Photo AI
6 (a) DNA is composed of four different DNA nucleotides - Edexcel - GCSE Biology - Question 7 - 2016 - Paper 1
Question 7
6 (a) DNA is composed of four different DNA nucleotides. (i) Which diagram represents the arrangement of the sugar, phosphate and the base in a DNA nucleotide? A B... show full transcript
Worked Solution & Example Answer:6 (a) DNA is composed of four different DNA nucleotides - Edexcel - GCSE Biology - Question 7 - 2016 - Paper 1
Step 1
(a) (i) Which diagram represents the arrangement of the sugar, phosphate and the base in a DNA nucleotide?
Answer
The correct diagram that represents the arrangement of sugar, phosphate, and the base in a DNA nucleotide is Diagram C. This diagram accurately portrays the structure of the nucleotide with the sugar (pentose), phosphate group, and nitrogenous base properly depicted.
Step 2
Step 3
(a) (iii) The length of one DNA nucleotide was measured at 3.3 × 10^{-10} metres. Calculate the approximate length of a gene containing 250 nucleotides in nanometres.
Answer
To calculate the length of a gene containing 250 nucleotides, use the formula:
Length in metres = number of nucleotides × length of one nucleotide
Substituting the values:
Length = 250 × 3.3 × 10^{-10} ext{ m} = 8.25 × 10^{-8} ext{ m} = 82.5 ext{ nm}.
Thus, the approximate length of the gene is 82.5 nanometres.
Step 4
(b) (i) State the genotype of Rani:
Answer
Since Rani is bred with Samba (a male white tiger), who has a recessive genetic trait (white fur), Rani must carry at least one dominant allele for orange/yellow fur pigmentation. Therefore, Rani's genotype is likely heterozygous (Aa), meaning she has one allele for normal pigmentation and one recessive allele.
Step 5
(b) (ii) The offspring with normal fur pigmentation was bred with a tiger that was heterozygous. Use A/a to represent the alleles for fur pigmentation. Predict, using the Punnett square, the percentage probability of the offspring from this cross having normal fur pigmentation.
Answer
Using a Punnett square:
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
From this cross, the offspring genotypes will be:
- 25% AA (normal pigmentation)
- 50% Aa (normal pigmentation)
- 25% aa (white fur)
Thus, the percentage probability of offspring having normal fur pigmentation is
75%.
Step 6
(b) (iii) Explain how two parents with a dominant phenotype can produce offspring expressing a recessive characteristic.
Answer
Two parents with dominant phenotypes can produce offspring expressing a recessive characteristic if both parents are heterozygous for the trait. For example, if both parents have the genotype Aa (dominant A for normal pigmentation and recessive a for white fur), there is a 25% chance that their offspring will inherit the recessive allele from both parents, resulting in the recessive phenotype. This allows for the appearance of traits not observable in the parents.