Lecture 2: Mitosis and meiosis 1. Chromosomes 2. Diploid life cycle 3. Cell cycle 4. Mitosis 5. Meiosis 6. Parallel behavior of genes and chromosomes
Basic morphology of chromosomes telomere short arm (p) centromere long arm (q) telomere End of the 19 th century: cytology studies of cells at the light microscopy level Discovery of chromosomes stained bodies (in Greek)
n = 3 2n = 6
A B A c b C d d Each chromosomes contains a long (up to 2 ) DNA molecule and many proteins
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 + XY female = 44 + XX One half of the set (23 chromosomes) come from father, and the other half from mother
Diploid life cycle Zygote formation Development It takes 2 50 mitoses to make an adult out of a zygote
Two types of cell division in the diploid life cycle Mitosis: - in many types of cells - produces identical cells - in haploid and diploid cells - one cell division Meiosis: - in germ line cells to produce gametes - reduces ploidy: 2n -> n - only in diploid cells - two cell divisions
Cell cycle = division (mitosis) + interphase Interphase = G 1 + S + G 2
Imagine a cell with just one pair of homologs (2n = 2) In G 1 there is only one DNA molecule (one chromatid) per chromosome, then DNA replicates during S, and in G 2 there are already two chromatids per each chromosome
Mitosis Interphase Late prophase Metaphase Early anaphase Telophase Mitosis is a continuous process with stage boundaries somewhat blurred
Snapshots of mitosis in a cell with 2n = 2 Prophase G 2 (interphase) Two chromosomes but four chromatids per cell Metaphase S (interphase) DNA replicates G 1 (interphase) centromere Two chromosomes and two chromatids per cell Telophase Anaphase A metaphase chromosome
The genetic consequence of mitosis is simple: it generates two identical copies of the parental cell
Stages of Prophase I Meiosis is a bit more complex
From meiocyte to gametes Meiosis is not a cycle, it is a linear process with no turning back
Snapshots of mitosis in a cell with two chromosomes (2n = 2) Interphase Prophase I 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. Notice in passing: cross-overs happen in Prophase I
Snapshots of mitosis in a cell with two chromosomes (2n = 2) Interphase Prophase I Metaphase I Anaphase I Telophase I 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 Completion of Meiosis I Prophase II Metaphase II Anaphase II Telophase II Peparation for Meiosis II Individual homologs line up in the equator Separation (disjoining) of the sister chromatids Completion of Meiosis II
The genetic outcome of meiosis is Interphase 2n Prophase I Metaphase I Anaphase I Telophase I 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 Completion of Meiosis I Prophase II Metaphase II Anaphase II Telophase II n n n n Peparation for Meiosis II Individual homologs line up in the equator Separation (disjoining) of the sister chromatids Completion of Meiosis II production of four haploid gamets (4 x n) out of one diploid (2n) meiocyte
Reduction of chromosome number from 2n to n occurs during the first division of meiosis (Meiosis I) A a
Using meiosis to explain Mendel s laws Consider the cross P: A/A x a/a F1: A/a Law I: equal segregation How can we explain formation of two gametic types with equal frequency (½ A, ½ a) in such F1 heterozygote? A/a
Using meiosis to explain Mendel s laws Consider 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 1/2 a 1/2 A/a Law I: equal segregation of alleles is due to orderly segregation of homologs in Anaphase I
Using meiosis to explain Mendel s laws Consider the cross P: A/A; B/B x a/a; b/b F1: A/a; B/b Law II: independent assortment 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
Using meiosis to explain Mendel s laws Consider the cross P: A/A; B/B x a/a; b/b F1: 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? ½ AB and ½ ab? A/a; B/b
Using meiosis to explain Mendel s laws Consider the cross P: A/A; B/B x a/a; b/b F1: A/a; B/b How can we explain formation of four gametic types (¼ AB, ¼ ab, ¼ Ab, ¼ ab) in F1 heterozygote? b B Alternative metaphase alignment of the second pair of homologs A/a; B/b
Using meiosis to explain Mendel s laws Consider the cross P: A/A; B/B x a/a; b/b F1: A/a; B/b How can we explain formation of four gametic types (¼ AB, ¼ ab, ¼ Ab, ¼ ab) in F1 heterozygote? b B Alternative metaphase alignment of the second pair of homologs A/a; B/b Law II: independent assortment of two pairs of alleles is due to two equally likely metaphase alignments of different homologs in Metaphase I