This document summarizes the process of protein synthesis, which occurs in two main steps - transcription and translation. In transcription, DNA in the nucleus is copied into messenger RNA (mRNA) using complementary base pairing. The mRNA then exits the nucleus and binds to ribosomes in the cytoplasm. In translation, the mRNA is read by the ribosome, which uses transfer RNA (tRNA) molecules to translate the mRNA codons into amino acids and assemble them into a polypeptide chain. Although all cells contain the same DNA, gene expression is regulated at the transcriptional and translational levels, allowing different proteins to be produced in different cell types.

2. • 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!

3. • 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

4. 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!

5. • 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

6. Process:
1. Unzip the gene that needs to be copied.
2. Use 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. Release the completed mRNA molecule.
4. DNA zips back up and the mRNA leaves the nucleus
and enters the cytoplasm.
TRANSCRIPTION

7. Transcription: DNA is copied into a
complementary strand of mRNA.
DNA
DNA
mRNA
DNA
mRNA

8. Example
• DNA sequence (template):
TACGCTAGTACGATT
• mRNA sequence:
AUGCGAUCAUGCUAA

9. 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)
Amino acid
–monomer
(building block)
for making
proteins, held
together by
peptide bonds
Translation Vocabulary

10. • Purpose: Read/follow the instructions carried
on the mRNA to make a polypeptide
• Location: Ribosomes
• Starts with: mRNA
• Ends with: Polypeptide
TRANSLATION
mRNA 🡪 polypeptide

11. Process:
1. mRNA attaches to a ribosome.
2. Ribosome reads the mRNA codons, starting at AUG.
– 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

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

13. The Genetic Code

14. Example
• DNA sequence (template):
TACGCTAGTACGATT
• mRNA sequence:
AUGCGAUCAUGCUAA
• Codons:
AUG CGA UCA UGC UAA
• Amino Acids:
Met – Arg – Ser – Cys – stop
• Anti-codons on tRNA:
UAC GCU AGU ACG AUU

15. Summary of Processes
Identical DNA
strands
A, T, C, and G
nucleus
mRNA from DNA
A, U, C, and G
nucleus
protein from
mRNA
Amino acids
ribosomes

16. 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?

17. • 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

18. • 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