Chapter 12, Section 3 – RNA & Protein Synthesis
GENES – coded DNA instructions that control the production of proteins within
the cell
The Structure of RNA
 RNA, like DNA, consists of long chain of nucleotides
o Made of 5-carbon sugar, a phosphate group, and nitrogenous base
 Three main differences between DNA & RNA
o Sugar in RNA is RIBOSE not deoxyribose
o RNA contains URACIL (U) in place of thymine
 Contains four different bases: A, U, C, G (Adenine, Uracil, Cytosine, and
Guanine)
 The ability to copy a single DNA sequence into RNA makes it possible for a
single gene to produce hundreds of even thousands of RNA molecules
Types of RNA
 Main job of RNA – PROTEIN SYNTHESIS
o Assembly of amino acids in proteins is controlled by RNA
 Three main types of RNA:
o MESSENGER RNA (mRNA) – RNA molecules that carry copies of
instructions for assembling amino acids
o RIBOSOMAL RNA (rRNA) – makes up majority of RNA, crucial in
protein synthesis
o TRANSFER RNA (tRNA) – transfers amino acids to the ribosome as it
is specified by coded messages in mRNA
Transcription
 TRANSCRIPTION – process where RNA molecules are produced by copying
part of the nucleotide sequence of DNA into a complementary sequence in
RNA
o RNA polymerase – required enzyme for transcription, similar to DNA
polymerase
 During transcription, RNA polymerase binds to DNA and separates the
DNA strands. RNA polymerase then uses one strand of DNA as a template
from which nucleotides are assembled into a strand of RNA
 RNA polymerase binds to specific points on DNA
o Only to regions of DNA called PROMOTERS (have specific base
sequences)
o PROMOTERS – signals in DNA that indicate to the enzyme where to
bind to make RNA. Once complete, signals are made to stop
transcription
RNA Editing
 RNA molecule is produced by copying DNA
 INTRONS – sequences of nucleotides not involved in coding for proteins,
contained in DNA of eukaryotic cells
o cut out of RNA molecules
 EXONS – the DNA sequences that code proteins; “expressed” in the
synthesis of proteins
o Form mRNA
 A single gene may produce several different forms of RNA
The Genetic Code
 Proteins are made by joining amino acids into long chains called
POLYPEPTIDES
 Properties of proteins are determined by the order in which different
amino acids are joined together to produce polypeptides
 GENETIC CODE – the “language” of mRNA instructions
o Read three letters at a time; code is read three bases at a time
o Each three letter “word” in mRNA is known as a codon
o CODON – consists of three consecutive nucelotides that specify a
single amino acid that is to be added to the polypeptide
 UCGCACGGU  read: UCG – CAC – GGU
 Different codons represent different amino acids
 Ex. UCG – Serine, CAC – Histidine, GGU – Glycine
 64 possible codons of genetic code
 Start (1) & Stop (3) commands are encoded
Translation
 Ribosomes are needed in order to read the “instructions” or sequence of
nucleotide bases in mRNA molecules
 TRANSLATION – the decoding of an mRNA message into polypeptide chain
(protein)
o Takes place on ribosomes
 During translation, the cell uses information from messenger RNA to
produce proteins
o Before translation occurs, messenger RNA is transcribed from DNA in
the nucleus and released into the cytoplasm
o Translation begins when an mRNA molecule in the cytoplasm
attaches to a ribosome
 As a codon of mRNA moves through the ribosome, the proper
amino acid is brought into the ribosome by tRNA. In the
ribosome, amino acid is transferred to the growing
polypeptide chain
 tRNA molecules carry only one kind of amino acid
 Have three unpaired bases
 ANTICODON – the three unpaired bases in tRNA
molecules, complementary to one mRNA codon
o Ribosome forms peptide bond between the first and second amino
acids
 At the same time, ribosome breaks the bond that held the first
tRNA molecule to its amino acid and releases the tRNA
molecule
 Ribosome moves to the third codon
o Polypeptide chain continues to grow until the ribosome reaches a
stop codon on mRNA molecule
 When stop codon is reached, ribosome releases the newly
formed polypeptide and mRNA molecule, completing
translation
The Roles of RNA & DNA
 DNA = “Master Plan”
o Remains within safety of the nucleus
 RNA = “Blueprints”
o Goes to the protein building sites in the cytoplasm – the ribosomes
Genes & Proteins
 Proteins are enzymes, which catalyze and regulate chemical reactions
 Genes for certain proteins may regulate the rate and pattern of growth
throughout an organism, controlling its size and shape
 Proteins are microscopic tools, each specifically designed to build or
operate a component of a living cell