6.1.4 - Coding v Non-coding DNA segments

Question 1

Non-coding DNA

Answer 1

Introns (in-between unnecessary) / “Junk DNA”

• Forms 98% of DNA in eukaryotic nuclei
• Junk DNA: Have no discernible purpose, as they are not involved in protein synthesis of any kind
• Often located in intergenic regions
• May be essential to embryonic development, as its removal results in an egg not developing properly

Question 2

Coding DNA

Answer 2

AKA genes

Question 3

Intron splice-site mutation

Answer 3

• As introns do not code for proteins, mutations within them often have no effect on the phenotype of the organism
• However, a mutation at a spliceosome binding site (splice site, e.g. 3’ or 5’ junctions, or the branchpoint) can affect intron splicing during mRNA maturation
• If the mutation resembles a splice site, splicing may occur at the wrong point
• If splicing fails to completely remove introns from the mRNA, intron sequences will be retained and read as a coding sequence during the mRNA translation process, resulting in pseudo-exons that cause the polypeptide to be assembled incorrectly
• Pseudo-exons cause genetic disorders e.g. beta thalassemia and Duchenne muscular dystrophy

Question 4

Non-coding RNA mutations (structural genes)

Answer 4

• Genes coding for RNA proteins, e.g. ribosomal RNA (rRNA) and transfer RNA (tRNA) are classified as non-coding as they are not based on a sequence of codons
• These are structural genes
• rRNA produces not a polypeptide chain, but a highly structured piece of RNA which binds to ribosomal proteins to form a ribosome
• tRNA produces a structure that allows amino acids to be attached to mRNA molecules
• Mutations in structural genes are not well tolerated because significant changes in some sites can result in the failure of cell function
• Mutations in the regulatory part of DNA causes non-infectious disease sich as heart disease, diabetes, cancer, obesity, among other disorders.

Question 5

Non-coding DNA mutations (frameshift mutation)

Answer 5

• Frameshift mutations in non-coding regions typically have no impact on phenotypic expression, due to the fact that introns are a buffer region that gets excised by spliceosomes during mRNA maturation.

Question 6

Junk DNA

Answer 6

• Have neither protein-coding nor regulatory functions. Comprises the majority of non-coding regions
• Contain repeats of DNA sequences that are a few hundred base pairs long
• Origin theory: repeat sequences are believed to be viruses that inserted copies of a segment of their genetic material into a host’s cell’s DNA. The repeats are called “selfish DNA” by some geneticists as they use host material to proliferate themselves, but give nothing back
• Evolutionary perspective: repetition may increased opportunity for adaptation by natural selection.

Question 7

Repetitive DNA

Answer 7

• Nucleotide sequences that reoccur multiple times in the genome
• Usually non-introns, and often appear due to duplication errors
• Can be short sequences and follow each other in the sequence, or long and scattered throughout the genome
• Trinucleotide diseases are diseases that involve expanded triplet blocks in or around certain genes. In introns, the particular repeats can form clusters that have toxic effects e.g. CAG trinucleotide repeat causes huntington

Question 8

Trinucleotide disease

Answer 8

Trinucleotide Repeat Expansion Diseases are genetic disorders caused by the abnormal expansion of three nucleotide repeat sequences in specific genes.

Question 9

Mutations in coding DNA

Answer 9

• Codes for wrong amino acid → faulty polypeptide chain → incorrect folding of protein → non functional protein → observable phenotypic impact
• Mutations in the genes of eukaryotes whose products are involved in DNA repair are also serious, as this impairs the ability of DNA to fix errors.
• Example of mutation in a gene for DNA repair is the mutated gene that causes xeroderma pigmentosum, a conditon that makes cell extremely vulnerable to damage when exposed to UV light from the sun or other sources.

Question 10

Exon shuffling

Answer 10

• Process involving exons duplicating or moving to create a new intron-exon sequence.
• This can form a new gene and lead to a new protein with a new combination of functions.
• Exons often duplicate and reposition in the genome.
• E.g. collagen protein is due to the highly repetitive amino acid sequence. It is a protein found in forming fibres in connective tissue and bones.

Question 11

Impact of exon shuffling on evolutionary history

Answer 11

Several mutations over evolutionary history involve exon duplication and deletion that led to the current gene coding