Lecture 6: Structure Of Eukaryotic Genes Flashcards
What is a gene?
Entire DNA sequence transcriptional unit required for synthesis of a functional protein or RNA molecule
What is interrupted genes?
A gene in which the coding sequence is not continuous due to the presence of introns
What do many genes encode?
Functional RNAs that are not translated into protein
What is primary (RNA) is
The original I modified RNA product corresponding to a transcription unit
What is RNA splicing?
The process of excising introns from RNA and connecting the Exxon’s into a continuous mRNA
What is an Exon?
A segment of DNA that is transcribed and retained in the mature RNA product
What is an Intron?
A segment of DNA that is transcribed, but later removed from within the transcript by splicing together the sequences on either side of it
What is mature transcript?
A modified RNA transcript. Modification may include the removal of intron sequences and alterations to the 5’ and 3’ ends
How are introns removed?
RNA splicing which occurs in cis in individual RNA molecules
Do exons remain in the same order in MRNA as in DNA?
Yes
What does mutation in the exons affect?
Polypeptide sequence
What does mutations in introns affect?
RNA processing - influence the sequence and/or production of a polypeptide
How can introns be detected?
When genes are compared with their RNA transcription products
What is cDNA?
Single-stranded DNA complementary to RNA, synthesised from it by reverse transcription in vitro
What are the methods that detect introns?
Restriction mapping Electron microscopy Sequencing
How many exons and introns does the ovalbumin gene contain?
8 exons and 7 introns
What does complementary regions between DNA and mRNA form?
DNA-RNA hybrid
What can genes be interrupted by?
Segments not present in mRNA
When are the positions of introns usually conserved?
When homologous genes are compared between different organisms
What do introns not usually encode?
Proteins
Genes that share a common ancestry have similar organisations with what?
The conservation of the positions of the introns
What is the organisation of the mammalian genes for DHFR?
Short exons and very long introns
What has been observed by doing comparisons of related genes in different specifies?
Sequencers of the corresponding exons are usually conserved but the sequences of the introns are much less similar
In what form can the homology of 2 genes be plotted?
Dot matrix comparison
Why does intron evolve much more rapidly then exons?
Lack of selective pressure to produce a polypeptide with a useful sequence
Under positive selection what sequence vary and what are conserved?
Exons- vary Introns - conserved
What are individuals under positive selection do?
Advantage survive (produce more fertile progeny)
What genes are uninterrupted but interrupted in multicellular eukaryotes?
S. Cerevisiae
What do genes show?
A wide distribution of sizes
What tends to be uniform in size between organisms?
Exons
Exons
Usually short, typically encoding fewer than 100 amino acids
What tends to vary in size between organisms?
Intron size
What is the overall length of a gene determined by?
It’s introns
What does the predominance of introns in multicellular eukaryotes mean?
Genes can be much larger than the sum total of the exons
What does a simple eukaryotic transcription unit produce?
Single monocistronic MRNA which is translated into a single protein which extends from the 5’ cap sire to the 3’poly(A) site
What lies between exons?
Introns and are removed during processing of primary transcript
How are multiple variants of a polypeptide chain produced?
The use of alternative start or stop codons
What is overlapping gene?
A gene in which part of the sequence is found within part of the sequence of another gene
How can 2 proteins be generated from a single gene?
Starting or terminating transcription at different points
What can complex eukaryotic transcription unit produce?
A primary transcript that can be processed in alternative ways
What happens when primary transcript has 2 poly (A) sites?
Processed into MRNAS with alternative 3’ exons
What is differential (alternative) splicing?
When certain exons are included or excluded
What does alternative splicing use?
Same pre-mRNA to generate mRNAs that have different combinations of exons
What is alternative splice sites?
mRNA has same 5’ and 3’ exons but different internal exons
What does alternative splicing produce?
Proteins with altered function
On average, how many expressed alternative transcripts per protein coding gene are there?
10-12
are alternative transcripts expressed at the same level?
No
What percentage of protein coding genes have at least 2 dominant/major alternative transcripts ?
75%
What are examples of Alternative Splicing?
Striated muscle mRNA Smooth muscle mRNA Fibroblast mRNA Brain mRNA
What can proteins consist of?
Independent functional modules, the boundaries which can be equated with those of exons
