1. The human gametes have about 3 billion bp of DNA in their chromosomes.
a. If the entire DNA was in relaxed B-DNA form, what would be the average length of a
chromosome in the cell?
2*(3*10^9 * 0.34*10^-9)=2*(1.02)= 2.04 m
b. On average, how many complete turns would be in each chromosome?
3*10^9 / 10 = 3*10^8 or 300 million turns
c. If there are around 30-40,000 genes in a human gamete, how many genes are there in an
60-80,000 genes in 46 chromosomes = 1300-1740 genes per chromosome. (2 alleles for
each gene, so 30-40,000 different genes in 46 chromosomes)
2. Define homologous chromosomes.
They are chromosomes that pair together at the metaphase of meiosis. They carry
different alleles for the same genes on the same loci.
3. Define non-homologous chromosomes.
They are chromosomes that aren’t members of the same pair. E.g chromosome 1 and
chromosome 18 would be non-homologous. They don’t pair together at metaphase of
meiosis and don’t carry alleles for the same genes.
4. How many homologous chromosomes are there in a germ cell of a woman?
23 (22 in males and 0 in gametes)
5. Distinguish between homologous chromosomes and sister chromatides.
Homologous chromosomes are chromosomes that pair together at the metaphase of
meiosis. They carry different alleles for the same genes on the same loci. Every human
cell has either 23 pairs (female) or 22 pairs (male) of homologous chromosomes except
for gametes that do not carry homologous chromosomes (n). They are only formed in
Meosis I. Their separation leads to the reduction in chromosomal count from 2n to 1n.
Sister chromatids are seen in Mitosis and Meiosis II. They separate into 2 identical cells in
mitosis and 2 gametes in meiosis. They are formed by DNA replication. Their separation
doesn’t affect the chromosomal count. (parent cell is 2n and daughter cells are 2n,
MeiosisI product is 1n and gametes are 1n)
6. What is the purpose of cell division in Prokaryotes? In Eukaryotes?
In prokaryotes, cell division is a means of reproduction.
In eukaryotes, it’s a method of tissue growth and repair (mitosis) as well as reproduction
7. Distinguish between DNA replication and cell division.
DNA replication occurs in the S phase of the cell cycle. One chromosome is replicated into
2 sister chromatids that separate at anaphase.
Cell division is of 2 types:
1. Mitosis: parent cell (2n) divides into 2 daughter cells (2n each). Each daughter cell
is capable of dividing.
2. Meiosis: consists of 2 stages. Germ cells (2n) are converted into gametes (1n).
Gametes can’t divide further. 8. Distinguish between reason/purpose for/of mitosis and reason/purpose for/of meiosis?
Chromosome number remains the same, Growth and tissue repair
so the genetic information is merely
duplicated. Therefore aids in growth and
Chromosome number is halved (n) and Formation of gametes
therefore will only return to normal (2n)
upon fertilization. In absence of meiosis,
the chromosome number will double with
every fertilization causing malfunctions
and death of the cell.
9. How is variability of genetic information attained by meiosis and fertilization?
1. The independent assortment of homologous chromosomes.
2. Crossing over.
1. Any of the female’s ova can be released during ovulation.
2. Millions of sp