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Protein Synthesis
If DNA resides in the nucleus, and proteins are made at the ribosomes, how can DNA direct protein production?
cell nucleus
ribosome ?
Summary of Protein Synthesis
DNA
Step 1:
Transcription Step 2:
Translation
a characteristic or trait
RNA
protein
polypeptide chain twists and folds
cell structure and function
amino acid sequence deoxyribonucleic
acid ribonucleic
acid
inside nucleus ribosomes (cytoplasm)
Important Note:
DNA is the cell’s official, permanent source of information. It never leaves the nucleus.
RNA is a temporary ‘copy’ of DNA information that can leave the nucleus. It is referred to as a ‘messenger.’ mRNA
determines
determines
DNA vs. RNA
Deoxyribose sugar
DNA (gene): C A G T T A A C G
RNA (‘copy’): G U C A A U U G C
Ribose sugar
Thymine Uracil
Messenger RNA (mRNA) Carries copies of instructions for making proteins
Ribosomal RNA (rRNA) Makes up ribosomes
Transfer RNA (tRNA) Carries amino acids to the mRNA (at the ribosome)
Step 1: DNA ‘unzips’ with the help of an enzyme.
Step 2: Free RNA nucleotides base pair with one side of DNA.
Transcription
incoming free RNA nucleotides
mRNA forming from gene
completed mRNA, carrying DNA information in the matching sequence of nitrogen bases
DNA re-zips
one gene
Step 3: RNA strand (now called mRNA) detaches from DNA and leaves the nucleus. DNA zips back up.
mRNA
cell nucleus pores
mRNA in cytoplasm, arriving at ribosome
ribosome
one gene
Transcription Review Questions
1. It is said that our DNA makes us unique, but how? What does DNA actually do?
2. What is an amino acid?
3. What does the m in mRNA stand for?
4. Explain the purpose of RNA.
5. mRNA is a temporary ‘copy’ of the DNA gene. Why is the word copy in quotes?
Sequence the following from smallest to largest:
• Cell
• Organ
• Chromosome
• Nucleus
• Nucleotide
• Organism
• Gene
• Nitrogen Base
• Genome
• Carbon
1
2
3
4
5
6
7
8 9
10
C
A, T, C, or G
Phosphate- Sugar- Adenine
Translation (Protein Synthesis – step 2)
• The mRNA ‘copy’ of the gene is used to align amino acids in a specific sequence to make a specific protein.
• Each amino acid is carried by a specific
transfer RNA (tRNA).
ribosome
Translation
base pairing!
This tRNA (UAC) will ONLY carry the amino acid Methionine (Met)
This tRNA (GGG) will ONLY carry the amino acid Proline (Pro)
ribosome
Translation
growing amino acid chain
ribosome
Translation
growing amino acid chain
ribosome
Translation
growing amino acid chain
ribosome
Translation
growing amino acid chain
ribosome
Translation
growing amino acid chain
Step 1: Each tRNA carries a specific amino acid. The tRNA molecules base pair with the mRNA in groups of threes.
• Codon= group of 3 mRNA bases
• Anticodon = group of 3 tRNA bases that base pair to mRNA
Translation
Step 1: Each tRNA carries a specific amino acid. The tRNA molecules base pair with the mRNA in groups of threes.
• Codon= group of 3 mRNA bases
• Anticodon = group of 3 tRNA bases that base pair to mRNA NOTE: there are only 20 amino acids and 64 different tRNA anticodons
•each tRNA can only carry one type of amino acid •one type of amino acid can be carried by more than one type of tRNA
Translation
Translation
Step 2: The amino acids, held in position at the ribosome, join by dehydration synthesis. The new bonds that connect them are called peptide bonds.
Peptide bond
Step 3: Amino acids are added one by one to the growing polypeptide chain. This chain will eventually twist and fold to make a protein. The tRNA molecules are released and will be used again.
Translation
This amino acid chain, once completed, will twist and fold to become a protein.
peptide bond
1) Structural protein 2) Enzyme (control reactions)
Determining amino acid sequence from DNA gene.
T
A
C
T
T
A
G
C
T
G
T
T
DNA A
U
G
A
A
U
C
G
A
C
A
A
mRNA tRNA Amino Acid U
A
C
U
U
A
G
C
U
G
U
U
mRNA translation table
Methionine
Asparagine
Arginine
Glutamine
A
T
G
A
A
T
C
G
A
C
A
A
template Complimentary strand
Start codon – AUG (Met) Stop codons – UGA, UAA, UAG