Meiosis
Page 27, question 1
1) Sexual reproduction involves the combining of two
cells from two parents. If meiosis didn’t happen, the
chromosome number in the zygote after fertilisation
would be too high. Meiosis makes sure each gamete
has only half the normal chromosome number so that
the new individual will inherit the right number when
the gametes have combined.
Question 2
Mitosis
Meiosis
Where does it occur?
In all cells
In the sex organs
What is involved?
Chromosomes
replicated, two
identical cells are
produced
Chromosomes are
replicated and the cell
divides twice to
create four gametes
with half the original
chromosome number.
What is the
purpose?
New cells for
-Gametes for
growth, repair and reproduction
replacement
- Unique
individual with a
new combination
of alleles
Question 3
• Meiosis would show some cells with only
half the number of chromosomes and
mitosis wouldn’t
Question 4
- Sex cells have only half the number of chromosomes so
that when they combine at fertilisation, the zygote will have
the normal number of chromosomes.
- If this didn’t happen, every generation would have twice
the number of chromosomes as the last one.
Page 28, question 8:
•
A scientist wanted to produce a hybrid crab by mating a fast growing crab
with another crab which had a good flavour. To do this, he injected the
sperm from a crab which had 194 chromosomes in each body cell into the
egg of a crab which had 100 chromosomes in each egg cell.
•
A) How many chromosomes would there be in a fertilised egg resulting from
this technique?
194 chromosomes in a body cell
Halve this for a gamete = 97 chromosomes in sperm
Egg has 100 chromosomes
So a fertilised egg would contain 100 + 97 = 197 chromosomes
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Question 9, page 29
• A crab, produced by artificial fertilisation,
has 183 chromosomes in each body cell.
In terms of cell division, explain why this
crab would be unable to produce sex cells.
• 183 / 2 = 91.5 chromosomes in gametes
• It would be infertile because you cannot
have half a chromosome
Why is an individual produced by
sexual reproduction not identical to
its parents?
• When a new individual is formed by sexual
reproduction it gains half its chromosomes
from one parent and a corresponding half
from the other.
• Therefore, the offspring has a unique set
of chromosomes that code for its
characteristics.
• They will have a unique genotype and
phenotype