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

n = 3 2n = 6

A A

B b

d d

c C

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 + 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

Cell cycle = division (mitosis) + interphaseInterphase = G1 + S + G2

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

The genetic consequence of mitosis is simple:it generates two identical copies of the parental cell

Meiosis is a bit more complex …

Stages of Prophase I

Meiosis is not a cycle,it is a linear process with no turning back

From meiocyte to gametes

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