DNA, Chromosomes & Replication
What are the three components of a DNA nucleotide?
Where does transcription occur in a eukaryotic cell?
In the nucleus.
What is the purpose of meiosis?
To produce haploid gametes containing half the chromosome number of the parent cell and generate genetic variation.
What is the difference between a dominant and recessive allele?
A dominant allele is expressed in the phenotype when at least one copy is present.
A recessive allele is generally expressed only when two copies are present in an individual with respect to a simple dominant/recessive inheritance pattern.
What is the difference between microevolution and macroevolution?
Microevolution: changes in allele frequencies within a population across generations.
Macroevolution: large-scale evolutionary change above the species level, resulting from accumulated microevolutionary changes.
Which nitrogenous bases pair together in DNA, and how many hydrogen bonds form between each pair?
What is the role of mRNA in protein synthesis?
mRNA carries the genetic information transcribed from DNA to the ribosome, where it provides the sequence used to determine the amino acid sequence of a polypeptide
Name the three processes associated with meiosis and fertilisation that contribute to genetic variation.
A heterozygous tall plant (Tt) is crossed with another heterozygous tall plant (Tt).
What proportion of offspring are expected to be tt?
Tt x Tt
Expected genotypes:
TT : Tt : Tt : tt
Therefore:
25%
Name three processes that can cause microevolutionary change
Natural selection also changes allele frequencies.
During DNA replication, what is the role of helicase and DNA polymerase?
Helicase: unwinds the DNA double helix and separates the two strands by breaking hydrogen bonds.
DNA polymerase: adds complementary nucleotides to the exposed template strand.
What happens during RNA splicing?
Introns are removed from the pre-mRNA and exons are joined together to produce mature mRNA.
What is the difference between a point mutation and a frameshift mutation?
Point mutation: involves a change to a single nucleotide/base pair.
Frameshift mutation: occurs when nucleotides are inserted or deleted in numbers that are not multiples of three, changing the reading frame of the genetic code.
A woman is a carrier for an X-linked recessive condition (XᴺXⁿ) and the father is unaffected (XᴺY).
What proportion of their sons would be expected to inherit the condition?
There is a 50% probability that a son inherits the affected X chromosome from his mother.
Therefore:
50%
of sons are expected to be affected.
A population has the following genotypes:
Genotype
AA - 20
Aa - 60
aa - 20
What is the frequency of the A allele?
100 x 2 = 200
A alleles: (20 x 2) +60 = 100
100/200 = 0.5 or 50%
Why are Okazaki fragments produced during DNA replication?
DNA polymerase can only synthesise DNA in one direction. Because the two DNA strands are antiparallel, the lagging strand must be synthesised discontinuously in short sections called Okazaki fragments, which are later joined together.
Explain how transcription factors can regulate gene expression.
Transcription factors are proteins that bind to regulatory regions such as the promoter and can increase or decrease transcription of a gene. This regulates how much mRNA, and therefore potentially how much protein, is produced.
Explain how crossing over increases genetic variation.
During meiosis, homologous chromosomes exchange corresponding sections of DNA. This produces chromosomes containing new combinations of alleles, resulting in genetically different gametes
A human karyotype contains 47 chromosomes, with three copies of chromosome 21.
What chromosomal abnormality is present and what condition is associated with it?
There is trisomy 21, meaning three copies of chromosome 21.
It is associated with Down syndrome.
A population of birds varies in beak size.
After a drought, large, hard seeds become the dominant food source. Over several generations, the average beak size increases.
What type of natural selection has occurred? Explain.
The environmental change favours birds with larger beaks. These birds have greater reproductive success, causing alleles associated with larger beaks to become more common and shifting the population phenotype toward larger beaks
A section of DNA has the sequence:
3' — TAC GGA CTT AGC — 5'
What is the complementary DNA sequence?
5' — ATG CCT GAA TCG — 3'
The complementary bases pair according to:
A ↔ T
C ↔ G
Two cells contain exactly the same DNA but develop into very different cell types.
Explain how this is possible.
Different cell types express different sets of genes. Chemical tags can alter chromatin structure, affecting whether DNA is accessible for transcription. Transcription factors can also activate or inhibit particular genes. Therefore, different patterns of gene expression result in different proteins being produced, causing cells to differentiate and develop different structures and functions.
A DNA sequence is:
THE CAT ATE THE RAT
A mutation removes the nucleotide corresponding to the second A.
Explain why this type of mutation can have a major effect on the resulting polypeptide.
Removing one nucleotide causes a frameshift, changing how all subsequent nucleotides are grouped into codons. This can change many amino acids in the polypeptide and may produce a premature stop codon, potentially resulting in a non-functional protein.
A couple has four children. The mother is heterozygous for an autosomal recessive condition and the father is also heterozygous.
What is the probability that at least one of four children will be affected?
P affected = 1/4
Unaffected = 3/4
Probability all 4 are unaffected = (3/4)^4 = 81/256
Therefore P (at least one affected) = 1 - 81/256
= 175/256 = 68.4%
A population becomes geographically separated by the formation of a mountain range. Gene flow between the two populations stops. Over many generations, the populations accumulate different mutations and experience different selection pressures.
Predict what may eventually occur and explain the mechanism.
The populations may become separate species through allopatric speciation.
Geographic isolation prevents gene flow. Mutation, genetic drift and natural selection cause the populations to become increasingly genetically different. If reproductive isolation eventually develops, individuals from the two populations may no longer be able to successfully reproduce, resulting in speciation