Lecture 5: Feb. 1, 2006 Chromosomes, mitosis and meiosis

Chapter 4 Chromosomes At 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 Anatomy Each duplicated chromosome Each duplicated chromosome Has two sister chromatids, which separate during cell division Has two sister chromatids, which separate during cell division 0.5 µm Chromosome duplication (including DNA synthesis) Centromere Separation of sister chromatids Sister chromatids Centromeres Sister chromatids A eukaryotic cell has multiple chromosomes, one of which is represented here. Before duplication, each chromosome has a single DNA molecule. Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid 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 stages 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 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 division

G 2 OF INTERPHASE PROPHASE PROMETAPHASE Centrosomes (with centriole pairs) Chromatin (duplicated) Early mitotic spindle Aster Centromere Fragments of nuclear envelope Kinetochore Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Kinetochore microtubule Figure 12.6 Nonkinetochore microtubules

Centrosome at one spindle pole Daughter chromosomes METAPHASEANAPHASETELOPHASE AND CYTOKINESIS Spindle Metaphase plate Nucleolus forming Cleavage furrow Nuclear envelope forming Figure 12.6

Details 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 I

Steps in meiosis II

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

Alternative alignments of paired chromosomes in metaphase I

Meiosis and fertilization 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 male and female Drosophila

Inheritance of the X-linked eye color gene in Drosophila This complex diagram shows the results of 2 separate crosses (reciprocal crosses) for the X-linked eye color gene (w + or w alleles)

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

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