Protein Synthesis

The Genetic Code

• Bases are read in triplets (groups of 3) called codons

• There are 64 codons (4³) but only 20 amino acids, so multiple codons can code for the same amino acid

  • Example: CCA, CCC, CCG, CCT all → proline

Important Features of Genetic Code

• Linear: read in one direction (5' → 3' on mRNA)

• Non-overlapping: each base belongs to only one codon

• Universal: almost all organisms use the same code

• Start codon: AUG (methionine)

• Stop codons: UAA, UAG, UGA (don’t code for an amino acid)

When a certain protein of the body is in high demand…

numerous RNA transcripts of its gene will be produced

ex: when we eat sugar → our body needs insultin protein

DNA is stored and protected in the…

nucleus

Protein Synthesis (chain of events)

DNA →RNA → Endoplasmic Reticulum → Ribosomes →Amino Acids →Protein

ONE GENE =

ONE POLYPEPTIDE

When multiple amino acids join together they form a

they form a polypeptide chain, which can then fold into a specific three-dimensional shape to become a functional protein. 

Genes (at a molecular level) are…

a sequence of nucleotides from DNA

**DISCLAIMER → this is just for clarification and not to confuse genes for pure DNA. nucleotides also differ from

What do proteins do?

They:

• Run cellular processes (like metabolism)

• Control physical traits

• Can cause genetic disorders if missing or changed

Archibald Garrod

• studied a disease called alkaptonuria — people’s urine turned black because it had a chemical called alkapton.

• He thought people with alkaptonuria had a defective enzyme that couldn’t break down alkapton.

• This defect was inherited.

🧩 Conclusion: A problem in a gene → defective enzyme → disease

Beadle and Tatum

→Discovery with Arginine/Mold

• Some mutant molds only grew when given arginine (an amino acid).

• This showed that one or more enzymes that make arginine were defective.

🧩 Conclusion: Each gene controls one enzyme in a chemical pathway.

Identify the roles of mRNA, tRNA, and rRNA

1. mRNA (messenger RNA)

• Acts as a template of the gene.(only one)

• Carries the instructions from DNA in the nucleus to the ribosome.

• Has codons (triplets) that tell the cell which amino acids to put in order.

➡ Role: Brings the genetic message to the ribosome.

2. rRNA (ribosomal RNA)

• Forms the ribosome, along with proteins.

• The ribosome is the place where the protein is built.

• rRNA helps:

  • hold the mRNA in place

  • connect amino acids together

➡ Role: Makes up the ribosome and helps build the protein.

3. tRNA (transfer RNA)

• Brings the correct amino acids to the ribosome.

• Each tRNA has an anticodon that matches with an mRNA codon.

➡ Role: Delivers the amino acids and matches them to mRNA codons.

When something goes wrong in protein synthesis it can lead to:

• No protein being made

OR

• Protein being made incorrectly → wrong shape and function

**5 key differences between DNA and RNA

1. Sugar

2. Bases

3. Strands

4. Length

5. Location

Genes don’t just code for proteins they also code for:

(probably not important)

• antibodies

• hormones

• structural proteins

→ these all affect an organism’s physical traits

4 scientists :

• Mendel

• Garrod Archibald

• Beadle & Tatum

• Vernon Ingram

Vernon Ingram’s Experiment

Vernon Ingram studied hemoglobin

In normal hemoglobin, the β chain has this sequence:

Valine – Histidine – Leucine – Threonine – Proline – Glutamic acid – Glutamic acid

• In sickle cell anemia, one glutamic acid is replaced by valine:
  Valine – Histidine – Leucine – Threonine – Proline – Valine – Glutamic acid

• This tiny change causes red blood cells to become sickle-shaped → blocking blood flow → serious disease.

🧩 Conclusion: A single amino acid change → major health effect.

mRNA

• carries the DNA “message” to the ribosome

• single stranded

• only contains the code for ONE GENE

• has a short life →destroyed when no longer needed

t-RNA

• 1st comes due to signal triggered by the mRNA and ribosome match/link up

• brings amino acid to the ribosome

• they come based on whatever codon needs its anti codon and they bring the corresponding amino acid

r-RNA

• binds to mRNA

• used to help build/make ribosomes cuz for some reason they need help being made

• act as a base for building the proteins

• varies in length?

DEFINE Transcription

Transcription is the process where a cell makes an mRNA copy of a gene.

• happens purely in the nucleus

STEPS of Transcription 

1. Initiation

2. Elongation

3. Termination

4. Capping and Tailing

5. Splicing

Translation

• happens in RIBOSOME

Initiation

1. ribosome clamps onto the mRNA

2. reads the code in codons (3 nucleotides at a time)

Elongation

1. t-RNA brings the correct amino acid

2. The amino acids join together form a polypeptide chain

Termination

1. Ribosome reaches a  stop codon

2. The completed protein is released

The genetic code///RNA polymerase both move in one direction

5’ →3’

Transcription Voice Note in detail

True or False: Introns are recycled

True. The diff nucleotides are recycled for more mRNA

Alternative Splicing

can join different combinations allowing one gene to make many different proteins

Initiaion

Elongation

Termination

Capping and Tailing

Splicing

After Transcription is complete

the MATURE mRNA leaves the nucleus through nucleus pore → to the cytoplasm to be translated