Chapter 17: Protein Synthesis Flashcards
“One Gene, One Enzyme”
Archibald Garrod in 1909 suggests that genes dictate phenotypes through enzymes
1940: Beadle & Tatums - grew strains of mold that were unable to grow in a certain medium. They lacked enzymes
Central Dogma
Flow of genetic information in cell
(replication)DNA-transcription-> RNA-translation->protein–>trait
What has metabolisms taught us about genes
inheritance of metabolic diseases
- suggested that genes coded for enzymes
- each disease (phenotype) is caused by non-functional gene product
Transcription
DNA nucleic acid language to RNA nucleic acid language
RNA
ribose sugar
N-bases
- U:A
- G:C
single stranded
lots of different types of RNA
-tRNA, mRNA, rRNA
Transcription
Making mRNA, the synthesis of complementary RNA
Template Strand
transcribed DNA strand
Coding Strand
untranscribed DNA strand(same squence as RNA except the T is now a U in RNA)
What is the enzyme that creates the mRNA
RNA polymerase
How many RNA polymerase enzymes are there?
3, each with their own specific promoter sequence it recognizes
RNA polymerase I
only transcribes rRNA genes
makes ribosomes
RNA polymerase II
transcribe genes into mRNA
RNA polymerase III
only transcribes tRNA genes
Are eukaryotic genes continuous?
no
Exons
the real gene
-expressed/coding DNA
Introns
the junk
-in between sequence
mRNA splicing
eukaryotic mRNA needs work after transcription. Edit out introns to make a mature mRNA transcript
Primary Transcript
pre- mRNA
What happens if there is a mistake in splicing?
a single base added or lost throws off the reading frame, which can then create a different protein than wanted
snRNPs
small nuclear RNA, proteins
Spliceosome
several snRNPs, recognize splice site sequence
- cut & paste gene
Alternative Splicing
Alternative mRNAs produced from same gene
-when is an intron not an intron
-different segments treated as exons
What’s the purpose of exons & introns?
One gene van code for different proteins
-depending on which sections of the mRNA template are considered exons during processing.
-explains why humans have a low number of genes for such complex organisms
How is the mRNA protected on its trip from nucleus to cytoplasm?
Enzymes in the cytoplasm attack mRNA
-protect the ends of the molecule by adding the 5’GTP cap and poly-A tail
-the longer the tail, mRNA lasts longer:produce more proteins
Translation
nucleic acid language to amino acid language
How does mRNA codes for proteins?
It codes for proteins in triplets
Codon System
3 letter codon system
Nirenberg & Khorana
determined mRNA-amino acid match
added fabricated mRNA to test tube of ribsomes, tRNA & amino acids
-created artifical UUUU mRNA
-found that UUU coded for phenylalanie
The Chart is filled with the code…
Code for all life
- strongest support for a common origin for all life
Code is redundant
-several condons for each amino acid
-3rd base wobble
Start Codon
AUG, methionine
Stop codons
UGA, UAA, UAG
What type of codon does tRNA have?
Anti-codon
Where is the genetic code found?
The genetic code is nearly universal and found in all organisms
Where does the universality of the genetic code split between organisms?
In the translation system
What does the near universality of the genetic code say about the history of life?
That it has been operating very early in the history of life.
“Clover Leaf” structure
-anticodon on “clover leaf” end
-amino acid attached on 3’ end
Aminoacyl tRNA synthetase
enzyme which bonds amino acid to tRNA
-bond requires energy
-ATP–>AMP
- bond is unstable
- so it can release amino acid at ribosome easily
How are ribosomes involved in the DNA replication process?
It facilitate coupling of tRNA anticodon to mRNA codon
Structure
- ribosomal RNA (rRNA) & proteins
- 2 subunits
- large
- small
How many sites are in a ribosomes ?
3
A site
P site
E site
A site (aminoacyl-tRNA site)
holds tRNA carrying next amino acid to be added to chain
P site (peptidyl-tRNA site)
holds tRNA carrying growing polypeptide chain
E site (exit site)
empty tRNA leaves from exit site
What are the steps to building a polypeptide?
Initiation
- brings together mRNA, ribosome sub-units, initiator tRNA
Elongation
-adding amino acids based on condon sequence
Termination
- end codon
Protein Synthesis in Prokaryotes
DNA in cytoplasm
circular chromosome
naked DNA
no introns
Protein Synthesis in Eukaryote Genes
DNA in nucleus
linear chromosomes
DNA wound on histone proteins
introns vs. exons
Translation in Prokaryotes
Transcription & translation are simultaneous in bacteria
-DNA is in cytoplasm
-no mRNA editing
-ribosomes read mRNA as it is being transcribed
Difference b/tw prokaryotes & eukaryotes
time & physical separation b/tw processes
- takes eukaryote ~ 1 hour from DNA to protein
- no RNA processing in prokaryotes
Mutations
Changes in the genetic material of a cell or virus
Point mutations
chemical changes in just one base pair of a gene
the change of a single nucleotide in a DNA temple strand can lead to the production of an abnormal protein
