Protein Synthesis

• The process of reading the instructions in DNA to

make a polypeptide.

Protein Synthesis

Amino acid
sequence
Primary structure

Alpha helix

or

beta sheet

Secondary structure

Polypeptide

chain is
bent and
folded into
3D structure

Tertiary
structure

2 or more
poly-pept

ide

chains
bound
together

Quaternary

structure

– Polypeptide

= a chain of
amino acids;
can bind to
others and
fold into a
protein

– Remember,

proteins are
the most
diverse
macro-molec
ule, and have
4 levels of
structure!

• DNA = instructions,

located in the
nucleus(which it
can’t leave)

• But proteins are

made in ribosomes.

• So protein synthesis

takes 2 steps:
- Transcription
- Translation

DNA

RNA

protein

transcription

translation

Central Dogma of Genetics

3 Essential Types of RNA

• mRNA (messenger):

copies instructions in
DNA and carries these
to the ribosomes in the
cytoplasm

• tRNA (transfer):binds

and carries specific
amino acids to the
ribosome

• rRNA (ribosomal):

along with proteins,
make up the ribosome
– They also help catalyze

the formation of peptide
bonds!

• Purpose: Carry the code/instructions out of

the nucleus
– (Remember: DNA never leaves the nucleus, and proteins

are made in the cytoplasm by ribosomes.)

• Location: Nucleus

• Starts with: DNA

• Ends with: mRNA

TRANSCRIPTION

DNA🡪mRNA

Process:
1.

RNA Polymerase binds to the DNA promoter where
transcription is to begin and unzips the gene that needs to
be copied.
– Looks for the TATA box as a signal

2.

RNA Polymerase uses complementary base-pairing rules to
match RNA nucleotides with the exposed DNA nucleotides.
– Remember: RNA has uracil, so A binds with U, and C still with G.

3.

Releases the completed mRNA molecule.

4.

DNA zips back up and the mRNA leaves the nucleus and
enters the cytoplasm.

TRANSCRIPTION

Transcription: DNA is copied into a

complementary strand of mRNA.

DNA

DNA

mRNA

DNA

mRNA

3’

5’

3’

5’

5’

3’

5’

3’

3’

5’

RNA is made in the 5’-3’ direction.

The DNA template is read in the 3’-5’ direction.

Example of Transcription

• DNA sequence (template):

3’ TACGCTAGTACGATT 5’

• mRNA sequence:

5’ AUGCGAUCAUGCUAA 3’

• RNA Splicing

– mRNA has introns (non-coding regions) and exons

(coding regions)

– This process removes introns and splices exons

together.

• Can create different combinations of exons and thus make

multiple polypeptides from 1 gene!

• 5’ cap (G cap, a modified guanine) is added to

the 5’ end 🡪 facilitates binding to a ribosome.

• poly A tail (50-520 As) is added to the 3’ end 🡪

helps the mRNA leave the nucleus.

Post-TRANSCRIPTION

Modifications

Codon: a set of
3 nucleotides on
the mRNA

Anticodon:
“complementary” 3
nucleotides on
tRNA

Genetic Code: code of instructions for how to make
proteins

mRNA

(messenger)

tRNA
(transfer)

Aminoacid
–monomer
(building block)
for making
proteins, held
together by
peptide bonds

Translation Vocabulary

• Purpose: Read/follow the instructions carried

on the mRNA to make a polypeptide

• Location: Ribosomes

• Starts with: mRNA

• Ends with: Polypeptide

TRANSLATION

mRNA 🡪polypeptide

Process:
1.

mRNA attaches to the small subunit of the ribosome.

2.

Ribosome reads the mRNA codons, always in the 5’ to 3’ direction,
starting at the AUG codon.
– 1 codon = 3 RNA nucleotides

3.

tRNAs act like taxis to pick up and drop off the amino acids that
match with each codon.

4.

tRNAs continue to drop off a.a., and the ribosome binds the a.a.
together with peptide bonds.

5.

When the “stop codon” is reached, the ribosome releases the
completed polypeptide chain.

TRANSLATION

Translation: interpreting
the RNA message into a
polypeptide to make a
protein.

The Genetic Code

Example

• DNA sequence (template):

3’ TACGCTAGTACGATT 5’

• mRNA sequence:

5’ AUGCGAUCAUGCUAA 3’

• Codons:

AUG CGA UCA UGC UAA

• Amino Acids:

Met – Arg – Ser – Cys – stop

• Anti-codons on tRNA:

UAC GCU AGU ACG AUU

Summary of Processes

Identical DNA

strands

A, T, C, and G

nucleus

mRNA from DNA

A, U, C, and G

nucleus

proteinfrom

mRNA

Amino acids

ribosomes

If all of our body cells have all of our DNA in
them, how come my cells are so different?

For example, why is a skin cell so different in
function from a muscle cell if they all have the
same DNA and thus the same instructions for

making proteins?

• Gene expression is a highly regulated

process.
– Turning genes “on” and “off” is CRITICAL for cell

differentiation.

– Can happen before, during, or after transcription

or translation.

– Transcription factors = regulatory proteins that

control gene activity

• Repressors decrease transcription, activators increase

transcription

– HIV is an example of a virus that disrupts

regulation.

Regulation

• The study of changes in gene expression that

are heritable.
– Different from mutations.

Epigenetics

• Mutation = a

change in a
DNA sequence.

• Epigenetics do

not affect the
actual DNA
sequence, just
how that DNA
sequence gets
expressed.

– Ex. Histone

modification