Chapter 13: RNA and Protein Synthesis

Chapter 13: RNA and Protein
The Genetic Code
 The first step in decoding genetic messages is
transcribing DNA into RNA
 The RNA contains the code for making a protein
 Proteins are made by joining together amino acids
into long chains called polypeptides
 There are 20 different amino acids commonly
found in polypeptides
The Genetic Code
 The specific amino acid sequence in a polypeptide
determines a protein’s properties – including its
shape and function
 RNA contains 4 bases – adenine, cytosine, guanine,
and uracil
 These bases are the letters of the special language
known as the genetic code
The Genetic Code
 The code is “read” in groups of three letters called
 Each codon in mRNA is like a word in the code made
of 3 consecutive bases specifying a single amino acid
to be added to the polypeptide chain
How to Read Codons
 4 bases in RNA means 64
possible codons
To read, start at middle
and work out
Most amino acids have
more than one codon
Methionine codon AUG
is the start codon
There are 3 stop
codons – UAA, UAG,
 Sequence of bases in mRNA = instructions for the
order of amino acids in a polypeptide
 More folding required to make a protein
 Ribosomes read codon sequence in mRNA to
assemble amino acids into polypeptides
 The decoding of an mRNA message into a protein is
a process known as translation
Steps in Translation
 mRNA is transcribed in nucleus, then enters
Steps in Translation
 Starts when a ribosome attaches to mRNA and begins
reading each codon, directing tRNA to bring the correct
amino acid
Each tRNA carries one amino acid – it gets matched with
the codon through an anticodon complementary to the
Ribosome helps form a peptide bond between the each
amino acid
Polypeptide chain continues to grow until the ribosome
reaches a “stop” codon
It releases both the newly formed polypeptide and the
mRNA molecule, completing the process of translation
Steps in Translation
The Roles of tRNA and rRNA in Translation
 Ribosomes are composed of ~ 80 proteins and three
or four different rRNA molecules
 rRNA molecules hold ribosomal proteins in place
and locate the beginning of the mRNA message
 May carry out chemical reaction that joins amino
acids together
The Molecular Basis of Heredity
 Most genes contain instructions for making proteins,
many of which are enzymes
Proteins are microscopic tools, each specifically
designed to build or operate a component of a living
Molecular biologists study life at the molecular level
The central dogma of molecular biology is that
information is transferred from DNA to RNA to
There are many exceptions to this “dogma”
The Molecular Basis of Heredity
 Gene expression is putting genetic information into
action in living cells, and involves DNA, RNA, and
 DNA carries information for traits and is used to
make mRNA, which is decoded into an amino acid
sequence to make a protein, which produces a trait
 Genetic code is near universal
 Some organisms show variations, but code is read
the same way

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