Evolution of eukaryotic genomes (Gene 342) Lecture 1

Lecture outline Regulation of genome replication Cell division cell cycle Cell division mitosis Meiosis

Regulation of eukaryote replication – cell cycle Series of events which coordinate replication in eukaryote cells consists of four phases G1 phase S phase Interphase G2 phase M phase - Mitotic phase

G1 phase Period before mitosis, cytokinesis and the S-phase Major phase for cell growth New organelles synthesized – great amount of protein synthesis Metabolic rate of the cell is high

S phase Synthesis phase – DNA replication and synthesis occur chromatids  sister chromatids Genetic material duplicated individual chromosomes cannot be seen (chromatin)

G2 phase Shortest part of interphase (4-5 hrs) Nucleus well-defined At least one nucleolus present Prepares the cell for mitosis

Go Cells metabolically alive but out of cell cycle Therefore, do not divide Irreparable cells eg. Brain, eye and heart muscle cells

M phase (mitosis) Chromosomes condense and become visible Chromosomes organised by microtubules Microtubules assemble in a complex array (spindle)

Cell Division - Mitosis growth and repair result is genetically identical cells

Cell division – Meiosis I Definitions Diploid - having two complete sets of chromosomes (2n) Haploid – having a single set of chromosomes (n) Homologous chromosomes – members of a chromosome pair, one from each parent Heterologous chromosomes – chromosomes from different pairs Gamete – haploid reproductive cells Zygote – fertilized egg cell

Meiosis II - progression Chromosome duplication Meiotic division I Meiotic division II Result is four, genetically different, haploid cells Diploid meiocyte 2n 4n Division I 2n Division II n Nonidentical haploid cells

Meiosis – Prophase I divided into 5 stages Leptonema – duplicated chromosomes condense and homologues associate Zygonema – synapsis and formation of the synaptonemal complex Pachytene – crossing over occurs Diplontene – chiasmata visible Diakenesis – chromosomes move to central plane perpendicular to axis of spindle

Meiosis I Metaphase I Anaphase I Telophase I paired chromosomes orient towards opposite poles Anaphase I chromosome disjunction (separation of bivalents) Telophase I Spindle dissembled Daughter cells separated by membrane Chromosomes condense and nucleus forms

Meiosis II Prophase II Metaphase II Anaphase II Telophase II Chromosomes condense and attach to new spindle Metaphase II Chromosomes move to equatorial plane Anaphase II Centromeres split and chromatids move to opposite poles Telophase II New nuclei form around chromatids in daughter cells

Meiosis III - recombination Prophase I (Pachytene) crossing over occurs exchange of genetic material between homologous pairs Anaphase I separation of maternally and paternally inherited chromosomes into different daughter cells

Meiosis Haploid products develop into gametes Females Males ¼ becomes an egg cell (oogenesis) others become polar bodies Males All four become sperm cells (spermatogenesis)