Ch.9 Gene Expression: The Flow of Information from DNA to RNA to Protein

Central Dogma

DNA →RNA → Protein

Transcription (DNA to RNA)

Translation (RNA to Protein)

Major Chemical Difference btwn RNA + DNA (3)

1. Ribose instead of deoxyribose

2. Nitrogenous base uracil instead of thymine (pairs with A)

3. Most RNA single stranded

Amplification of Gene

(Transcription) single gene is transcribed into many RNA molecules → (Translation) Each RNA molecule is then translated into many protein molecules

Codon

group of 3 nucleotides

• EX: 3 nucleotide code = 4³ = 64 combinations

  
  

Colinear

The sequence of nucleotides in a gene directly matches the sequence of amino acids in the protein

Non-overlapping

Each nucleotide is part of only one codon—the reading frame moves in steps of three without overlapping

Reading frame

Translation starts at a specific "START" codon and continues in triplet groups, establishing a strict reading frame until it hits a "STOP" codon

Crick + Brenner

Added or deleted 1/2 bases = frameshift + scrambled protein sequence

Added or deleted 3 bases = normal protein

Key Concepts of Genetic Code (6)

1. Triplets + Nonoverlapping

2. Stop codon: AUG

3. Start codon: UAA, UAG, UGA

4. Degenerate (yet ambiguous)

5. Mutations: frameshift, missense, nonsense

6. Almost universal

Transcription

One of the two strands of the DNA double helix serves as a template for a single-stranded RNA transcript

• RNA strand formed by complementary base pairing

Promoter

(Transcription in prokaryotes)

DNA sequences that act as a "start" signal, telling RNA polymerase where to bind and begin transcription

• RNA polymerase builds 5’-3’

Terminators

RNA sequences that provide the signal to RNA polymerase for stopping transcription

• Form hairpin loops (intramolecular H-bond)

Transcription in prokaryotes: Initiation

• Region of DNA is unwound to form open promoter complex

• Phosphodiester bonds formed between first two nucleotides

Transcription in prokaryotes: Elongation (RNA copy)

Core RNA polymerase loses affinity for promoter, moves in 3’-to-5’ direction on template strand

• Within transcription bubble, NTPs added to 3’end of nascent mRNA

Eukaryotic mRNA transcription

transcript must be processed (adding a 5' cap, a 3' poly-A tail, and splicing out introns) before becoming mature mRNA

• prokaryotic happens immediately

5' methylated cap

adds a “backward” G to the 1st nucleotide of a primary transcript (not from the DNA template)

Poly-A polymerase.

adds the string of adenine bases to the 3' end of eukaryotic mRNA, creating the poly-A tail

• the longer the message; longer the tail

Exons

expressed regions; sequences found in a gene’s DNA and mature mRNA

Introns

intervening regions; sequences found in DNA but not in mRNA

• Some eukaryotic genes have many introns

Untranslated Regions (UTRs)

regions of mRNA located at the 5' and 3' ends (outside of the protein-coding region) that are not translated into amino acids

Alternative splicing

process where a single primary RNA transcript is spliced in different ways to produce multiple distinct mRNAs and proteins from the same gene

Charged tRNA

molecule that is covalently coupled to its specific amino acid, ready to deliver it to the ribosome during translation

• 74-95 nt, anticodon (comp to mRNA codon)

Wobble Position

Some tRNAs recognizemore than one codon

• genetic code is degenerate (5’ anticodon pairs with various 3’ codon)

Translation: Initiation (Stage 1)

small ribosomal subunit binds to the 5' cap of the mRNA and migrates to the first AUG start codon. An initiator tRNA carrying Methionine (Met) then establishes the starting point for protein synthesis

Translation: Elongation (Stage 2)

Charged tRNAs are ushered into the ribosome's A site, new amino acids are added to the C-terminus of the polypeptide, and the ribosome moves down the mRNA in the 5'-to-3' direction

Translation: Termination (Stage 3)

When the ribosome reaches a stop codon, release factors bind to it. This triggers the release of the ribosomal subunits, mRNA, and completed polypeptide chain

Polyribosome

structure where several ribosomes translate the same mRNA simultaneously

• allows cell to rapidly produce many copies of a polypeptide from a single mRNA transcript

Mutations in coding sequences (4)

1. Silent: do not alter amino acid seq

2. Missense: replace 1 amino acid with other

3. Nonsense: changes codon into stop codon (to early)

4. Frameshift: insertion/deletion of nucleotides

Null mutations

completely block function of a gene product (e.g. deletion of an entire gene)

• recessive to wild-types

Hypomorphic mutations

gene product has weak, but detectable, activity

• recessive to wild types

Hypermorphic mutations

generate more gene product or the same amount of a more efficient gene product

• dominant to wild-types

• EX: Achondroplasia (D allele of FGFR3)

Neomorphic mutations

generate gene product with new function or that is expressed at inappropriate time or place

• dominant to wild types

• EX: Antennapedia (Drosophila) causes ectopic expression = legs where antenna should be