DNA is a polymer of nucleotides - AQA - GCSE Biology - Question 4 - 2020 - Paper 1

A nucleotide consists of three main components:

1. A phosphate group, which is attached to a sugar.
2. A five-carbon sugar, specifically deoxyribose in the case of DNA.
3. A nitrogenous base, which can be one of four types: adenine (A), cytosine (C), guanine (G), or thymine (T). The essential structure of the nucleotide can be illustrated as follows:

• The phosphate group connects to the sugar molecule.
• The sugar is linked to one of the four nitrogenous bases.

To calculate the total length of the DNA double helix in a human body cell, follow these steps:

1. Each pair of nucleotides contributes 0.34 nm to the length.
2. The total number of nucleotides in a human cell is given as 1.2 × 10^{10}.
3. Therefore, the number of base pairs is half the total nucleotide count: $\text{Number of base pairs} = \frac{1.2 \times 10^{10}}{2} = 6.0 \times 10^{9}$
4. Now, calculate the total length by multiplying the number of base pairs by the increase in length per base pair: $\text{Total Length} = 6.0 \times 10^{9} \times 0.34 \, \text{nm} = 2.04 \times 10^{9} \, \text{nm}$
5. Convert this to meters (1 m = 1 × 10^{9} nm): $\text{Total Length in meters} = \frac{2.04 \times 10^{9}}{1 \times 10^{9}} = 2.04 \, \text{m}$

Non-coding parts of DNA can switch genes on or off, thereby affecting gene expression. These regions may contain regulatory elements that interact with transcription factors or other proteins, which in turn can enhance or inhibit the transcription of nearby coding genes. This regulation is crucial for processes such as cellular differentiation and response to environmental stimuli.