross
P: A/A x a/a F1: A/a
How can we explain formation of two gametic types with equal frequency (½ A, ½ a) in such F1 heterozygote?
A/a
A 1/2
a 1/2
Law I: equal segregation of alleles is due to orderly segregation of homologs in Anaphase I
Using meiosis to explain Mendel’s lawsConsider the cross
P: A/A; B/B x a/a; b/bF1: A/a; B/b
How can we explain formation of four gametic types (¼ AB, ¼ ab, ¼ Ab, ¼ aB) in such F1 heterozygote?
A/a; B/b
Law II: independent assortment
Using meiosis to explain Mendel’s lawsConsider the cross
P: A/A; B/B x a/a; b/bF1: A/a; B/b
How can we explain formation of four gametic types (¼ AB, ¼ ab, ¼ Ab, ¼ aB) in such F1 heterozygote?
A/a; B/b
½ AB and½ ab
?
A/a; B/b
Using meiosis to explain Mendel’s lawsConsider the cross
P: A/A; B/B x a/a; b/bF1: A/a; B/b
How can we explain formation of four gametic types (¼ AB, ¼ ab, ¼ Ab, ¼ aB) in F1 heterozygote?
A/a; B/b
Alternative metaphase alignment of the second pair of homologs
B
b
Using meiosis to explain Mendel’s lawsConsider the cross
P: A/A; B/B x a/a; b/bF1: A/a; B/b
How can we explain formation of four gametic types (¼ AB, ¼ ab, ¼ Ab, ¼ aB) in F1 heterozygote?
A/a; B/b
Alternative metaphase alignment of the second pair of homologs
B
bLaw II: independent assortment of two pairs of alleles is due to two equally likely metaphase alignments of different homologs in Metaphase I