BIOL 2030 Lecture Notes - Lecture 14: Start Codon, Intron, Operon

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Gene Organization
February 22-24, 2016
Colinearity between genes and protein:
Continuous sequence of nucleotides encodes continuous sequence of amino acids
Number of nucleotides in the gene is proportional to the number of amino acids on the
protein
The discovery of split genes:
mRNA is shorter than its DNA template
mRNA is not encoded as an equal collinear segment of DNA
mRNA is derived from segments of DNA called Exons (separated by noncoding introns)
introns are transcribed and therefore, introns must be spliced from pre-mRNA to produce
mRNA
Gene includes DNA sequence that codes for all exons, introns and those sequences at the
beginning/end of the RNA that are not translated into a protein
Structure of mRNA:
5’ untranslated region [start codon] protein coding region [stop codon] 3’
untranslated region
Shine-Dalgarno sequence in prokaryotes only
Prokaryotic protein-coding genes: usually found in a contiguous array in the DNA called
an Operon. Operon operates as a unit utilizing a single transcription start site for multiple
genes.
Eukaryotic protein-coding genes: each gene is transcribed from its own start site to yield
a pre-mRNA that is processed into a functional mRNA encoding a single protein.
In prokaryotic DNA genes contain little to no noncoding gaps (introns) and the DNA is
transcribed directly into colinear mRNA, which then is translated into protein while the
mRNA is still being produced.
In eukaryotic cells, transcription and translation occur in different parts of the cell.
Messenger RNAs (mRNAs), which encode the amino acid sequences of proteins, are modified
after transcription.
Modifications:
1) Capping of the 5’ end
2) Polyadenylation of the 3’ end
3) Splicing (removal) of introns
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Capping a methylated guanine nucleotide is attached to the 5’ end of pre-mRNA
1 st step phosphate group removed from 5’ end of pre-mRNA.
2 nd step GMP is added.
3 rd step - Methyl groups (CH3 ) are added to the guanine base of GMP and the 2’ sugar position
of the first 2 RNA nucleotides. Base of the 1st RNA nucleotide may also be methylated.
The guanine nucleotide is joined to the pre-mRNA by an unusual 5’-5’ linkage involving
3 phosphate groups
Modification is necessary for efficient translation, transport of mRNA from nucleus and
protects mRNA from degradation.
Polyadenylation
~50 to 150 adenine (A) nucleotide are added to the 3’ of the pre-mRNA
Splicing removal of introns from pre-mRNA
requires three consensus requences in the pre-mRNA: 5’ splice site, 3’ splice site and
branch point
consensus sequences are used by the spliceosome to recognize and remove introns
introns are removed in the form of a lariat and exons are spliced together by two
successive reactions
the phosphodiester bond is broken and a new one is formed between a nucleotide of the
5’ splice site (G) and the branch point (A)
Splicing occurs on a spliceosome
a ribonucleoprotein complex [300 proteins and 5 small snRNAs]
contains 5 snRNPs
snRNA + protein snRNP
U1, U2, U4, U5 and U6
snRNP are central to the activity of the spliceosome
Spliceosome Assembly:
U1 binds to 5’ splice site
Specific binding also assists
U2 binds to branch point site
A complex of U4, U5 and U6 bind
*See assembly diagram
RNA Polymerase II
functional coupling of mRNA transcription and mRNA processing by RNA polymerase
CAP is hen added as soon as 5’ of pre-mRNA emerged from polymerase
Capping enzymes are recruited to the C-terminal domain (CTD) of RNA polymerase
Termination of RNA Polymerase:
Polyadenylation factors are recruited to CTD of Pol II
RNA is cleaved at 3’ site
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