Ribosomes and Protein Synthesis

Unit: 2
Topic: 13.2 Ribosomes and
Protein Synthesis
EQ: How would you build a
system to read the
messages that are coded in
genes and transcribed into
RNA?

Think about it

• How would you build a system to read the

messages that are coded in genes and
transcribed into RNA?

•

Would you read the bases one at a

time, as if the code were a language with
just four words—one word per base?

•

Perhaps you would read them as

individual letters that can be combined to
spell longer words.

This Photo by Unknown Author is licensed under CC BY

The Genetic

Code

• What is the genetic code, and how is it read?

• The genetic code is read three “letters” at a time, so

that each “word” is three bases long and
corresponds to a single amino acid.

• The first step in decoding genetic messages is to

transcribe a nucleotide base sequence from DNA to
RNA.

The Genetic

Code

• This transcribed information contains a code for

making proteins.

• Proteins are made by joining amino acids together

into long chains, called polypeptides.

• As many as 20 different amino acids are commonly

found in polypeptides.

The Genetic
Code

• The specific amino acids in a

polypeptide, and the order in
which they are joined,
determine the properties of
different proteins.

• The sequence of amino acids

influences the shape of the
protein, which in turn
determines its function.

• RNA contains four different

bases: adenine, cytosine,
guanine, and uracil.

The Genetic
Code

• These bases form a

“language,” or genetic code,
with just four “letters”: A, C,
G, and U.

• Each three-letter “word” in

mRNA is known as a codon.

• A codon consists of three

consecutive bases that
specify a single amino acid to
be added to the polypeptide
chain.

Multiple Choice

What is the relationship between a protein, the cell, and DNA?

• DNA is produced by protein which is produced in the cell

• Protein is composed of DNA which is produced in the cell

• DNA controls the production of protein in the cell

• A cell is composed of DNA and protein

Multiple Choice

Which of these nitrogenous bases is NOT in RNA?

• Uracil

• Thymine

• Cytosine

• Guanine

Multiple Choice

A polypeptide (protein chain) is a sequence of ____________________ held together by a _______________ bond.

• amino acids; peptide

• proteins; hydrogen

• fatty acids; ionic

• fatty acids; covalent

Multiple Choice

When RNA is being made, the RNA base ____ always pairs with the base ___ in DNA.

• U-T

• T-G

• U-A

• A-U

Multiple Choice

How many nucleotides make up a codon?

• 1

• 3

• 5

• 15

Multiple Choice

How many codons: AAU GCG AUG

• 9

• 1

• 3

• 15

How to read codons

• Because there are four different bases in

RNA, there are 64 possible three-base
codons (4 × 4 × 4 = 64) in the genetic
code.

• This circular table shows the amino acid to

which each of the 64 codons corresponds.
To read a codon, start at the middle of the
circle and move outward.

How to read codons

• Most amino acids can be specified by more

than one codon.

• For example, six different codons—UUA,

UUG, CUU, CUC, CUA, and CUG—specify
leucine. But only one
codon—UGG—specifies the amino acid
tryptophan

Multiple Choice

In mRNA, each codon specifies for a particular

• nucleotide

• Enzyme

• Amino Acid

• Monomers

Multiple Choice

What amino acid is represented by the codon UUA?

• Phenylalanine
• Tyrosine
• Leucine
• Stop codon

Start and stop Codons

• The genetic code has punctuation

marks.

• The methionine codon AUG serves

as the initiation, or “start,” codon for
protein synthesis.

• Following the start codon, mRNA is

read, three bases at a time, until it
reaches one of three different “stop”
codons, which end translation.

Multiple Choice

Arginine

• AAG

• AGG

• ACU

• CUG

• UAG

Multiple Choice

Stop

• CUG

• UAG

• ACU

• CCC

• UAG

Multiple Choice

AUG

• Arginine

• Serine

• Lysine

• Threonine

• Methionine

Fill in the Blank

What is the amino acid sequence for the following mRNA strand:

AUG-GAC-ACU-UAG

Fill in the Blank

What is the amino acid sequence for the following mRNA strand:

UGU-UGG-AUA-CCC

Fill in the Blank

What is the amino acid sequence for the following mRNA strand:

UCG-GAA-CAC-AAU

Translation

• What role does the ribosome play in

assembling proteins?

• Ribosomes use the sequence of codons

in mRNA to assemble amino acids into
polypeptide chains.

• The sequence of nucleotide bases in an

mRNA molecule is a set of instructions that
gives the order in which amino acids should
be joined to produce a polypeptide.

Translation

• The forming of a protein requires the

folding of one or more polypeptide chains.

• Ribosomes use the sequence of codons in

mRNA to assemble amino acids into
polypeptide chains.

• The decoding of an mRNA message into a

protein is a process known as translation.

Steps in Translation

Messenger RNA is transcribed in the
nucleus and then enters the cytoplasm
for translation.

Steps in translation

• Translation begins when a ribosome

attaches to an mRNA molecule in the
cytoplasm.

• As the ribosome reads each codon of

mRNA, it directs tRNA to bring the
specified amino acid into the
ribosome.

• One at a time, the ribosome then

attaches each amino acid to the
growing chain.

• Each tRNA molecule carries just one

kind of amino acid.

•

Steps in translation

• In addition, each tRNA molecule has

three unpaired bases, collectively called
the anticodon—which is
complementary to one mRNA codon.

• The tRNA molecule for methionine has

the anticodon UAC, which pairs with the
methionine codon, AUG.

• The ribosome has a second binding site

for a tRNA molecule for the next codon.

• If that next codon is UUC, a tRNA

molecule with an AAG anticodon brings
the amino acid phenylalanine into the
ribosome.

Steps in Translation

• The ribosome helps form a

peptide bond between the first
and second amino
acids—methionine and
phenylalanine.

• At the same time, the bond

holding the first tRNA molecule
to its amino acid is broken.

• That tRNA then moves into a

third binding site, from which it
exits the ribosome.

Steps in Translation

• The ribosome then moves to the

third codon, where tRNA brings it
the amino acid specified by the
third codon.

• The polypeptide chain continues

to grow until the ribosome
reaches a “stop” codon on the
mRNA molecule.

• When 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 polypeptide chain continues

to grow until the ribosome
reaches a “stop” codon on the
mRNA molecule.

• When the ribosome reaches a

stop codon, it releases both the
newly formed polypeptide and
the mRNA molecule, completing
the process of translation.

The Roles of tRNA and rRNA
in Translation

• Ribosomes are composed of roughly 80

proteins and three or four different rRNA
molecules.

• These rRNA molecules help hold ribosomal

proteins in place and help locate the
beginning of the mRNA message.

• They may even carry out the chemical

reaction that joins amino acids together.

Multiple Choice

Translation takes place in the 

• ribosome

• chloroplast

• nucleus

• mitochondria

Multiple Choice

Brings the amino acids to the ribosome

• mRNA

• tRNA

• rRNA

• DNA

Multiple Choice

What would be the tRNA anticodon for the mRNA codon CAG?

• GTC

• GUC

• CUG

• CAG

Multiple Choice

Which phrase best describes translation?

• converts mRNA into a polypeptide

• catalyzes bonds between amino acids

• produces RNA from DNA molecules

• recycles tRNA molecules for reuse

Multiple Choice

What type of bonds are forming during translation?

• Peptide bonds

• Hydrogen bonds

• Acid bonds

Multiple Choice

Which phase of which process does this picture show?

• Transcription

• Translation

• Replication

Multiple Choice

The codon is located on the 

• DNA

• mRNA

• tRNA

• rRNA

Multiple Choice

The job of tRNA is to

• carry amino acids to the ribisomes

• carry ribisomes to the ER

• carry glucose to mitochondria

• carry mRNA out of the nucleus

Multiple Choice

What would be the tRNA anticodon for the mRNA codon CAG?

• GTC

• GUC

• CUG

• CAG

The Molecular Basis
of Heredity

• What is the “central dogma” of

molecular biology?

• The central dogma of molecular

biology is that information is
transferred from DNA to RNA to
protein.

The Molecular Basis
of Heredity

• A gene that codes for an enzyme to

produce pigment can control the color
of a flower. Another gene produces
proteins that regulate patterns of tissue
growth in a leaf. Yet another may
trigger the female or male pattern of
development in an embryo.

• Proteins are microscopic tools, each

specifically designed to build or
operate a component of a living cell.

The Molecular

Basis of
Heredity

• Molecular biology seeks to explain living

organisms by studying them at the molecular
level, using molecules like DNA and RNA.

• The central dogma of molecular biology is that

information is transferred from DNA to RNA to
protein.

• There are many exceptions to this “dogma,”

but it serves as a useful generalization that
helps explain how genes work.

• Gene expression is the way in which DNA,

RNA, and proteins are involved in putting
genetic information into action in living cells.

• DNA carries information for specifying the traits

of an organism.

The Molecular

Basis of
Heredity

• The cell uses the sequence of bases in DNA as a

template for making mRNA.

• The codons of mRNA specify the sequence of

amino acids in a protein.

• Proteins, in turn, play a key role in producing an

organism’s traits.

• One of the most interesting discoveries of

molecular biology is the near-universal nature of
the genetic code.

• Although some organisms show slight variations in

the amino acids assigned to particular codons, the
code is always read three bases at a time and in
the same direction.

• Despite their enormous diversity in form and

function, living organisms display remarkable unity
at life’s most basic level, the molecular biology of
the gene.

Multiple Choice

Which of the following sequences of processes correctly reflects the central dogma?

• protein synthesis, transcription, translation

• protein synthesis, translation, transcription

• transcription, translation, protein synthesis

• translation, transcription, protein synthesis

Multiple Choice

Which process is shown?

• diffusion

• repiration

• recombination

• translation

Multiple Choice

What is the relationship between a protein, the cell, and DNA?

• DNA is produced by protein which is produced in the cell

• Protein is composed of DNA which is produced in the cell

• DNA controls the production of protein in the cell

• A cell is composed of DNA and protein

Multiple Choice

The diagram shows the process of

• Transcription

• Translation

• Replication

• Mitosis

Multiple Choice

Where does transcription take place?

• nucleus

• ribosome

• cytoplasm

• smooth ER

Multiple Choice

Where does translation take place?

• Nucleus
• Ribosome
• Golgi Body
• Cytoplasm