Lecture 3: Cell Division

Question 1

Phases of the cell cycle

Answer 1

• G0: cell cycle arrest, non-replicative (many differentiated cells remain in G0)
• G1: cellular contents, excluding the chromosomes are duplicated (prep for division)
• S: DNA synthesis/replication, each of the 46 chromosomes is duplicated by the cell
• G2: a short resting phase before cell division
• Mitosis: cell division

Question 2

Describe the difference in cell division between somatic and gametic cells

Answer 2

• somatic: division occurs by mitosis to produce diploid cells (2n)
• gametic: meiosis produces gametes which are haploid (n)

Question 3

Describe chromosomes

Answer 3

• made of 2 chromatids

- held together at centromere

Question 4

Describe the cell during G1, S and G2 compared to mitosis

Answer 4

• during G1, S, and G2 the nuclear envelope is intact and no chromosomes are visible
• nuclear envelope disintegrates during mitosis

Question 5

Centrosome

Answer 5

microtubule organising centre, seeds outward growths of polar fibers

Question 6

Centromere

Answer 6

specific DNA sequence that is essential for segregation during cell division

Question 7

Meiosis

Answer 7

• pattern of cell division producing gametes
• germline cells are haploid
• DNA is copied once into 46 chromosomes and then meiosis splits this back to 23 chromosomes and then you have another division to form 4 daughter cells (n) (single chromatid/cell)

Question 8

What are the steps of meiosis?

Answer 8

• Interphase: chromosomes duplicate
• Prophase I: homologous chromosomes pair and exchange segments (recombination between maternal and paternal chromosomes (homologs))
• Metaphase I: tetrads line up at metaphase plate (2 chromatids x 2 homologous chromosomes), random segregation of paternal and maternal chromosomes (independent assortment)
• Anaphase I: pairs of homologous chromosomes split up via spindle fibres pulling on complete chromosome (2 chromatids) to either pole
• Telophase & Cytokinesis: two haploid cells form (chromosomes are still double)
• Meiosis II: separates sister chromatids (results in four haploid cells with each cell containing a single chromosome)

Question 9

Genetic diversity in meiosis

Answer 9

generated by 2 processes:

• recombination (physical breakage of DNA duplex in one maternal and one paternal chromatid and joining of the ends, sequences are exchanged but DNA sequences are neither gained nor lost unless a mistake is made (non-disjunction)
• independent assortment: each bivalent is a 4 stranded structure (4 chromatids), the mitotic spindle pulls 2 chromatids towards each pole randomly

Question 10

Chiasmata

Answer 10

• each chiasma marks a crossing over point
• on avg 55 chiasmata/cell in human male meiosis
• important for correct chromosome segregation at meiosis I
• holds homologs together until align along plate and formation of spindle until anaphase

Question 11

What is usually the cause of children with incorrect chromosome numbers?

Answer 11

often the products of gametes where bivalent lacked crossovers

Question 12

Fertilisation

Answer 12

sperm and egg chromosomes combine during the first mitosis forming a diploid zygote. Further rounds of mitosis and differentiation form the whole organism

Question 13

Describe the production of primary oocytes and primary spermatocytes

Answer 13

the germ-line develops by repeated mitotic divisions of diploid cells, leading to the production of primary oocytes and spermatocytes, which then undergo meiosis

Question 14

Describe primary oocytes

Answer 14

primary oocytes form during fatal life but arrest at prophase of meiosis I until puberty. after puberty, one oocyte/month completes meiosis. they also display asymmetric cell division

Question 15

Describe the distribution of genetic material within diploid cells

Answer 15

2 copies of each gene, 1 per chromosome copy, each gene is called an allele

Question 16

Describe allelism

Answer 16

alleles can vary slightly in their DNA base sequence, so an individual can have different alleles of a gene (A vs a)