MIC1008 Tutorial 13 Nov 2024

What are exons?

• Set of genes that are adjacent to one another in the genome and are coordinately controlled

• Genetic information coding for an amino acid sequence that will form a functional polypeptide/protein

• Intervening non-coding segments of DNA

• Site where repressor proteins bind

Answer explanation

Exons contain the actual genetic information coding for protein. In contrast, introns are intervening non-coding segments of DNA. During the splicing process of pre-mRNA modification, introns are removed from the sequence. Alternative splicing patterns allow multiple exon sequences to be created from a single gene, resulting in multiple proteins.

An operon is a set of genes that are adjacent to one another in the genome and are coordinately controlled. The silencer region is a site of a gene where repressor proteins bind to regulate gene expression.

What is the role of promoter region in the regulation of gene expression?

• Alter gene expression by binding transcription factors

• Bind negative regulators to inhibit gene expression

• Recruit RNA polymerase and transcription factors to DNA

• Allow for alternative splicing and recombination of genetic components

Answer explanation

The promoter region is the site of a gene where RNA polymerase and other transcription factors bind to DNA, upstream from the gene locus. A mutation in this region commonly results in a decrease in the amount of gene transcribed.

An enhancer region is a stretch of DNA that alters gene expression by binding transcription factors, while a silencer region is a site on the gene where repressor proteins bind. Introns are intervening non-coding segments of DNA that are not expressed in the final protein. Alternative splicing patterns of introns and exons allows for multiple proteins to be generated from a single gene.

During transcription of a eukaryotic cell, what determines the template strand (of DNA)?

• The location of the AUG start codon

• The location of the MET start codon

• The base sequence of the enhancer

• The base sequence of the promoter

Colour blindness is an X-linked recessive trait. A man and a woman, each of whom has normal vision, have two children, one of whom is colour blind. Which of the following statement is true?

• The colour-blind child received the allele for colour blindness from its father

• The colour-blind child must be male

• Daughters of this couple have a 0.5 chance of being colour blind

• If the couple have another child the chance that it will be colour blind is 0.5

What is the first stage of meiosis during which a cell is considered haploid?

• Metaphase I

• Anaphase I

• Prophase II

• Telophase II

Answer explanation

During meiosis I, the cell is diploid because the homologous chromosomes are still located within the same cell membrane. Only after the first cytokinesis, when the daughter cells of meiosis I are fully separated, are the cells considered haploid. Following this first division, the cell begins meiosis II with prophase II, making this the first haploid meiotic stage.

During translation, which site in the ribosome allows for tRNA molecules to enter the complex?

• A site

• P site

• E site

• R site

Answer explanation

The ribosomal complex has three sites where tRNA molecules can be oriented during the process of translation: the A site, the P site, and the E site. During polypeptide elongation, a tRNA with an attached amino acid will enter at the A site. It will then move to the P site, now holding the growing polypeptide chain. All tRNAs no longer holding an amino acid will exit the ribosome at the E site.

A mitochondrial disorder exhibits variable severity among siblings, even though they all inherited the mutation from their mother. What is the most likely explanation for this variation in symptom severity?

• Random assortment of mitochondrial DNA during meiosis leads to different levels of mutation inheritance among siblings

• The severity of the disorder depends on the father’s mitochondrial DNA contribution

• Heteroplasmy results in different proportions of mutated and normal mitochondria in each sibling

• Mutations in mitochondrial DNA are corrected at varying rates in different family members

Answer explanation

Heteroplasmy is the presence of both mutated and normal mitochondrial DNA within a cell. Because mitochondria are randomly distributed to daughter cells during cell division, different tissues (and individuals if germ cell during meiosis) can have varying proportions of mutated versus normal mitochondria. This varying ratio affects the severity of symptoms in individuals with the same mitochondrial mutation. Mitochondria DNA do not undergo meiosis.

Which of the following is a recognized mode of alternative splicing?

• Exon skipping

• Alternative acceptor site

• Alternative donor site

• All of the above

Answer explanation

The correct answer is 'all of the above'. Exon skipping is the most common mode in mammals and occurs when an exon is spliced out of the primary transcript. Mutually exclusive exon splicing occurs when one of two exons is retained, but not both. Alternative donor site occurs when an alternative 5' splice junction is used which will change the 3' end of the upstream exon. Alternative acceptor site occurs when there is an alternative 3' split junction and the 5' end of the downstream exon is changed.

Diagram above depicts alternative acceptor site.

If the mRNA sequence is AGA UUU CGA GCC, what are the anticodon of corresponding tRNAs?

Evaluate responses using AI:

OFF

Answer explanation

UCU AAA GCU CGG

Red snapdragons were crossed with white snapdragons. All of the offspring (F1) were pink. When these pink snapdragons were allowed to interbreed, three phenotypes were observed in the second generation in a ratio of 1 red : 2 pink : 1 white.

It is reasonable to conclude from these data that

• both of the parental snapdragons are heterozygous

• two gene loci are involved in determining the colour in these crosses

• a backcross of the F1 pink snapdragons to the red parent would produce red and pink snapdragons

• this is an example of recessive epistasis