Lecture 27 Flashcards
What are the code for stop codons
UAG
UAA
UGA
What happens when a stop codon on the mRNA is encountered in the A site
Translation is terminated
How does translation termination work
Encounter stop codon
(No tRNA with an anticodon complementary to stop codons)
Instead stop codons in empty A site are recognized by release factor proteins
Release factor proteins enter the site , bind to stop codon, and stimulate releases of the polypeptide chain
After release, release factor protein and ribosomal subunits separate from the mRNA
Polysome/ polyribosome
An mRNA and associated ribosomes are referred to as polysome or polyribosome
Speeding up the process (translation)
Once a ribosome has moved a certain distance along an mRNA, another initiation complex can form at the start codon
Therefore. Several ribosomes migrate along an mRNA at the same time speeding up the synthesis of the required polypeptide
Myoglobin
One polypeptide chain = one gene expressed
Some polypeptides form a functional protein by themselves: here, just one gene is expressed to produce the functional protein
Hemoglobin
2 (alpha) - globins & 2 (Beta)-globins
= 2 genes expressed
Some polypeptides are joined together with other different polypeptides in order to form the functional protein. In this case 2 or more different genes must be expressed
Protein synthesis: produced from polypeptide chains
- it’s the functional protein that produces the phenotypic expression of the gene or genes
- the individual polypeptides in hemoglobin are not proteins
- most polypeptides are folded or modified in some way after they leave the ribosome
- this process is called ‘post-translational modification’ of proteins
Post- translational modification of proteins
Most polypeptides are folded or modified in some way after they leave the ribosome
What does it mean by saying every cell in a multicellular organism is genetically identical
They all contain the same genome
How do you get the differentiation of many different cell types
Each cell has the same genome
So to get a different cell type
In each cell different sets of genes are turned on and off resulting in differentiation of many different cell types
How do u get different mature mRNAs
The same pre-mRNA transcribed from a single gene can be spliced differently in different cells of the body, producing different mature mRNAs
Each of the different mature mRNAs derived from the same gene may code for different polypeptides
Cell specific expression of genes allows for the differentiation of different cell types in multicellular organisms
Definition of a gene used in early inheritance studies
Early: Gene is a unit of heredity
One gene on enzyme hypothesis
Each gene coded for a particular enzyme
One gene one protein hypothesis
Genes can code not only for enzymes but also for other types of proteins
One gene one polypeptide hypothesis
Some genes code for just one of the several different polypeptides that make up a functional protein
Current gene definition
We know now that genes also encode for functional RNAs like tRNA or rRNA in addition to mRNA
A gene is now defined as
A unit of heredity on a chromosome that contains either:
-the code for a protein molecule or one of its parts
OR
-the code for functioning RNA molecules such as tRNA or rRNA
The definition of a gene timeline
-one gene one enzyme hypothesis
- one gene one protein hypothesis
- one gene one polypeptide hypothesis
-Current
Mutation
Is a permanent heritable change in the genetic material
Mutations are the only source of new alleles
One of the several factors providing genetic variation in populations upon which natural selection can act
What is the only source of new alleles
Mutation
Types of mutations
Large and small scale
Type of small scale mutation
Point mutations
Point mutations
Small scale mutation
Only affect one base pair of DNA
Types of point mutations
Base substitution mutation. Where an incorrect base is substituted that then results in a change in the base sequence of both dna strands
OR
Insertion or deletion of a single base pair
Missense mutation
Point mutations
Change in DNA sequence causes a change in a sense codon producing a different amino acid
This generates a different polypeptide that may alter function
Sickle cell disease is caused by a missense mutation producing a change from glutamic acid to Valine in polypeptides of hemoglobin
Nonsense mutation
Point mutation
Change in DNA sequence causes a change in a NONsense (stop) codon. Producing a shorter polypeptide chain
Very unlikely this shortened polypeptide with be functional
About 10% of cystic fibrosis cases are caused by a nonsense mutation. Where CFTR protein isn’t fully manufactured
Silent mutation
Point mutation
Change in DNA sequence causes a change of a codon to a different sense codon, but the new codon specifies the same amino acid as in the normal polypeptide
This means function of the polypeptide will be unchanged
Frame shift mutation
Point mutation
Deletion or insertion in DNA sequence causes a change in the reading frame of the resulting mRNA
after mutation point the codon code will almost assuredly produce a different amino acid sequence
This will typically result in a non functional polypeptide
Example of deletion:
THE DOG SAW THE CAT RUN
if u delete the E it is unreadable
THD OGS AWT HEC ATR UN
EXAMPLE OF INSERTION: same sentence
TRH EDO GSA WTH ECA TRU N
Cause of crohns disease
Frameshift mutation
NOD2 gene encodes for a protein important for immune function.
Frameshift mutation NOD2 3020insC encodes a truncated protein which makes those with this mutation less able to respond properly to gut bacteria
