and Genes How They Work
Chapter 15 Part 1
Outline
Cells Use RNA to Make Protein Gene Expression Genetic Code Transcription Translation Spliced Genes
Introns and Exons
Prokaryotic and Eukaryotic Gene Expression
Central Dogma of Gene Expression
Ribosomes Ribosomes are a complex of proteins and RNA where the proteins are made. Composed of 2 subunits Small subunit
30S (prokaryotes) 40S (eukaryotes)
Large subunit
40S (prokaryotes)/ 60S (eukaryotes)
Synthesis is carried out in 3 sites (E,P,A)
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Large subunit
Large ribosomal subunit E site P site A site
Small subunit
E
P
A mRNA Binding site
Ribosomes
Small ribosomal subunit
3 types of RNA molecules
Ribosomal RNA (rRNA) is found in the site of polypeptide assembly. Transfer RNA (tRNA) transports and positions amino acids to the ribosome. Messenger RNA (mRNA) directs which amino acids are assembled into polypeptides. Central Dogma
DNA RNA Protein
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OH 39 59
Amino acid attaches here
Transfer RNA
Anticodon
Gene Expression
Transcription
DNA sequence is transcribed into RNA sequence
initiated when RNA polymerase binds to promoter binding site
moves along DNA strand and adds corresponding complementary RNA nucleotide disengages at stop signal
Gene Expression
Translation
nucleotide sequence of mRNA transcript is translated into amino acid sequence in the polypeptide
rRNA recognizes and binds to start sequence
moves three nucleotides at a time disengages at stop signal
Gene expression - collective of transcription and translation
Genetic Code
Genetic code consists of a series of information blocks called codons.
DNA reading frame (triplet) each codes for one amino acid Example: AGA codons codes for arginine in bacteria and humans.
genetic code is nearly universal 4 nucleotides 43 = 64 different combinations of threes that will for 20 amino acids
AUG (start codon) codes for methionine Stop codons UAA UAG UGA
Example
DNA triplet AAA RNA codon UUU Amino acid Phenylalanine Redundancy of the code DNA triplet AAT RNA codon UUA Amino acid Phenylalanine
Effects of base pair substitution in DNA
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Frameshift alteration
Delete 1 base WHYDIDTHEREDBATEATTHEFATRAT? Hypothesis A : Delete T WHY DID HER EDB ATE ATT HEF ATR AT? unpunctuated (Nonsense) WHYODIDOTHEOREDOBATOEATOTHEOFATORAT? Delete T O O R B E T F R WHY DID HEO EDO ATO ATO HEO ATO AT? (Nonsense)
Hypothesis B : punctuated
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Delete 1 base WHYDIDTHEREDBATEATTHEFATRAT? Hypothesis A : Delete T WHY DID HER EDB ATE ATT HEF ATR AT? unpunctuated WHYODIDOTHEOREDOBATOEATOTHEOFATORAT? (Nonsense) Delete T O O R B E T F R WHY DID HEO EDO ATO ATO HEO ATO AT? (Nonsense)
Hypothesis B : punctuated
After deleting one single nucleotide, the reading frame was shifted, therefore the genes that were transcribed downstream the deletion resulted in non sense.
Transcription
RNA polymerase
only one of two DNA strands (template or antisense strand) is transcribed non-transcribed strand is termed coding strand or sense strand In both bacteria and eukaryotes, the polymerase adds ribonucleotides to the growing 3' end of an RNA chain.
synthesis proceeds in 5' 3' direction
Transcription
Promoter
Transcription starts at RNA polymerase binding sites called promoters on DNA template strand. Promoters (short sequence that serves as attachment for RNA polymerase and is not transcribed) Example TTGACA and TATAAT both are located upstream the transcription starting point.
Initiation
Other eukaryotic factors bind, assembling a transcription complex.
RNA polymerase begins to unwind DNA helix.
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subunit Coding strand 5' 3' Promoter RNA polymerase Bacterial DNA
Template strand TTGACA TATAAT 35 sequence 10 sequence
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subunit
RNA polymerase
Unwinding of ds DNA by alfa subunit to expose the template strand so transcription can start.
Transcription Bubble
Transcription
Elongation
Transcription bubble moves down DNA at constant rate leaving growing RNA strands protruding from the bubble.
Termination
Stop sequences at the end of the gene cause phosphodiester bond formation to cease, transcription bubble to dissociate, and RNA polymerase to release DNA.
Transcription
Eukaryotic transcription differs from prokaryotic transcription:
three RNA polymerase enzymes initiation complex forms at promoter with RNA polymerase II RNAs are modified after transcription (post transcriptional modification)
Addition of 5' Cap and 3' Poly-A- tail
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Eukaryotic initiation complex.
Transcription factor TATA box Eukaryotic DNA
TATAAA 1. A transcription factor recognizes and binds to the TATA box sequence which is part of the core promoter.
2. Other transcription factors are then recruited, and the initiation complex begins to build. RNA polymerase II mRNA 3. Ultimately, RNA polymerase II associates with the transcription factors and the DNA, forming the initiation complex, and transcription begins.
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Posttranslational modifications
5' cap HO OH P P P
CH2 N+ CH3
Methyl group P GP CH3 P
mRNA
ail 3' -A t ly ' po A 3 A A A A
5'