DNA and Protein Synthesis

DNA:

• double polynucleotide chain ; double helix structure
• largest and most stable molecule of the three
• found mostly in the nucleus and quantity is constant for all cells of a species

mRNA:

• single polynucleotide chain ; single helix
• smaller than DNA, but larger than tRNA ; the least stable molecule
• manufactured in the nucleus but found throughout the cell ; quantity varies from cell to cell
• pentose sugar = ribose ; bases = A U C G

tRNA:

• single polynucleotide chain ; clover leaf shaped
• smallest molecule ; more stable than mRNA but less than DNA
• pentose sugar = ribose ; bases = A U C G
• manufactured in the nucleus but found throughout the cell ; quantity varies from cell to cell

Transcription: the process of making pre-mRNA, using part of the DNA as a template.

1 . DNA helicase unwinds the DNA double helix at the gene to be coded for and exposes the bases.

2. RNA polymerase lines up free RNA nucleotides alongside the exposed DNA bases on the template strand, and the free bases are attracted to their complementary pairs, replacing T with U. RNA polymerase bonds the nucleotides together creating a strand of mRNA.

3. RNA polymerase moves along the DNA, separating the strands and assembling the mRNA strand.

4. When the RNA polymerase reaches a particular sequences on the DNA strand called a stop codon, it stops making pre-mRNA.

5. Splicing occurs where the introns are removed and the exons are bonded together. The mRNA molecule leaves the nucleus.

Translation: the process of joining amino acids together to create a polypeptide chain.

1. The mRNA attaches itself to a ribosome.

2. A tRNA molecule carrying an amino acid, with an anticodon complemenatry to the first codon on the mRNA, attaches itself to the mRNA by specific base pairing.

3. A second tRNA attaches itself to the next codon on the mRNA in the same way.

4. The two amino acids attached to the tRNA are joined by a peptide bond and the tRNA moves away, leaving behind the amino acid.

5. This process continues producing a chain of linked amino acids until theres a stop codon.

6. The polypeptide chain moves away from the ribosome.