Observation

This question was the least answered among the candidates. Many candidates were only able to answer the questions on chromosomes while other questions were vaguely answered. Many candidates could not explain gene replication in living things, neither could they answer the question about the reason the number of chromosomes in a gamete is always half the number of chromosomes in the somatic cells.

On the corresponding complementary base pair of the DNA molecule, the diagram was either awfully drawn and the base pairs not being appropriate or not even drawn at all.

The expected answers are:
(a) Process of gene replication in living things

• The DNA in a chromosome consists of two/double strands;
• of nucleotides held together by hydrogen bonds;
• between the bases;
• before cell division, the DNA has to replicate;
• this involves the double strands of DNA unwinding /unzipping to two separate/single strands;
• this is done by an enzyme;
• for each single strand, the Adenine of a nucleotide will pair with the Thymine;
• the Guanine will pair with the cytosine to form two joined DNA strands again.                                                                                                     

(b)(i)  Chromosome
Is a thread – like structure; in the cell nucleus; of a living organism; it contains/ carries the genes/hereditary information.

(ii) Function of a chromosome
It carries genes; hereditary information/ hereditary factor.                                                                                                                                                               
(iii)  Sex Chromosomes in Humans
X chromosome
Y chromosome                                                                       
Note: spelling of chromosome must be correct to score.

(c) Reason why the number of chromosomes in agamete is always half the number in a somatic cell

• Only one chromosome of each pair goes into the gamete;
• one of each pair of chromosomes comes from the male gamete and one from the female gamete;
• when there is a fusion of the male and a female gametes during fertilization, a zygote is formed;
• the gametes must each contain only half of each of the chromosomes;
• if not the chromosome number will double each time an organism reproduces sexually;
• while in mitosis, after cell division in somatic cells, the exact same number of chromosomes are maintained.                                                          

(d) Construction of complementary strand of DNA