DNA Replication
What enzyme unwinds and separates the two DNA strands?
Helicase.
What is mutation?
A change in the DNA nucleotide sequence.
How many genetically different haploid cells are produced at the end of meiosis?
Four.
What is random fertilisation?
The random fusion of a sperm and egg, each containing a different combination of alleles.
What is aneuploidy?
An abnormal number of chromosomes caused by errors in chromosome separation during meiosis.
What enzyme adds complimentary nucleotides to the growing DNA strands?
DNA polymerase.
What is the difference between a point mutation and a frameshift mutation?
A point mutation involves a change to one nucleotide/base, while a frameshift mutation results from the insertion or deletion of nucleotides that changes the reading frame.
During which stage of meiosis does crossing over occur?
Prophase 1.
Name the three processes that contribute to genetic variation in offspring.
Crossing over, independent assortment and random fertilisation.
A selection of a chromosome is lost. What type of chromosomal abnormality is this?
Deletion.
What are Okazaki fragments and why are they produced?
Short sections of DNA produced on the lagging strand because DNA polymerase can only add nucleotides in one direction.
Name two environmental factors that can cause DNA damage?
UV radiation, X-rays and certain chemicals.
What is crossing over?
The exchange of DNA between non-sister chromatids of homologous chromosomes, producing new combinations of alleles.
Parent A can produce 4 genetically different gametes.
Parent B can also produce 4 genetically different gametes.
How many possible combinations can occur when their gametes fuse?
16 possible combinations.
A section of a chromosome is copied twice. What is this called?
Duplication.
What enzyme joins Okazaki fragments together?
DNA ligase.
Original:
ATG – CCA – TTT – GGC
Mutated:
ATG – CCA – TCT – GGC
What type of mutation has occurred?
Point mutation — one nucleotide has changed.
What is independent assortment?
The random orientation and separation of homologous chromosome pairs during meiosis, resulting in different combinations of chromosomes in gametes.
Two siblings have the same biological parents but are genetically different. Explain why.
Their parents produce genetically different gametes due to crossing over and independent assortment. Random fertilisation means different sperm and egg combinations can fuse, producing different offspring genotypes.
During meiosis, a pair of homologous chromosomes fails to separate correctly.
a) What is this error called?
b) What type of chromosomal abnormality can result?
c) Explain how this error can affect the chromosome number of the resulting gametes.
a) The error is called nondisjunction.
b) It can result in aneuploidy.
c) If homologous chromosomes fail to separate, some gametes may receive an extra chromosome, while others may receive one fewer chromosome. If one of these gametes is involved in fertilisation, the resulting offspring may have an abnormal chromosome number.
Explain how DNA replication produces two DNA molecules that are genetically identical to the original DNA molecule.
Helicase separates the DNA strands. DNA polymerase adds complementary nucleotides to each original strand. This produces two DNA molecules, each containing one original strand and one newly synthesised strand.
A point mutation changes one nucleotide in a gene. However, the resulting polypeptide remains unchanged.
Explain how this is possible, using the genetic code.
This is possible because the genetic code is degenerate, meaning that more than one codon can code for the same amino acid. Therefore, a change in one nucleotide may produce a different codon that still codes for the same amino acid. As a result, the amino acid sequence and polypeptide may remain unchanged.
Key terms: degenerate genetic code → different codon → same amino acid → unchanged polypeptide.
Explain how meiosis produces genetically different gametes?
Crossing over creates new combinations of alleles, while independent assortment produces different combinations of maternal and paternal chromosomes in gametes.
Two siblings have the same biological parents but are genetically different.
Explain how crossing over, independent assortment and random fertilisation contribute to the genetic differences between the siblings.
Crossing over exchanges DNA between homologous chromosomes during meiosis, creating new combinations of alleles. Independent assortment randomly distributes maternal and paternal chromosomes into gametes, producing different combinations of chromosomes. Random fertilisation means that any sperm can fuse with any egg, creating further combinations of alleles. Together, these processes produce genetically different offspring.
Key terms: crossing over → independent assortment → different gametes → random fertilisation → different offspring genotypes.
A section of chromosome 1 breaks off and attaches to chromosome 4. The genetic material has not been lost or duplicated, but it is now located on a different chromosome.
What type of chromosomal abnormality has occurred? Explain your answer.
A translocation has occurred.
A section of one chromosome has broken off and become attached to a different, non-homologous chromosome. Because the DNA segment has moved rather than simply being lost or duplicated, this is classified as a translocation.