Simultaneous transcription and translation in prokaryotes
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Transcript of Simultaneous transcription and translation in prokaryotes
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