gene expression what is a gene? mendel –unit of inheritance conferring a phenotype modern...
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GENE EXPRESSION
What is a gene?• Mendel
– Unit of inheritance conferring a phenotype
• Modern definition– Unit of DNA directing
the synthesis of a polypeptide or functional RNA molecule
GENE EXPRESSION
How is a gene expressed?
DNA RNA protein
• DNA RNA– Transcription
• RNA protein– Translation
TRANSCRIPTION
• Synthesis of RNA from a DNA template
• Occurs in the nucleus of a eukaryotic cell
• Similar to DNA synthesis
• Some differences
TRANSCRIPTION
• How is it different from DNA synthesis?• Start at promoter • Unidirectional
– Single strand transcribed
• Continuous (not discontinuous)• Catalyzed by RNA polymerase• Termination sequence• No mismatch repair or ligation• Thymine is replaced with Uracil
RNA PROCESSING
• RNA transcripts must be modified prior to transport from the nucleus to the cytoplasm
RNA PROCESSING
• Splicing– Removal of introns (junk)
• Capping– Backwards G-residue
at 5’ end
• Polyadenylation– Addition of
A-residues to 3’ end
TRANSLATION
• Synthesis of a protein from an mRNA template
• Occurs in the cytoplasm of eukaryotic cells
• Requires ribosomes, tRNAs, and various other factors
• Utilizes genetic code
TRANSLATION
Messenger RNA• mRNA• Temporary copy of gene• Information deciphered
using the genetic code• Functional unit is codon
– Three consecutive nucleotides
– “Triplet”
TRANSLATION
mRNA codons
• 3 consecutive nucleotides– 64 different codons
(43)• 61 encode specific
amino acids
• 61 > 20; some duplication
TRANSLATION
mRNA codons• 3 consecutive nucleotides
– One of these (AUG) is a start codon
– 3 are stop codons (UAG, UGA, UAA)
• Terminate translation• Recognized by tRNAs
TRANSLATION
Transfer RNA (tRNA)• Short segments of RNA
– ~80 nucleotides long• Two important regions
– Anticodon• Nucleotide triplet• Complementary to codon of
mRNA– Amino acid attachment site
• Amino acid corresponding to codon is covalently attached to the tRNA
TRANSLATION
Ribosomes
• Cellular organelles
• Workhorse
• Site of translation
• Facilitate interaction between mRNA and tRNA
• Facilitate polymerization of amino acids
MUTATION
Mutation
• Heritable– Change in DNA sequence
• Relatively rare
• Typically occurs during DNA replication or repair
• May affect gene expression
• Spontaneous or mutagen
MUTATION
Mutation• Different effects
– Harmful– Neutral– Beneficial
• Most are neutral or harmful• A minority are beneficial• Ultimate source of genetic
variation– Natural selection acts upon
variation
MUTATION
Single gene mutations• How do mutations
affect gene expression?
• A change in DNA can give rise to a change in RNA codons, which can encode different amino acids
MUTATION
Base substitutions mutations
• Affect only a single base
• “Point mutation”– Affects a single base pair
• One base replaced by another, does not change “downstream” triplet
MUTATION
Base Substitution mutations
• Can cause change in amino acids
• e.g., GAA GTA (glu val)
• Phenotype may be affected
MUTATION
Frameshift mutations
• Insertion or deletion of base(s)– e.g., GGA GAGA (gly glu)
• Alter reading frame– All downstream amino acids altered
• Protein function affected– Effect similar to nonsense mutation
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