Gene Structure 5 Flashcards
What is an overlapping gene?
Adjacent genes located on either the same or the opposite DNA strand, sharing one or more nucleotides of coding sequence. Genes can also overlap regulatory regions.
What do overlapping genes allow?
Multiple proteins encoded from the same sequence, coding regions to be on both strands and for co-regulation of related proteins.
Where were overlapping genes first seen?
ssDNA in phage genomes.
Where are overlapping genes found?
In mitochondria, microbes and eukaryotes. Seen in 10% of human genes.
Give 3 types of overlapping gene.
Same strand, different strand and partial/terminal overlaps.
Describe a same strand overlap.
Unidirectional. The 3’end of one gene overlaps with the 5’ end of another gene on the same strand. Genes may be regulated by common promoter.
Where are same strand overlaps commonly found?
In bacteria, for example E.coli has 586 same strand overlap gene pairs.
What are the two types of different strand overlap?
Convergent and divergent.
Describe a convergent different strand overlap.
Overlap of the 3’ ends of two genes on different strands, Overlap contains polyadenylation sites.
Where are convergent different strand overlaps commonly found?
In drosophila
Describe a divergent different strand overlap.
Overlap of the 5’ ends of two genes on opposite strands,. Transcription of both genes may be controlled by a bidirectional promoter.
Describe a partial/terminal overlap.
Involves a small 5’/3’ overlap (often 1bp) of coding sequence. Terminator site for one gene overlaps with the initiator site of another. Common in prokaryotes.
Give an example of partial/terminal overlap found in bacteria.
1 base overlap between TrpE and TrpD. Shine-Dalgarno of TrpA within TrpB. Allows proteins to be synthesised in equal ratios. Translation coupling is dependent on partial overlaps.
What is the result of in phase overlaps?
Coincident reading frames.
Where are in phase overlaps commonly found?
In bacteria and viruses.
What are the two types of in phase overlaps?
Different initiation or different termination.
Describe in phase overlaps with different initiation sites.
Alternative transcription start sites, can have the promoter for one gene within the coding region on an adjacent gene. Genes share the same terminator- proteins have dissimilar N termini and identical C termini.
Give an example of an in phase overlap with different initiation sites.
Aspartokinase in Thermus flavus-initiation differs, askB subunit overlaps with the 3’end of the askA subunit.
Describe in phase overlaps with different termination sites.
Genes have the same initiator codon but termination occurs at distinct codons in each gene. Proteins have dissimilar C termini and identical N termini.
Give an example of an in phase overlap with different termination sites.
Pili gene C53 in E.coli- termination differs, produces 5 different polypeptides due to overlap.
What results from out of phase overlapping genes?
Overlapped genes are in different ORFs to one another. Short overlaps are often in phase 2 and large overlaps are often in phase 1.
Where are out of phase overlaps commonly seen?
In prokaryotes and phages.
Give an example of an out of phase overlap.
Mouse lnk4a/Arf- alternative first exons (1α and 1β) transcribed from different promoters. Both spliced to same acceptor site in exon 2, which gets translated in alternative frames. Produces two different tumour suppressor proteins.
What is ribosomal frameshifting?
The ribosome pauses on the mRNA and shifts before continuing- usually by 1 base. Codons read after the shift are in a different reading frame.
