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(a) DNA is composed of four different DNA nucleotides - Edexcel - GCSE Biology - Question 7 - 2016 - Paper 1
Question 7
(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? (ii) ... show full transcript
Worked Solution & Example Answer:(a) DNA is composed of four different DNA nucleotides - Edexcel - GCSE Biology - Question 7 - 2016 - Paper 1
Step 1
Which diagram represents the arrangement of the sugar, phosphate and the base in a DNA nucleotide?
Answer
The correct diagram representing the arrangement of the sugar, phosphate, and base in a DNA nucleotide is option C. The key indicates that the sugar is represented by a pentagon, the phosphate by a circle, and the base by a rectangle.
Step 2
Step 3
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 with 250 nucleotides:
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Convert the length of one nucleotide from metres to nanometres: 1 metre = 1 imes 10^9 nanometres, hence: Length of one nucleotide = 3.3 imes 10^{-10} ext{ m} = 3.3 imes 10^{-10} imes 1 imes 10^9 ext{ nm} = 0.33 ext{ nm}.
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Therefore, the length of a gene with 250 nucleotides: Length = 250 imes 0.33 ext{ nm} = 82.5 ext{ nm}.
Step 4
Step 5
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 the Punnett square, there are 3 combinations that lead to normal pigmentation (AA, Aa) out of 4 total combinations.
Hence, the percentage probability of the offspring having normal fur pigmentation is:
rac{3}{4} imes 100 = 75 ext{ %}.
Step 6
Explain how two parents with a dominant phenotype can produce offspring expressing a recessive characteristic.
Answer
Two parents displaying a dominant phenotype can produce offspring showing a recessive phenotype if both parents are heterozygous for the trait. For example, if both parents have the genotype A/a, they can pass on the recessive allele 'a' to their offspring. The combinations (AA, Aa, Aa, aa) from the Punnett square indicate that there is a 25% chance of producing an offspring with the genotype 'aa', resulting in the recessive phenotype.