Week 7 Multicellular organisms depend on cell division for: - Development of an organism from a single fertilised egg. - Growth through various stages in life - Repair of damaged tissues, or replenishing the cells that die e.g. new blood cells, skin cells. - Production of gametes. Prokaryote Eukaryote · a circular DNA molecule · naked - no associated proteins · plasmids often present · one chromosome only · Contains a linear DNA molecule Nuc · associated with histone proteins · no plasmids · two or more different chromosomes KEY TERMS: Histones - Positively charged proteins attracted to negatively charged phosphate groups of DNA. Nucleosome - Complex of DNA and histone proteins Karyotype - Complete array of chromosomes within an individual. Sister chromatids - Joined copies of the original chromosome (two replicas of a single chromosome). Centromere - Region where chromatid is attached to sister chromatid. Homologous chromosomes - Maternal and paternal copies of the same chromosome. Eukaryotic cells consist of chromatin
Chromosome karyotypes Somatic (non reproductive) cells - These have two sets of chromosomes (diploid) - E.g Humans - 2n = 46, This means we have two sets of chromosomes, each has 23 chromosomes. Rice 2n = 24 Gamete cells - These are reproductive cells (sperm and egg): They have half as many chromosomes as somatic cells (haploid). - Human egg or sperm have 23 chromosomes; rice or pollen have 12 chromosomes. Homologous Chromosomes - Two chromosomes of a pair are VERY SIMILAR in length, position of centromere, the way they stain with certain dyes etc. but are NOT IDENTICAL - The homologous pair has the same genes on each chromosome but the properties of a gene pair may differ. - E.g a gene on the chromosomes 1p (paternal chromosome 1:dad) may encode black hair, but it's copy on 1m (maternal chromosome 1: mum) may encode red hair. - Hence black and red hair are ALLELES of each other. - The gene on 1p may be functional, it's copy on 1m may not be functional: normal versus mutant allele. - Each of the 23 different chromosomes is non-homologous to all other chromosomes. Eg chromosome 1 is very different in the above properties compared to all others. They all have completely different genes. The concept of cell cycle Most cells spend the longer part of their life (about 90% of time taken for one round of cell cycle) in interphase: - Growing, carrying out their particular functions, preparing for division. Divided into 3 further phases (G1, S, G2). - Chromosomes are duplicated during S phase - Then mitosis occurs (shorter phase of actual division) includes - Separation of chromosomes (the actual mitosis process)
- Sharing of cytoplasm between the two daughter cells (cytokinesis) CELL CYCLE (EUKARYOTES) - G1 (Gap Phase 1): Primary growth phase of cell - S (Synthesis) : Cell synthesises replica of genome - G2 (Gap phase 2) : Second growth phase (preparing for separation of newly replicated genome) - Mitosis: Separation of two daughter genomes - Cytokinesis: