1. Protein Synthesis – Review

2. Review!!

3. An Overview…
Genes on chromosomes need to be transcribed into RNA, and then translated by ribosomes into protein.
Genes act by directing the production of only ONE protein each.
This is known as the one gene-one enzyme hypothesis.
An error in gene sequence can result in a change in protein sequence, shape, and therefore function.
TRY THIS: Activity p 233, Translating the Code.

4. Overview…
DNA in the nucleus is TRANSCRIBED into a complementary RNA message that moves to the cytoplasm. (copied from one medium to another)…
Numerous RNA copies can move to different ribosomes.

5. Overview
The ribosomes TRANSLATE the message into Polypeptide chains and process into proteins (convert into a different language).
Transcription involves the copying of information in DNA into mRNA
Translation involves ribosomes using mRNA as blueprint to synthesize a protein composed of amino acids.

6. A Cool Visual

7. RNA ribose sugar with a hydroxyl on 2’ Carbon
uracil replaces the base thymine and pairs with adenine
single stranded

8. 3 Types of RNA
Messenger RNA (mRNA): varies in length, end product of transcription of a gene, moves to the ribosome
Transfer RNA (tRNA): delivers amino acids to the ribosomes for translation. Shaped like a cloverleaf; very short.
Ribosomal RNA (rRNA): binds with protein to make ribosomes

9. Step 1 - Transcription Within the nucleus, the
DNA of a particular gene
unwinds and separates.
One strand of the DNA is
transcribed into
messenger RNA (mRNA)
in a process called
transcription.

10. Transcription – Making mRNA

11. Transcription divided into three sequential processes…
Initiation
RNA polymerase binds to DNA at specific site known as promoter, near the beginning of the gene. (usually high [] of A & T…only 2 bonds to break!)
Elongation
Enzyme RNA polymerase puts together the appropriate ribonucleotides and builds the mRNA transcript

12. Termination
Shortly after RNA polymerase passes the end of a gene, recognizes signal to stop transcribing
mRNA transcript then is completely released from the DNA and will exit the nucleus

13. The Final Touches…
the mRNA must be prepared to leave the nucleus. A 5’ cap is added (for protection), and a poly A tail is added to the 3’ end
The mRNA contains regions of code that do not make the protein (introns). Introns are removed by spliceosomes.
Spliceosomes join exons (coding regions) together.
The mRNA strand leaves the nucleus and binds with ribosomes in the cytoplasm.

14. P 249, Q 1-6

15. Step 2 Translation
Once mRNA enters cytoplasm, it is ready to be translation
Ribosomes: made up of two parts: a large subunit (60s) and a small subunit (40s). Bind to the mRNA (recognizing the 5’ cap)
Must be lined up correctly to the proper reading frame (reading triplets correctly)

16. tRNA
tRNA delivers amino acids to the polypeptide building site (ribosome)
tRNA is a small single stranded nucleic acid, resembles a cloverleaf
one arm: anticodon (sequence of three bases complementary to mRNA)
3’ end has acceptor site for a particular amino acid
this recognition by tRNA of mRNA is facilitated through complimentary base pairing.
every tRNA carries only one specific amino acid
therefore there must be at least 20 (20-64) different tRNA’s

17. Translation

18. Translation
The mRNA consists of nucleotides that code for a specific amino acid. The code is in triplet called a CODON (3 nucleotides = 1 amino acid). Amino acids are brought into place by a molecule known as transfer RNA (tRNA). This process is known as translation. Peptide bonds occur b/w amino acids.

19. Building the Chain…
First codon recognized is start (AUG-methionine)- aligns the reading frame
ribosome contains two sites for tRNA: an A (acceptor) site and a P (peptide) site.
tRNA (Met- AUG) enters the P site.
the rest of the tRNA enter at the A site and form peptide bonds between the amino acids as the chain forms.

20. Ribosome translocates one codon over and continues the chain until it reaches a stop codon. (UGA, UAG, UAA)
A release factor protein dismantles the complex, releasing the polypeptide chain.
Enzymes may cleave the chain at specific places

24. Protein Synthesis
Once all the triplet codes (Codons) are read and the amino acids are brought into place, the completed protein travels through the ER to the Golgi.
The Golgi modifies the structure and packages it into a vesicle.
Vesicle moves to the membrane and is released by exocytosis.

25. More Homework...
P 241 Q1-9
P 254 Q1-4