Lecture 5:Lecture 5:Feb. 1, 2006Feb. 1, 2006

Chromosomes, Chromosomes, mitosis and meiosismitosis and meiosis

Chapter 4 Chapter 4 ChromosomesChromosomesAt right: A newt lung cell in culture at an intermediate stage of mitotic spindle formation, when most of the chromosomes (blue) have already connected to spindle microtubules emanating from the centrosomes but have not yet congressed to the spindle equator. Immunofluorescence highlights:

alpha-tubulin (green) gamma-tubulin (magenta)keratin (red)

Image: A. Khodjakov

Chromosome AnatomyChromosome Anatomy Each duplicated chromosomeEach duplicated chromosome

Has two Has two sister chromatidssister chromatids, which separate during , which separate during cell divisioncell division

0.5 µm

Chromosomeduplication(including DNA synthesis)

Centromere

Separation of sister

chromatids

Sisterchromatids

Centromeres Sister chromatids

A eukaryotic cell has multiplechromosomes, one of which is

represented here. Before duplication, each chromosome

has a single DNA molecule.

Once duplicated, a chromosomeconsists of two sister chromatids

connected at the centromere. Eachchromatid contains a copy of the

DNA molecule.

Mechanical processes separate the sister chromatids into two chromosomes and distribute

them to two daughter cells.

Figure 12.4

A cell cycle showing the 4 A cell cycle showing the 4 stagesstages

The M-stage is the shortest stage. In addition to the 4 stages, two checkpoints and the 2 kinds of protein complexes (cyclin+kinase)

needed to drive through the checkpoints are also shown.

The chromosome number does not change.

Two daughter cells are produced in the end.

Steps in a mitotic cell divisionSteps in a mitotic cell division

Two successive cell divisions occur

resulting in 4 daughter cells. Pairing of

homologous chromosomes occurs and the chromosome

number is halved in the first division. The second division is

identical to mitosis.

Steps in a meiotic cell divisionSteps in a meiotic cell division

G2 OF INTERPHASE PROPHASE PROMETAPHASE

Centrosomes(with centriole pairs)

Chromatin(duplicated)

Early mitoticspindle

Aster

CentromereFragmentsof nuclearenvelope

Kinetochore

Nucleolus Nuclearenvelope

Plasmamembrane

Chromosome, consistingof two sister chromatids

Kinetochore microtubule

Nonkinetochoremicrotubules

Centrosome at one spindle pole

Daughter chromosomes

METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Spindle

Metaphaseplate

Nucleolusforming

Cleavagefurrow

Nuclear envelopeforming

Details of the steps in meiosis prophase IDetails of the steps in meiosis prophase I

During this phase, exchange of parts of chromatids (visible as X-shaped structures called chiasmata)

follows chromosome pairing.

There is a mistake in this diagram. Each chromosome in

leptotene, zygotene and

pachytene should have 2 chromatids. The paired

homologous chromosomes

in zygotene and early pachytene should have 4

strands.

Subsequent steps in meiosis ISubsequent steps in meiosis I

Steps in meiosis IISteps in meiosis II

Pairing of homologous chromosomes and Pairing of homologous chromosomes and chiasma formation [due to crossing over chiasma formation [due to crossing over (recombination) between sister chromatids](recombination) between sister chromatids]

Alternative alignments of paired Alternative alignments of paired chromosomes in metaphase Ichromosomes in metaphase I

Meiosis and fertilization Meiosis and fertilization in females (XX) and males (XY)in females (XX) and males (XY)

The female produces only

one kind of gamete

while the male produces 2

kinds of gametes.

The chromosome composition of The chromosome composition of male and female Drosophilamale and female Drosophila

Inheritance of the X-linked eye color Inheritance of the X-linked eye color gene in Drosophilagene in Drosophila

This complex diagram shows the results of 2

separatecrosses

(reciprocal crosses) for the X-linked eye color gene

(w+ or w alleles)

Inheritance of the X-linked gene for the disease Inheritance of the X-linked gene for the disease hemophilia in the descendants of Queen Victoriahemophilia in the descendants of Queen Victoria

Consequences of nondisjunction (lack of segregation) of the Consequences of nondisjunction (lack of segregation) of the X-chromosomes during meiosis in DrosophilaX-chromosomes during meiosis in Drosophila