cell division

Question 1

why does cell replication and division occur

Answer 1

• growth, repair and reproduction
• genetic information passed onto next generation
  DNA needs to undergo replication before dividing
• prokaryotes divide by binary fission / conjugation and eukaryotes divide by either mitosis or meiosis

Question 2

what is mitosis

Answer 2

• body cells, growth and development

- diploid (2n): two copies of each chromosome, 46, sister chromatids

Question 3

what is meiosis

Answer 3

• sex cells (gametes), reproduction

- haploid (n): single copy of each chromosome, 23, homologous chromosomes (alternative forms of genes)

Question 4

what are the phases of the cell cycle

Answer 4

• interphase: G1 phase (gap), synthesis phase (chromosomal replication), G2 phase (gap)
• miotic phase: mitosis (nuclear division), cytokinesis (cytoplasmic division)

Question 5

describe different terminology used for DNA

Answer 5

• chromatin: chromosomal material, makes up chromosomes, loose filaments of DNA, easy access to DNA for replication
• chromosome: DNA and associated proteins (histones)
• chromatid: copy of a chromosome formed by DNA replication, tightly coiled around histones
• sister chromatids: pair of chromatids joined at the centromere (X shape)
• homologous chromosomes: two copies of each chromosome, similar but not identical

Question 6

what occurs in interphase

Answer 6

• G1: cell commitment to division, accumulate building blocks of chromosomal DNA
• G0 resting phase: cell is not dividing nor preparing (inactive), differentiating cells
• synthesis phase: DNA replication, long chromatin threads, parent double helix is unwound, two precise templates, complementary strands are created, two identical copies
• G2 phase: rapid cell growth, ends when cell enters miotic phase, final checkpoint
• checkpoint: G1 (favourable environment), G2 (undamaged DNA), M (miotic spindle attached)

Question 7

what is the importance of DNA being packaged into chromosomes

Answer 7

• double helix DNA: sequence of nucleotides (determines genes)
• histones: DNA wraps around histones (proteins) forming nucleosomes (beads on a string)
• chromatin: thin fibres, loosely dispersed in G1, condenses into chromosomes in G2
• importance: division of chromatids neatly, condensed, discrete organised chromosomes

Question 8

describe what occurs in the mitotic phase

Answer 8

• P: chromatin condenses to chromosomes spindle begins to assemble (microtubules extend)
• PM: nuclear membrane degrades, chromosomes = obvious, spindle attaches to kinetochore (centromere), sister chromatids migrate to equator
• M: centromere of chromosomes lined up on metaphase plate, kinetochore attached to each chromatid, spindle fibres pull towards opposite poles
• A: chromosomes split at centromeres, sister chromatids migrate to opposite ends, kinetochore microtubules shorten, cell lengthens (non-kinetochore microtubules) to prepare for chromosome segregation
• T: new nuclear membrane forms (recycled materials), nucleoli reform, spindle fibres deconstructed, chromosomes uncoil (chromatin threads)
• C: proteins act to constrict the centre of the cell and the cleavage furrow forms between two nuclei and pinch cell into two, same time as telophase

Question 9

what occurs during meiosis

Answer 9

• reduction division producing haploid reproductive cells (gametes)
• responsible for halving number of chromosomes
• reproductive organs of animals and plants
• M1: interphase (replication), prophase (homologous chromosomes), metaphase (crossing over), anaphase (1 homologous chromosome) and telophase
• M2: interphase (no replication), prophase, metaphase (crossing over), anaphase and telophase (4xn), one sister chromatid from each homologous pair

Question 10

how does genetic variation occur

Answer 10

• diversity: generated in sexually reproducing organisms through recombination
• mixing: of maternal and paternal genomes, original chromosomes reshuffled into new combinations
• crossing over: of genetic information during meiosis (produce new hybrids)

Question 11

what are the differences in plant and animal cytokinesis

Answer 11

• A: proteins constrict centre of cell and cleavage furrow forms between two nuclei, pinch cell into two
• P: no centrosomes or centrioles, spindle is barrel shaped, cell wall (no cleavage), cell plate built from membrane components (cellulose laid down)