Ch. 11 Gene Expression: Translation and Genetic Code Flashcards
Sickle cell anemia
First inherited human disease to be studies on a molecular level.
Discovery in 1957: normal red blood cells: Hemoglobin contains 2 alpha and beta globin chains (and the iron containing heme group). The 6th amino acid in the beta chain in a healthy hemoglobin is glutamic acid. In sickle cell disease, this is replaced by valine.
Difference between healthy and sickle shaped blood cell is based on?
One single amino acid
Proteins Structure
Complex macromolecules composed of?
20 different amino acids
Proteins are made of
Polypeptides
Polypeptide
a long chain of amino acids
Amino acids
Have a free amino group. a free carboxyl group, and a side group (R).
There are 20 different side groups, hence there are 20 different amino acids.
Different kinds of side groups
Hydrophobic or nonpolar.
Hydrophilic or polar.
Acidic.
Basic.
Amino acids are joined by?
Peptide bonds. (to make a polyPEPTIDE)
The carboxyl group of one amino acid is covalently attached to the amino group of the next amino acid.
Levels of protein structure
Primary structure - amino acid chain=polypeptide chain.
Secondary structure - 2 types, helix or beta sheet.
Tertiary structure - further folding.
Quaternary structure - several polypeptide interacting with each other. Non protein proteins can be added - heme group.
Molecular interactions determining tertiary structure (side chains)
The side chains (R) of the amino acids in the polypeptide chain react with each other. This determines (and stabilizes) the tertiary structure of a protein.
Molecular interactions determining tertiary structure (side chains)
What would happen if you exchange the amino acids? (instead of arginie you insert leucine)
Diff side chains - different interaction or possible no interaction at all
In proteins, the function is determined through?
The structure
The structure of a protein is determined through through?
The sequence of amino acids in the primary structure.
If you change this sequence, you change the structure, you change the function of the protein (or the protein loses it’s function)
Genes encode polypeptides
Classic experiments revealed that genes specify the structure of polypeptides by means of a code composed of fundamental units called codons, and that each codon is 3 nucleotides long.
The Beadle and Tatum experiment
One gene - one enzyme hypothesis.
(later restated as the one gene - one protein/polypeptide hypothesis)
Every gene carries info for 1 polypeptide
Crick and colleagues: each amino acid is specified by 3 nucleotides
(cracked the genetic code)
In the 1950s it became apparent that genes were made of DNA and the info was transcribed into mRNA, which when directed the process of polypeptide synthesis.
There are 20 diff amino acids and 4 diff nucleotides.
How does the encoding work?
Crick and colleagues: Cracking the genetic code
64
Groups of nucleotides must act as a coding unit.
How many nucleotides are in the coding unit (codon)?
20 amino acids means there must be at least 20 diff codons.
There must be at least 3 nucleotides in 1 codon (4x4x4=64)
4x4=16 but have to have 20 aa so not enough.
4x4x4=64 enough to cover the 20 aa.
4 bc each position can have A,T,C,G = 4 options.
Crick and colleagues: Cracking the genetic code
Reading frame
The coding sequence is a string of codons, read from 5’ to 3’ and it is nonoverlapping.
The beginning of the sequence is called the reading frame.
In a triplet codon there can be 3 reading frames - different results of aa from where you start.
Crick and colleagues: Cracking the genetic code
Experiment
Induce mutations that disrupt the reading frame by inserting or deleting a single base pair in a gene.
Bacteriophage T4: rII gene; mutants in that gene can only grow in E.coli B but not in E.coli k12.
Treat T4 with a chemical which causes a single bp insertion or deletion.
The reading frame will be thrown off.
Reading frame
The reading frame gets shifted with insertion and deletions. Every insertion (+) is suppressing every deletion (-). ( 1 time shift to right then back to the left, vise versa. Combination of 3 insertion and 3 deletions resorted the reading frame resulting in the wild type phenotype. (proved code is indeed 3 nucleotides)
The Genetic code is a triplet code
The triplet code is a set of 3 nucleotides (codons) that have the instructions to make a specific amino acid (or to start or stop the making of the polypeptide chain).
The Genetic code is a triplet code
64 codons
All 64 codons were deciphered by the mid-1960s.
Of the 64 triplets:
61 code for amino acids
3 triplets are “stop” signals to end translation process.
Start and stop codons
AUG - Met (initiator)
Stop codons - UAA, UAG, UGA
Properties of the genetic code
The genetic code is composed of nucleotide triplets.
The genetic code contains start and stop codons.
The genetic code is nonoverlapping.
The genetic code is degenerate (redundant).
The genetic code is no ambiguous.
The genetic code is nearly universal.
The genetic code is degenerate (redundant).
More than one codon may specify a particular amino acid (AAA and AAG both code for Lysine) (Wobble: 1st and 2nd bases of the codon follow strict rules, but the 3rd base pairs weakly with flexibility)