In women, the first division of meiosis produces one daughter cell that has almost all of the cytoplasm - AQA - A-Level Biology - Question 3 - 2018 - Paper 3

Tick 'Crossing over' as it is the process that results in genetic recombination, leading to the allelic combinations seen in the chromosomes.

First, calculate the volume of a polar body and the nucleus using the formula:

$V = \frac{4}{3} \pi r^3$

For the polar body, the radius is ( r = \frac{10.4}{2} = 5.2 ) μm, thus:

$V_{polar body} = \frac{4}{3} \times 3.14 \times (5.2)^3 = 32.73 \, \mu m^3$

For the nucleus, the radius is ( r = \frac{7.0}{2} = 3.5 ) μm:

$V_{nucleus} = \frac{4}{3} \times 3.14 \times (3.5)^3 = 17.14 \, \mu m^3$

Subtracting the volume of the nucleus from the volume of the polar body:

$Volume_{mitochondria} = V_{polar body} - V_{nucleus} = 32.73 - 17.14 = 15.59 \, \mu m^3$

Using the density of mitochondria in the polar body (0.08 mitochondria per μm³):

$Number \, of \, mitochondria = Volume_{mitochondria} \times 0.08 = 15.59 \times 0.08 = 1.248 \approx 1 \, mitochondrion$

The treatment prevents inheritance of mitochondrial diseases by replacing the faulty mitochondria from the affected woman's egg cell with healthy mitochondria from a donor's egg cell. By using a donor nucleus, the egg cell now contains normal mitochondria, which decreases the likelihood of passing on mitochondrial DNA mutations.

One reason is that if most of the mitochondria are faulty, the energy production is significantly reduced. This leads to insufficient ATP being generated for enzyme-catalyzed reactions, which are critical for cellular function and metabolism.