M&M Flashcards
Describe Interphase
90% of Cell Cycle
1. G1 phase
- Synthesise cellular organelles, RNA, protein, ATP for DNA replication
2. S phase
- DNA replication occurs here to produce identical sister chromatids
3. G2 phase
- Cellular synthesis of organelles, spindle proteins and ATP
Explain what is meant by homologous chromosomes
- Chromosomes are made up of 2 tightly woven sister chromatids bound at centromere by kinetochore proteins
- Homologous chromosomes determine the same characteristics
- Diploid (2n) organisms have 2 complete sets of chromosomes in homologous pairs, one from each parent
Prophase
- Chromatin threads condense and shorten and thicken to form distinct chromosomes. Each chromosome has 2 kinetochore microtubules attached, 1 from each centriole
- Nucleolus disappears
- Nuclear envelope disintegrates
- Centrioles move to opposite poles of cell
Metaphase
- Mitotic spindle or metaphase plate is fully formed
- Chromosomes line up singly at equator of spindle
Anaphase
- Centromere divides and sister chromatids separate
- Kinetochore microtubules shorten and pull sister chromatids to opposite poles of the cell
- Non-kinetochore microtubules r lengthen to elongate cell
Telophase & Cytokinesis
Reverse of prophase to form 2 daughter nuclei
- Chromosomes decondense to form thin elongated chromatin
- Nucleolus reappears and nuclear envelope reappears
- Spindle fibres disintegrate
Cytokinesis is division of cytoplasm to actually form 2 daughter cells
- In animals, cleavage furrow forms
- In plants, cell plate forms
Significance of Mitosis
- Growth
- Increase in size and complexity of cells to form tissues, organs, systems and organisms - Repair
- Damaged cells replaced, damaged tissues repaired - Asexual Reproduction
- Genetic Stability — faithful transfer of info via genetically identical cells
Meiosis
- Two haploid gametes fuse to form a diploid zygote, requiring meiosis to produce 4 haploid daughter cells, known as reduction division to produce genetically different cells
- Meiosis I: Homologous chromosomes paired as bivalents and separated
- Meiosis II: Sister chromatids separated
Meiosis I
Prophase I: Crossing over occurs between non-sister chromatids of homologous chromosomes. Chiasmata form when non-sister chromatids pair up by synapsis, breaking and rejoining to exchange corresponding alleles, resulting in new combinations of alleles
Metaphase I: Bivalents arrange themselves at metaphase plate with only one spindle fibre attached to each centromere
Independent arrangement of bivalents at metaphase plate
Anaphase I: *Homologous chromosomes separate** with each chromosome consisting of 2 sister chromatids
Cytokinesis produces 2 haploid daughter cells
Meiosis II
- Exactly same as mitosis except it starts from 2 haploid daughter cells instead of diploid (which doesn’t matter in mitosis anyway)
- Start cell of Meiosis II has same amount of DNA molecules (23 X 2 sister chromatids) as non-dividing diploid cell (46 X 1 chromatid)
Importance of Meiosis
Meiosis produces haploid, genetically different gametes, generating genetic variation
1. Crossing Over during Prophase I where chiasmata form between non-sister chromatids of homologous chromosomes to generate new combinations of alleles
2. Independent assortment of homologous chromosomes in metaphase I and independent separation of homologous chromosomes in anaphase I
3. Independent assortment of sister chromatids in metaphase II and independent separation of sister chromatids in anaphase II
4. Random fusion of gametes
Chromosome Mutations
- Variation in chromosome structure
- Deletion
- Duplication
- Inversion
- Translocation moves segment from 1 to a non homologous one - Variation in chromosomal number (Aneuploidy)
- Non-disjunction in Anaphase I = n+1, n+1, n-1, n-1
- Non-disjunction in Anaphase II = n, n, n+1, n-1
- Down Syndrome caused by Trisomy 21 in meiosis I
