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