1. Protein Synthesis
Chapter 10

2. From DNA to RNA Single strand Contains ribose sugar
Double helix
Contains deoxyribose sugar
Contains the bases:
Guanine
Cytosine
Adenine
Thymine
Single strand
Contains ribose sugar
Contains the bases:
Guanine
Cytosine
Adenine
Uracil

3. Three Types of RNA Messenger RNA (mRNA) Transfer RNA (tRNA)
Copies the DNA nucleotides from a gene and carries this “message” to the ribosomes
Transfer RNA (tRNA)
Transports amino acids to the mRNA once it is attached to the ribosomes
Ribosomal RNA (rRNA)
Used to manufacture ribosomes
Contains enzymes for forming peptide bonds between the amino acids brought by the tRNA to the ribosomes

4. Protein Synthesis Protein synthesis is also known as “gene expression”
This is because DNA contains nucleotide bases in a specific order that serves as a recipe for putting amino acids together in the correct sequence for each individual
A segment of DNA that tells the cell how to make a polypeptide (small protein) is a gene.
The function of a protein is controlled from the beginning by the order of the amino acids that it contains.

5. Protein Synthesis
Protein synthesis is divided into two groups of reactions:
Transcription
The process by which the mRNA copies the nucleotide sequence of one gene on the DNA
Translation
The process by which tRNA delivers amino acids to the mRNA attached to a ribosome in order to assemble the protein (polypeptide)

6. Protein Synthesis: Transcription
Step 1
RNA polymerase binds to a promoter, a specific sequence on the DNA that marks the beginning of a gene
The promoter acts as a “start” signal for transcription
Step 2
The enzyme, RNA polymerase, unwinds the DNA and breaks the hydrogen bonds between the base pairs of the gene to be copied

7. Transcription continued
Step 3
RNA polymerase adds mRNA nucleotides in a 5’  3’ direction along the DNA according to the base-pairing rules
Exception: mRNA contains no thymine; therefore, uracil from mRNA bonds to the adenine in DNA
RNA polymerase reaches a sequence at the end of the gene that signals it to stop transcribing the DNA
The mRNA is now released from the DNA
The DNA goes back together and rewinds

8. Transcription vs. Replication
Only one side of DNA is transcribed
Purpose is to copy a gene in order to make a protein
Accomplished by RNA polymerase
Both sides of DNA are replicated
Purpose is to make a complete copy of the DNA for cell division
Accomplished by DNA polymerase and others

9. mRNA Processing
Before mRNA leaves the nucleus, it must be protected from enzymes in the cytosol that will destroy it
A small string of nucleotides that all contain the base guanine is attached to the 5’ end
This is known as the G-cap
A long string of nucleotides that all contain the base adenine is attached to the 3’ end
This is known as the poly-A tail

10. mRNA Processing
In addition to protecting the mRNA from enzymatic destruction, the G-cap and poly-A tail also help to:
Pull the mRNA transcript out of the nucleus &
Attach the mRNA transcript to a ribosome

11. mRNA Processing Not all of our DNA contains codes for amino acids
Some of our DNA contains sequences known as introns (interfering sequences) that are not part of the gene
These introns are unique to each person
There are enzymes that must cut out the introns before the mRNA leaves the nucleus
These enzymes that remove the introns from the DNA form a spliceosome

12. mRNA Processing: Spliceosome

13. mRNA Processing The remaining mRNA transcript consists of exons
Exons are the portions of the mRNA that must be EXPRESSED (translated into amino acids)
When the introns are removed, the exons are spliced together to make a complete mRNA transcript of the gene that is to be translated
Once the introns are removed and the exons are spliced together, the mRNA is ready to leave the nucleus

14. mRNA Processing