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-Histidine-Glycine

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 anti-codon 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