Study Questions Set 3

Question 1

The human gametes have about 3 billion bp of DNA in their chromosomes. a) if the entire DNA was in relaxed B-DNA, what is the length of 1 chromosome in a cell? b) on average how many complete turns would be in a chromosome? c) if there are 30-40k genes in a gamete, how many genes are there in the average chromosomes?

Answer 1

a) 3 billion bp/DNA * 1 DNA/23 chromosomes * 0.34 nm/bp * 1m/10^9nm = 0.044m
b) 0.044m * 10^9nm/m * 1 turn/3.4nm = 13,043, 478 turns
c) 35000/23 = 1521

Question 2

Define homologous chromosomes

Answer 2

chromosomes that contain the same genes in the same gene order but with different alleles

Question 3

Define non-homologous chromosomes

Answer 3

chromosomes that contain different genes

Question 4

how many chromosomes in a germ cell?

Answer 4

there are 46 homologous chromosomes in the germ cell of a woman, or 23 pairs

Question 5

distinguish between homologous chromosomes and sister chromatids.

Answer 5

homologous chromosomes have the same genes in the same gene order with different alleles, but sister chromatids have the same genes in the same gene order with the same alleles. sister chromatids are identical

Question 6

what is the purpose of cell division in prokaryotes and eukaryotes?

Answer 6

prokaryotes: reproduction
eukaryotes: tissue growth, cell repair, reproduction (specialized germ cells)

Question 7

distinguish between DNA replication and cell division

Answer 7

• DNA replication: the process of a double helix DNA molecule being replicated into two identical daughter DNA molecules. Occurs during S phase
• Cell Division: the process of a cell splitting into two daughter cells, and where the previously replicated DNA divides between the daughter cells. Occurs during M phase

Question 8

Distinguish between reason/purpose for/of mitosis and reason/purpose for/of meiosis?

Answer 8

• Mitosis: to produce two identical, diploid daughter cells. Necessary for tissue growth, repair of damaged tissue
• Meiosis: reduction of chromosome number - to produce two haploid daughter cells with variability in their DNA. Necessary for reproduction (production of gametes), and for evolution of the species

Question 9

how is the variability of genetic information attained by meiosis and fertilisation?

Answer 9

• A: variability is attained by:
  o Independent assortment of homologous chromosomes in meiosis (number of combinations is 2^n)
  o Crossing over during meiosis

Question 10

what is cell cycle? and what are the stages?

Answer 10

• Cell cycle: orderly set of events that take place between the formation of a new cell and the division of that cell into two new daughter cells
  o Interphase: where most of the cell spends its life. Normal cell functions are carried out
   G1: cell grows, carries out normal metabolism, organelles duplicate
   S: DNA replication, chromosome duplication
   G2: cell grows, prepares for mitosis
  o M Phase: mitosis: prophase, prometaphase, metaphase, anaphase, telophase, nuclear division (karyokinesis), and cytokinesis
  o Go: cells that have stopped dividing either temporarily or permanently

Question 11

list and briefly describe the checkpoints in cell division

Answer 11

• Checkpoints: specific times during the cell cycle where specific proteins stop the cell cycle to make an assessment. If everything is deemed ‘normal’ it continues. If something is wrong, it will either try to fix it, and if unfixable will undergo apoptosis
  o G1 checkpoint: is the cell big enough? Is the environment favourable?
  o S checkpoint: is all DNA replicated?
  o G2 checkpoint: is the cell big enough? Is the environment favourable?
  o Metaphase checkpoint: are all chromosomes aligned on spindle?
  • Their purpose is to conserve energy and resources – no point in using them to make a damaged cell

Question 12

what can trigger arrest in the cell cycle?

Answer 12

damage to the cell, DNA replicating more than once, improper separation of chromosomes, cell dividing before replication is complete, cell mass not the right size

Question 13

how many chromosomes are there in the somatic cell of a person with down’s syndrome? a) how many autosomes? b) how many sex chromosomes does this person have in a somatic cell? c) germ cell? d) gamete? e) spermatozoid? f) ovum? g) zygote?

Answer 13

they have 47 chromosomes

a) 45 b) 2 c) 2 d) 1 e) 1 f) 1 g) 2

Question 14

how many DNA molecules does a person with Down’s have a) in mitotic metaphase? b) in G1?

Answer 14

a) 94 b) 47

Question 15

how many telomeres are there in a person’s somatic cell in G2?

Answer 15

188

Question 16

there are 4 alleles for a certain gene carried by chromosome 21. how many alleles does a person with Down’s have in a) G1? b) how many different alleles could for this gene could the SAME person have?

Answer 16

a) 3 b) 3

Question 17

there are 13 alleles for a certain gene carried by chromosome 21.
a) how many alleles does a person with down’s syndrome have in a cell which is in meiosis I anaphase? b) how many different alleles could they have in meiosis I anaphase?

Answer 17

a) 6 b) 6

Question 18

when does chromosome segregation happen in a) mitosis? b) meiosis?

Answer 18

a) mitotic anaphase

b) meiosis I anaphase

Question 19

when does chromatid segregation happen in a) mitosis b) meiosis

Answer 19

a) anaphase

b) meiosis II anaphase