Topic 1

Question 1

Describe a human chromosome

Answer 1

• found inside the nucleus
• there are 46 chromosomes (23 pairs)
• 1 inherited from the mother and 1 other from the father
• 22 pairs of autosomes (chromosomes 1-22)
• 1 pair of sex chromosome (XX,XY) - determine sex
• each pair of chromosome (chromosome 1-22) is said to be homologous because they are said to be identical

Question 2

when are chromosomes visible under microscope?

Answer 2

• during metaphase of mitosis
• visible as the chromosomes are in high condensed state
• 22 pairs of autosomes and 1 pair of sex chromosome

Question 3

what is a chromosome?

Answer 3

• a chromosome consists of DNA molecules wrapping around his tone proteins and folding into a compact structure
• the DNA of a single chromosome is one long , continuous, double stranded molecule
• human chromosomes are different in length and carry different number of genes on it

for example:
Chromosome 1: 246 million bases long and contains 2968 genes

Chromosome Y: 50 million bases long and contains 231 genes

Question 4

what are genes?

Answer 4

Genes are segments of a DNA that code for a protein

Question 5

what are the levels of a eukaryotic chromosomal organisation?

Answer 5

level 1: DNA double helix

Each chromosome contains a long
continuous double stranded DNA molecule.

Further packaging is needed to make sure the chromosome fits in the nucleus

level 2: Nucleosome

• nucleosomes are the basic structure when DNA wraps around his tone protein
• made up of DNA duplex coiled around 8 histone proteins (histone octamer) every 147 base pairs (bp)
• nucleosomes are separated by a variable length of DNA of 20-80 bp called linker DNA
• histones are positively charged and are attracted to the negatively charged phosphate group of the DNA
• Nucleosomes wrapped into higher order coils are known as solenoids

level 3: solenoids

• Nucleosomes are further coiled into a solenoid

level 4: Chromatin loop

• The solenoid is then organised into looped domains

level 5: Roessettes of Chromatin loops

• The final organisation of the chromosome is unknown but it appears to involve further radial looping into rosettes around a pre existing scaffold of protein

level 6: mitotic chromosome

highly condensed chromosome ps before cell divides

Question 6

describe the cell cycle

Answer 6

• cell cycle depicted as a circle
• first gap phase, G1, involves growth and prepares cell for DNA synthesis , organelles duplicate, carries out normal metabolism
• during S phase, a copy of the genome is synthesised, replication of DNA to form sister chromatid
• The second gap phase, g2, prepares cell grows and prepares for mitosis
• during mitosis, replicated chromosomes are partitioned
• Cytokinesis divides the cell into 2 cells with identical genomes

Question 6

what is interphase?

Answer 6

• period between cell division
• time when cell grows and engages in diverse metabolic activities
• may extend for days or weeks depending on cell type (while M phase lasts for about 1 hour in mammalian cells)
• Can be further divided into G1 phase, S phase, G2 phase

Question 7

M phase

Answer 7

can be further divided into Mitosis and cytokinesis

Question 8

Mitosis

Answer 8

• Cell division process for producing two
  identical daughter
  cells that are identical to the parent cell
• Important for the growth and maintenance of an animal
• can be divided into 5 phases

+ prophase
+ prometaphase
+ metaphase
+ anaphase
+ telophase

Question 9

cytokinesis

Answer 9

parent cell divided into 2 identical daughter cells