Section 5, Chapter 4

Transcription & Translation

Transcription

RNA synthesis from DNA template

RNA molecule

1. single-stranded nucleic acid

2. sugar phosphate backbone

3. sugar = ribose

4. Uracil (U) replaces Thymine (T)U & A = complimentary base pair

Properties of RNA

4

There are several kinds of RNA

Transfer RNA (tRNA):Transfers amino acids to the ribosomes during translation.

Ribosomal RNA (rRNA):Provides structure and enzyme activity for ribosomes

Messenger RNA (mRNA): Conveys genetic information from DNA to the ribosomes

Messenger RNA (mRNA):

Delivers genetic information from the DNA inside the nucleus to the cytoplasm

mRNA is formed beside a strand of DNA

RNA nucleotides are complementary to DNA nucleotides with one exception – no thymine in RNA; replaced with uracil)

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DNA

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mRNA

Transcription

1. Synthesis of messenger RNA (mRNA) from DNA template

2. Occurs in nucleus

3. Only 1 of the DNA strands is transcribed

4. Transcription is catalyzed by RNA Polymerase

5. mRNA undergoes further processing & leaves the nucleus

Properties of mRNA

A Codon is the 3 letter nucleotide sequence of mRNA that encodes for 1 amino acid.

AUG is the first codon in protein synthesis, so it’s it’s called the start codon

The codon sequence of mRNA determines the amino acid sequence of a protein.

Figure 4.23

Protein Synthesis

Examples of Codons

amino acid

The start codon marks the site at which translation into protein sequence begins, and the stop codon marks the site at which translation ends.

Translation = Ribosomes synthesize proteins by reading the codons of the mRNA transcript.

1. Occurs on Ribosomes in cytoplasm

2. transfer RNA (tRNA) transports amino acids to mRNA

3. anticodons on tRNA align with codons on mRNA

tRNA

Anticodon

Amino acid binding site

Clover-leaf shape RNA with 2 important regions

Small particle of protein & ribosomal RNA (rRNA)

Ribosomes have 2 subunitsLarge subunit holds tRNA & amino acidsSmall subunit binds to mRNA

Small subunit has 2 binding sites for adjacent mRNA codons

Ribosomes link amino acids by peptide bonds

Ribosomes

large subunit

small subunit

Binding sites with codons

anticodons

Peptide bond forming

Ribosomes

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

4. The Ribosome links the amino acids together

5. Polypeptide chain lengthens

Sequence of Translation

Translation- Figure 4.24

Anchors polypeptide.

Translation

tRNA released

TRANSCRIPTION

Figure 4.23 overview of protein synthesis

Once translation is complete chaperone proteins fold the protein into its configuration

enzymes may further modify proteins after translation phosphorylation – adding a phosphate to the protein

glycosylation – adding a sugar to the protein

post-translational modification

End of Chapter 4