Simultaneous transcription and translation in prokaryotes

Green arrow = E. coli DNA

Red arrow = mRNA combined with ribosomes

Eukaryotic RNADifferences

RNA processing– Primary transcript produced in the

nucleus– Processed before transported to the

cytoplasm

A cap consisting of 7-methylguanosine is added to the 5’ end of the transcript

3’ poly (A) tail

Eukaryotic RNARNA processing– 5’ cap• Protects RNA from degradation• Required for binding to the ribosome during

initiation of protein synthesis (translation)

– 3’ poly (A) tail• Protects RNA from degradation by

nucleases

Eukaryotic RNARNA processing– Splicing• Removes intervening sequences in RNA

Many eukaryotic genes contain internal sequences that do not encode amino acids – introns (light colored areas)

Sequences that encode amino acids – exons (darker colored areas)

Splicing removes the introns and brings together the coding regions

Gene Splicing• Consensus sequence at intron-exon

junction

• snRNAs pair complementarily with the splice site

• Splicing enzymes can then cut-out introns

Gene Splicing• Sometimes, different introns are

spliced-out determining the function (type) of protein that is made

The Central Dogma(Francis Crick, 1958)

(Transcription) (Translation)

DNA RNA Protein(Gene) (Phenotype)

An informational process between the genetic material (genotype) and the protein (phenotype

Proteins

• Proteins are just long polymers of amino acids– So, the basic unit of a protein is an

amino acid– 20 different amino acids

Proteins

• Amino acids in a protein are held together by peptide bonds– Facilitated by peptidyltransferase

Proteins

• A long string of amino acids is called a polypeptide

• A protein has an amino (the first amino acid in the chain) and a carboxyl (the last amino acid in a chain) ends

Aminoacyl site:new amino acid brought in

Peptidyl site:peptidyltransferase attaches amino acid to chain

Ribosome moves in this direction

Translation (protein synthesis)

Animation of protein synthesis

• http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter12/animation_quiz_2.html

Cells have adapter molecules called tRNA with a three nucleotide sequence on one end (anticodon) that is complementary to a codon

of the genetic code. • There are different transfer RNAs

(tRNAs) with anticodons that are complementary to the codons for each of the twenty amino acids.

• Each tRNA interacts with an enzyme (aminoacyl-tRNA synthetase) that specifically attaches the amino acid that corresponds to its anticodon.

• For example, the tRNA to the right with the anticodon AAG is complementary to the UUC codon in the genetic code (mRNA). That tRNA would carry the amino acid phenylalanine (see genetic code table) and only phenylalanine to the site of protein synthesis.

• When a tRNA has its specific amino acid attached it is said to be “charged.”

Proteins

Protein can have a

• Primary structure

• Secondary structure

• Tertiary structure

• Quaternary structure

Primary structure•The order of the amino acids•The order is the primary determinant of protein function•The primary structure is determined by the code on the DNA/RNA

synthesized

Tryptophane Synthase A Protein 268 amino acids long

Primary structure

Amino end

Carboxyl end

Alpha Helix

Secondary structure•Interaction of side groups, giving polypeptides a periodic structure•Stabilized by hydrogen bonds

Alpha Helix

Beta Pleated Sheet

Tertiary structure•The folding or bending of the polypeptide

Tertiary structure can be affected by environmental factors such as temperature

Enzymes are proteins: if the tertiary structure is changed (mutation or temperature) the enzyme cannot carry out its function

Bovine Insulin Protein

Quaternary structure•Two or more polypeptides combine to form a functional protein

Proteins

• The order of the amino acids (the primary structure) can affect the secondary, tertiary and quaternary structures– Possibly affecting the function of the

protein

Hemoglobin

Alpha chains each have 141 amino acids

Beta chains each have 146 amino acids

Change in beta chain at amino acid 6 out of the 146 amino acids (change in codon from GAG to GUG)

Proteins

• The order of the amino acids in a polypeptide is like the order of words in a sentence

Proteins

• If you change one word you can change the meaning significantly– John only punched Jim in his eye.

Proteins

• If you change one word you can change the meaning significantly– John only punched Jim in his eye.– John only punched Jim in his dreams.

Proteins

• This is what happens in mutations– If the code changes (DNA), new amino

acids can be put in the polypeptide, changing “the meaning” of the polypeptide

Genetic Code• One fundamental question: How can

DNA and RNA, each consisting of only four different nucleotides (bases), encode proteins consisting of 20 amino acids?– Solving the genetic code became the

most important biological question of the late 1950s and early 1960s