Simultaneous transcription and translation in prokaryotes Green arrow

Simultaneous transcription and translation in prokaryotes Green arrow = E. coli DNA Red arrow = m. RNA combined with ribosomes

Eukaryotic RNA Differences 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 RNA 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 RNA 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 • sn. RNAs 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) DNA (Gene) (Translation) RNA Protein (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

Translation (protein synthesis) Peptidyl site: peptidyltransferase attaches amino acid to chain Aminoacyl site: new amino acid brought in Ribosome moves in this direction

Animation of protein synthesis • http: //highered. mcgrawhill. com/sites/0072556781/student_view 0/c hapter 12/animation_quiz_2. html

Cells have adapter molecules called t. RNA with a three nucleotide sequence on one end (anticodon) that is complementary to a codon of the genetic code. • There are different transfer RNAs (t. RNAs) with anticodons that are complementary to the codons for each of the twenty amino acids. • Each t. RNA interacts with an enzyme (aminoacyl-t. RNA synthetase) that specifically attaches the amino acid that corresponds to its anticodon. • For example, the t. RNA to the right with the anticodon AAG is complementary to the UUC codon in the genetic code (m. RNA). That t. RNA would carry the amino acid phenylalanine (see genetic code table) and only phenylalanine to the site of protein synthesis. • When a t. RNA 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

Primary structure Amino end Carboxyl end Tryptophane Synthase A Protein 268 amino acids long

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

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

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

Beta chains each have 146 amino acids Alpha chains each have 141 amino acids Hemoglobin

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 1950 s and early 1960 s