Transcription & Translation

Section 5, Chapter 4

3 RNA Molecules

• Messenger RNA (mRNA):

• Transcribed from DNA in nucleus

4

• Transfer RNA (tRNA):

•Translates a codon of MRNA into an amino acid

•Carries amino acids to mRNA

•Anticodons on tRNA are complimentary to codons of mRNA

•• Ribosomal RNA (rRNA):

• Provides structure and enzyme activity for ribosomes

mRNA Molecules

Messenger RNA (mRNA):

•Delivers genetic information from

nucleus to the cytoplasm

• Single polynucleotide chain

DNA

SA

S

U PP

DNA mRNA

• Single polynucleotide chain

•Formed beside a strand of DNA

• RNA nucleotides are complementary

to DNA nucleotides (exception – no

thymine in RNA; replaced with uracil)

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A C T A C T A A C G G A T

A T T G C C T AT G A T G C T A G

G A T C

Step 1. RNA Polymerase attaches to DNA strands

& breaks Hydrogen bonds

Strand 1

RNA

Polymerase

A T T G C C T AT G A T G C T A G

Strand 2

A C T A A

T A C C G G A T

T G A T T

G A T C

Step 2. Strands Separate

A UG G C CU A C U A GRNA

Polymerase

mRNA

A T G G C C T A

T G A T T

C T A G

Replication bubble

Step 3. RNA Polymerase synthesizes mRNA

using DNA strand as a template

A C T A A

T G A T T

T A C C G G A T G A T C

A UG G C CU A C U A G

mRNA

RNA

Polymerase

T G A T T

Step 4. RNA Polymerase releases mRNA

& DNA resumes original structure

A T G G C C T A C T A G

A C T A A

T G A T T

T A C C G G A T G A T C

A T G G C C T A C T A GT G A T T

Step 5. mRNA is undergoes further processing & leaves nucleus

A T G G C C T A C T A G

A UG G C CU A C U A G

mRNA

• Codon = 3 letter sequence that encodes for an amino acid

•All mRNA begin with AUG “Start Codon”

• Codon = 3 letter sequence that encodes for an amino acid

•All mRNA begin with AUG “Start Codon”

A UG G C CU A C U A G

Start Codon

mRNA

Note:

• Codons are redundant - Each amino acid corresponds to more than

one codon

• e.g. UCU, UCC, and UCA all encode for Serine

Note:

• Codons are redundant - Each amino acid corresponds to more than

one codon

• e.g. UCU, UCC, and UCA all encode for Serine

•Start Codon (AUG)

initiates translation

•Stop Codons terminate

translation

•Start Codon (AUG)

initiates translation

•Stop Codons terminate

translation

The codon sequence of mRNA

determines the amino acid sequence

of a protein.

Protein Synthesis

of a protein.

Figure 4.23

tRNA

2. Amino acid

binding site

Clover-leaf shape

RNA with 2

important regions

1. Anticodon

Ribosomes

• Small particle of protein & ribosomal RNA (rRNA)

• Ribosomes have 2 subunits

• Small subunit binds to mRNA

• Small particle of protein & ribosomal RNA (rRNA)

• Ribosomes have 2 subunits

• Small subunit binds to mRNA• Small subunit binds to mRNA

• Large subunit holds tRNA& amino acids

• Small subunit has 2 binding sites for adjacent mRNA codons

• Ribosomes link amino acids by peptide bonds

• Small subunit binds to mRNA

• Large subunit holds tRNA& amino acids

• Small subunit has 2 binding sites for adjacent mRNA codons

• Ribosomes link amino acids by peptide bonds

large subunit

Peptide bond forming

Ribosomes

large subunit

small subunit

Binding sites with codons

anticodons

1. mRNA binds to the small subunit of a Ribosome.

2. The ribosome ‘reads’ the mRNA sequence

3. tRNA brings amino acids to the ribosomes,

aligning their anticodons with mRNA codons

1. mRNA binds to the small subunit of a Ribosome.

2. The ribosome ‘reads’ the mRNA sequence

3. tRNA brings amino acids to the ribosomes,

aligning their anticodons with mRNA codonsaligning their anticodons with mRNA codons

4. The Ribosome links the amino acids together

5. Polypeptide chain lengthens

aligning their anticodons with mRNA codons

4. The Ribosome links the amino acids together

5. Polypeptide chain lengthens

Anchors polypeptide.

tRNA released

TRANSLATIONFigure 4.23

TRANSCRIPTION

After translation Chaperone proteins

fold protein into its configuration

After translation Chaperone proteins

fold protein into its configuration

Enzymes may further modify proteins

after translation = post-translational modification

• Phosphorylation – adding a phosphate to the protein

• Glycosylation – adding a sugar to the protein

Enzymes may further modify proteins

after translation = post-translational modification

• Phosphorylation – adding a phosphate to the protein

• Glycosylation – adding a sugar to the protein

End of Section 5, Chapter 4