Chaper 17: From gene to protein Flashcards
DNA stores genetic information in its sequences. But how do cells use the information?
- DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins.
- Proteins are the links between genotype and phenotype.
What is gene expression?
The process by which DNA directs protein synthesis, includes two stages: transcription and translation.
The central dogma
is the concept that cells are governed by a cellular chain of command: DNA-RNA-Protein
What are the steps required to produce proteins from genes?
RNA -Transcription -Primary transcript -mRNA Translation -Ribosomes
What is Transcription?
- The synthesis of RNA under the directions of DNA.
ex. writing a script for RNA from DNA.
What is a primary transcript?
The initial RNA transcript from any gene.
What does mRNA do?
Carries a genetic message from DNA to the protein-synthesizing machinery of the cell.
-produced by protein-coding genes
What is Translation?
synthesis of a polypeptide, which occurs under the direction of mRNA
How are Ribosomes in relation to Translation?
- They are the sites of translation.
- They are complex particles that facilitate the orderly linking of amino acids into polypeptide chains.
How are the instructions of assembling amino acids into proteins encoded into DNA?
There are 20 amino acids, but only four nucelotide bases in DNA.
How many bases on DNA correspond to an amino acid?
64
How does each codon specify an amino acid?
By triplet code.
What is a codon?
- A triplet
- a series of nonoverlapping, three nucleotide words.
ex) AGT on a DNA strand codes for the amino acid serine.
Template strand
The DNA strands that provides a template for making an RNA transcript.
How are codons read?
From 5’ to 3’
Are amino acids placed at the corresponding position along a polypeptide?
Each codon specifies the amino acid to be placed at the corresponding position along a polypeptide.
Molecular components of transcription
- RNA synthesis is catalyzed by RNA polymerase.
- Follows the same base-pairing rules as DNA except Uracil sub. for Thymine.
Where does transcription began?
The promoter
Where does it end?
The terminator
only in bacteria
Transcription Unit
The stretch of DNA that is transcribed
What is the process of RNA transcription?
- Initiation
- Elongation
- Termination
Initiation
- after RNA polymrase binds to the promoter, the DNA strands unwind.
- polymerase starts RNA synthesis at the strart point on the template strand
Elongation
- Polymerase moves downstream (reading from 5’-3’) unwinding DNA and elongating RNA transcript.
- In the wake of transcription DNA strands reform a double helix
Termination (prokaryotic)
- RNA transcript is released
- polyermase detaches from the DNA
Transcription factors
In eukaryotes, a collection of proteins mediate the binding of RNA polymerase and the initiation of transcription.
Transcription Initiation complex
A promoter where transcription factors and polymerase II bind together
TATA box
is a DNA sequence that indicates where a genetic sequence can be read and decoded
What is the eukaryotic promoter?
- Includes the TATA box
- nucleotide sequence containing TATA
- 25 nucleotides upstreaming from transcriptional start point.
- nucleotide sequences are given as they occur on the non template strand.
Several transcription factors
One recognizing the TATA box, must bind to the DNA before RNA polymerase II can bind in the correct position and orientation.
Additional transcription factors
- bind to the DNA along w/ RNA polymerase II, forming the transcription initiation complex.
- RNA polymerase II unwinds the DNA double helix, and RNA synthesis begins at the start point on the template strand.
Initiation of Transcription
- promoters
- transcription factors
- transcription initiation complex
Elongation of the RNA strand
- RNA polymerase moves along DNA, untwist double helix, 10 to 20 bases at a time.
- Transcription progresses at a rate of 40 nucleotides per second in eukaryotes.
- A gene can be transcribed many times by several RNA polymerases.
Termination of Transcription
- bacteria, polymerases stops transcription at the end of the terminator.
- eukaryotes, polymerase continues transcription after the pre-mRNA is cleaved from growing RNA chain; the polymerase eventually falls off the DNA.
RNA processing
- enzymes in eukaryotic nucleus modify pre-mRNAs before sent to cytoplasm as mRNAs
- 5’ and 3’ ends are usually altered.
- some interior parts of the molecule are cut out and other parts are sliced together.
Alteration of mRNA ends
- the 5’ end receives a modified nucleotide 5’ cap.
- the 3’ end gets a poly-A tail
Why are 5’ end and poly-A tail important?
- Facilitate the export of mRNA
- protect mRNA from degradation
- help ribosomes attach to the 5’ end for translation.
5’ end
a modified guanine nucleotide added to the 5’ end
poly-A tail
50-250 adenine nucleotides added to the 3’ end
Introns
- noncoding regions of a gene
- some contain sequences that may regulate gene expression.
exons
coding regions of a gene that will be translated into amino acid sequences.
RNA splicing
removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence.
- some genes can encode more than one kind of polypeptide, depending on which segments are treated as exons during splicing.
Transfer RNA (tRNA)
- the message is a series of codons along an mRNA molecule, and translator is (tRNA)
- carries a specific amino acid on one end.
- has an anticodon on the other end.
- anticodon base pairs w/ complementary codon on mRNA
Structure of tRNA
- consist of a single RNA strand that only has about 80 nucelotides long.
- b/c of hydrogen bonds, tRNA twists and folds into a three dimensional L-shaped molecule.
- when flattened to reveal base pairing, tRNA looks like a cloverleaf.
What are the two steps that require accurate translation?
- correct match between a tRNA and a amino acid, done by enzyme aminoacyl-tRNA synthetase.
- correct match between tRNA anticodon and mRNA codon.
wobble
flexible pairing at the third base of a codon and allows some tRNAs to bind to more than one codon.
tRNA systheization process
- start w/ enzyme, add amino acid
- add ATP, breakdown leaves one phosphate.
- tRNA comes from environment and joins enzyme, then tRNA and amino acid joins.
- enzyme can fall off. complete complex.
Riboaomes
- have specific coupling tRNA anticodons with mRNA codons in protein synthesis.
- made of protein and rRNA
- eukaryotic and prokaryotic have similar ribosome make up, some bacterial antibiotic target specific bacterial ribosomes w/o harming eukaryotic ribosomes.
What are the three binding sites of tRNA?
- P site
- A site
- E site
P site
holds tRNA that carries the growing polypeptide chain.
A site
holds tRNA that carries the next amino acid to be added to the chain.
E site
the exit site, where discharged tRNAs leave ribosome
What are the 3 stages of translation?
- Initiation
- Elongation
- Termination
- all three stages require protein “factors” that aid in the translation process.
Initiation (translation)
- brings together mRNA and tRNA w/ the first amino acid and two ribosomal subunits.
1. small ribosomal subunit binds w/ mRNA and a special initiator tRNA
2. small subunit moves along mRNA until it reaches the start codon (AUG)
3. initiation factors bring in the large subunit that completes the translation initiation complex.
Elongation (translation)
- amino acids are added 1 by 1 to the preceding amino acid.
- each addition involves proteins called elongation factors.
- codon recognition
- peptide bond formation
- translocation
Termination of Translation
- occurs when a stop codon in the mRNA reaches A site of ribosome.
- A site accepts a protein called release factor.
- release factor causes addition of water molecule instead of amino acid.
- that reaction releases polypeptide and the translation assembly then comes apart.
