DNA and Chromosomes
What three components make up a DNA nucleotide?
A phosphate group, deoxyribose sugar and nitrogenous base.
What enzyme unwinds and separates DNA strands during replication?
Helicase
What type of cell division produces haploid gametes?
Meiosis
What process copies the information in a gene into mRNA?
Transcription
What inheritance pattern occurs when one allele masks the expression of another?
Dominant/recessive inheritance. A dominant allele can determine the phenotype when only one copy is present, while a recessive phenotype generally requires two recessive alleles
Which DNA bases are complementary, and what holds them together?
A pairs with T and C pairs with G. Complementary bases are held together by hydrogen bonds.
What enzyme adds complementary DNA nucleotides during DNA replication?
Name three processes associated with sexual reproduction that increase genetic variation.
Crossing over, independent assortment and random fertilisation.
What three modifications occur to pre-mRNA before it leaves the nucleus?
Addition of a 5′ cap, RNA splicing to remove introns, and addition of a poly-A tail.
What enzyme cuts DNA at specific recognition sequences when making recombinant DNA?
A resitriction enzyme
Explain the difference between introns and exons and identify the function of the promoter region.
Introns are non-coding sections removed from pre-mRNA during RNA processing, while exons are retained in the mature mRNA. The promoter is a DNA region where transcription machinery, including transcription factors and RNA polymerase, binds to initiate transcription.
A nucleotide is added to the middle of a protein-coding DNA sequence. What type of mutation is this, and why can it have a major effect on the protein?
An insertion frameshift mutation. Adding a nucleotide shifts the reading frame, potentially changing every codon after the mutation and therefore many amino acids in the resulting polypeptide.
Compare the number and size of functional gametes produced through spermatogenesis and oogenesis.
Spermatogenesis generally produces four functional sperm cells of approximately equal size from one original cell. Oogenesis generally produces one large functional ovum and polar bodies because the cytoplasm divides unequally.
Explain the roles of mRNA and tRNA during translation.
mRNA carries the genetic code from DNA to the ribosome as a sequence of codons. tRNA carries specific amino acids to the ribosome. Its anticodon binds to a complementary mRNA codon, allowing amino acids to be assembled in the correct sequence.
A mother is a carrier of an X-linked recessive condition and the father is unaffected. What percentage of their sons are expected to have the condition?
50% of sons. Each son receives his Y chromosome from his father and has a 50% chance of receiving the affected X chromosome from his carrier mother.
Compare the organisation of DNA in prokaryotic and eukaryotic cells, including chromosomes, histones and plasmids.
Prokaryotes generally contain a circular chromosome located in the cytoplasm/nucleoid and may also contain smaller circular DNA molecules called plasmids. Eukaryotic nuclear DNA is organised into linear chromosomes and packaged around histone proteins. Mitochondria and chloroplasts also contain circular DNA.
Why are Okazaki fragments produced during DNA replication, and what must happen to them to produce a continuous DNA strand?
DNA polymerase can only synthesise DNA in the 5′ → 3′ direction. Because the two DNA strands are antiparallel, the lagging strand is produced discontinuously as Okazaki fragments. These fragments are subsequently joined to form a continuous DNA strand.
During meiosis, homologous chromosomes fail to separate correctly. Name this error and explain how it can result in aneuploidy.
The error is nondisjunction. Homologous chromosomes or sister chromatids fail to separate correctly, producing gametes with too many or too few chromosomes. Following fertilisation, this can produce an aneuploid individual, such as one with a trisomy or monosomy.
Explain how chromatin structure and transcription factors can regulate whether a gene is expressed.
Chemical modifications can alter how tightly DNA is packaged. Euchromatin is less condensed and generally more accessible for transcription, whereas heterochromatin is tightly condensed and generally less transcriptionally active. Transcription factors bind to regulatory DNA regions, including promoters, and can increase or decrease transcription.
Explain how PCR and gel electrophoresis can be used together to produce a DNA profile.
PCR amplifies selected regions of DNA, producing enough DNA for analysis. DNA fragments can then be separated using gel electrophoresis, where fragments move through a gel according to size. Differences in DNA sequences can produce fragments of different lengths, creating characteristic banding patterns that can be compared between samples.
Explain how homologous chromosomes, sister chromatids, gene loci and alleles are related to one another.
Homologous chromosomes are chromosome pairs containing the same genes at the same gene loci, although they may carry different alleles of those genes. After DNA replication, each chromosome consists of two genetically identical sister chromatids joined at the centromere.
A substitution changes a DNA triplet from TAC to TTC. Explain how you would determine whether this mutation changes the resulting polypeptide.
First transcribe the original and mutated DNA sequences into the corresponding mRNA codons. Use the genetic code to determine the amino acid specified by each codon. Compare the amino acids to determine whether the substitution is silent, missense or nonsense, and therefore whether the polypeptide is changed.
Explain how crossing over and independent assortment during meiosis, followed by random fertilisation, generate genetically unique offspring.
Crossing over exchanges DNA between homologous chromosomes, producing new allele combinations. Independent assortment randomly distributes maternal and paternal chromosomes into gametes. Random fertilisation combines one genetically unique sperm with one genetically unique ovum. Together, these processes produce enormous variation in offspring genotypes.
Explain how HOX genes can cause different cells with the same DNA to contribute to different body structures during development.
HOX genes encode transcription factors that regulate the expression of other genes involved in body development. Different HOX genes are expressed in different locations and developmental stages. This activates different sets of genes in cells, contributing to the formation and positioning of different body structures despite cells containing the same genome.
A child has DNA bands at 120, 250, 400 and 510 bp. The mother has bands at 120, 300, 400 and 620 bp. Candidate A has 200, 250, 450 and 510 bp, while Candidate B has 250, 300, 510 and 620 bp. Which candidate is consistent with being the other biological parent? Justify your answer.
Candidate A. The child shares 120 bp and 400 bp bands with the mother. The unexplained 250 bp and 510 bp bands must be consistent with the other biological parent. Candidate A contains both 250 bp and 510 bp bands, whereas Candidate B also contains these two bands—so as written, both candidates are actually consistent. More DNA markers would be required to distinguish between them reliably.