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GENE EXPRESSION
Gene Expression
Our phenotype is the result of the expression of proteins
Different alleles encode for slightly different proteins
Protein variation is the basis for normal phenotypic variation - blue or brown eyes
It is also the basis for abnormal phenotypes- cystic fibrosis
In the late 1930s, early 40s, Beadle and Tatum
- working with a mold – Neurospora
- observed that a mutation in a single gene caused the loss of a single enzyme, and that this resulted in a mutant phenotype
This established that genes produce phenotypes through the action of proteins
Awarded the Nobel Prize in Medicine in 1958
Cytoplasm
Nucleus
DNA
Central Dogma
RNA
Protein
Replication
Transcription
Translation
Gene Expression
Together transcription and translation are called gene expression.
The genetic information encoded in the DNA of an embryo includes all of the genes needed to develop and maintain the organism.
Different cell types express different subsets of genes.
Transcription
DNA is used as a template for creation of RNA using the enzyme RNA polymerase.
DNA
5’
3’
5’
3’
G T C A T T C G G
C A G T A A G C C
Transcription
RNA polymerase reads the nucleotides on the template strand from 3’ to 5’ and creates an RNA molecule that looks like the coding strand.
DNA template strand
DNA
5’
3’
5’
3’
G T C A T T C G G
C A G T A A G C C
DNA coding strand
Transcription
The new RNA molecule is formed by incorporating nucleotides that are complementary to the template strand.
DNA coding strand
DNA template strand
DNA
5’
3’
5’
3’
G T C A T T C G G
C A G T A A G C C
G
RNA
5’
GG U C A U U C3’
Two types of nucleic acids
RNA
Usually single-stranded
Has uracil as a base
Ribose as the sugar
Carries protein-encoding information
Can be catalytic
DNA
Usually double-stranded
Has thymine as a base
Deoxyribose as the sugar
Carries RNA-encoding information
Not catalytic
# of strands
kind of sugar
bases used
Types of RNA
Abbrev. Function
mRNA Messenger RNA - encodes protein
rRNA Ribosomal RNA - part of ribosome
- used to translate mRNA into protein
tRNA Transfer RNA - couples the region which binds the mRNAcodon and its amino acid
rRNA is part of ribosome, used to translate mRNA into protein
tRNA is a connection between anticodon and amino acid
Transcription
Occurs in three steps:
Initiation
Elongation
Termination
RNA processing
mRNA transcripts are modified before use as a template for translation:
- Addition of capping nucleotide at the 5’ end - Addition of polyA tail to 3’ end
Important for moving transcript out of nucleus And for regulating when translation occurs
Splicing - the removing internal sequences - introns are sequences removed - exons are sequences remaining
RNA processing
• The process of reading the RNA sequence of an mRNA and creating the amino acid sequence of a protein is called translation.
Transcription
Codon Codon Codon
Translation
DNA
T T C A G T C A G
DNAtemplatestrand
mRNA
A A G U C A G U C MessengerRNA
Protein Lysine Serine ValinePolypeptide(amino acidsequence)
Translation
The genetic code
There is a 3 to 1 correspondence between RNA nucleotides and amino acids.
The three nucleotides used to encode one amino acid is called a codon.
The genetic code refers to the codons that encode each amino acids.
What is the correspondence between the mRNA nucleotides and the amino acids of the protein?
Proteins are formed from 20 amino acids in humans.
Codons of one nucleotide:AGCU
Can only encode 4 amino acids
Codons of two nucleotides:AA GA CA UAAG GG CG UGAC GC CC UCAU GU CU UU
Can only encode16 amino acids
Codons of three nucleotides:
AAA AGA ACA AUA AAG AGG ACG AUGAAC AGC ACC AUC AAU AGU ACU AUUGAA GGA GCA GUA GAG GGG GCG GUGGAC GGC GCC GUC GAU GGU GCU GUUCAA CGA CCA CUA CAG CGG CCG CUGCAC CGC CCC CUC CAU CGU CCU CUUUAA UGA UCA UUA UAG UGG UCG UUGUAC UGC UCC UUC UAU UGU UCU UUU
Allows for 64 potential codons => sufficient!
A codon of three nucleotides determines choice of amino acid
The genetic code is non-overlapping
- All known organisms use the same genetic code. (Rare organisms use one codon for an additional amino acid.)
The genetic code is universal
The genetic code is degenerate
Some codons encode the same amino acid. e.g. GGU, GGC, GGA, and GGG all encode glycineDegeneracy is mostly at the third base of the codon.
Some codons have additional functions
AUG encodes methionine. Methionine can be used within a protein sequence and
is often the first amino acid cueing the beginning of translation.
UAA, UAG, and UGA do not encode an amino acid.These codons signal termination of the protein.