Transcript RNA and protein synthesis

RNA
and the business of making proteins
RNA structure
• RNA is the principle molecule that carries
out the instructions coded in DNA
• RNA is a nucleic acid
• RNA is different than DNA in three ways
– Sugar is ribose
– RNA is single stranded
– RNA contains uracil instead of thymine
• Thymine forms H bonds with adenine
Transcription
• This is the process by which RNA molecules are
made
• Part of the nucleotide sequence of DNA is copied
into RNA
• Enzyme responsible is called RNA polymerase
• “Start” sequences in DNA tell RNA polymerase
where to start copying
• “Stop” sequences tell RNA polymerase where to
stop
Forms of RNA
• Section of DNA code for three different kinds of
RNA
• Messenger RNA (mRNA)
– Serves as a temple for the assembly of the protein
– It’s a long chain of RNA
• Ribosomal RNA (rRNA)
– Makes up the organelle where proteins are assembled
(remember?????)
• Transfer RNA (tRNA)
– Carries specific single amino acids to the site of protein
assembly (one tRNA for every type of amino acid)
Genetic Code (1)
• Proteins are made of polypeptides, which are
made of amino acids
• There are 20 different amino acids
• The order of the amino acids determines the
properties of the protein
• The order of the nucleotides in DNA controls the
order of the amino acids in the finished protein.
• This is called the genetic code
Genetic Code (2)
• The language is written in only 4 letters
(AUCG)
• Nucleotides in mRNA are read in groups of
3 letters
• The group of 3 nucleotides is called a codon
Genetic Code (3)
• The language is written in only 4 letters
(AUCG)
• Nucleotides in mRNA are read in groups of
3 letters
• The group of 3 nucleotides is called a codon
Genetic Code (4)
• For example: m RNA sequence
AAACACGGU
• Is read like this: AAA CAC GGU
• Which translates as: Lysine-HistidineGlycine
Genetic Code (5)
• The start signal is AUG, which stands for the
amino acid methionine
• Thus methionine is usually the first amino acid in
all proteins
• Sometimes enzymes cut this off after the complete
protein is made
• There are three stop signals that do not code for
any amino acid
– UAA, UAG, UGA
Genetic Code (6)
• Translation is the process in which mRNA
is decoded into an amino acid sequence
• Ribosomes match up the appropriate tRNA
with the sequence on the mRNA
• Every tRNA has a three nucleotide anticodon which matches up with the mRNA
codon
Genetic Code (7)
• The ribosome binds two codons at a time
• When a tRNA binds its amino acid to the
polypeptide chain it is released from the ribosome
to bind to another one of its specific amino acid
• The ribosome moves along the mRNA reading
two codons at a time until it comes to one of the
three stop codons. The result is a protein, which is
released from the ribosome
Mutations
• Inheritable change in genetic information
• Gene mutations
– Substitution (no effect on codon)
– Insertion or deletion (effects codon)
• Chromosome mutations
– Deletion, duplication, inversion, translocation
Effects of mutations
• So what’s the big deal?
– Some have little or no effect
– Some produce beneficial variation
– Some negatively affect gene function