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5.3 Transcription and Translation
How does DNA control a cell?
This control is through protein synthesis.
Protein Synthesis requires RNA.
DNA
Sugar Structure
Type of sugar
Nitrogenous
Bases
# of strands
RNA
Pentose (5 C)
Deoxyribose
Ribose
ATCG
AUCG
2
1
What is a gene?
- the section of DNA that codes for a polypeptide
- it is a specific sequence of nitrogenous bases located in
a specific location on the DNA
So why not just copy the gene directly into a protein?
DNA, which holds the code, must remain in the nucleus.
Protein synthesis, which requires ribosomes, takes place
in the cytoplasm.
SO…
mRNA is required (messenger)
What are the two parts of protein synthesis?
1. transcription
2. translation
What is transcription?
mRNA makes a copy of the gene which is the section of
DNA required to make a specific polypeptide.
How Does it happen?
- Helicase unzips the DNA but only a little… just the
distance of one gene
- RNA polymerase moves along one strand making a
single stranded, complementary copy of the nucleotides
required for one gene.
- the mRNA detaches and the DNA rezips itself
What kind of RNA is there?
mRNA – carries a complementary copy of the genetic
code information for one gene from the nucleus to the
ribosome.
tRNA – transfers 1 of the 20 possible amino acids to the
ribosome for polypeptide formation
rRNA – carries the genetic information to make a
ribosome
How does a tRNA know which of the 20 amino acids to
pick?
It reads the codon (AKA triplet) which is a set of 3
nitrogenous bases that codes for a particular amino
acid.
What is translation?
- creation of a polypeptide by a ribosome using the
code from mRNA and individual amino acids from
tRNA
How does translation occur?
- mRNA aligns with a ribosome
- 2 codons fit within the ribosome
- 2 tRNAs attach (whose anticodons are complementary
to the mRNA)
- a condensation reaction joins the 2 a.as creating a
peptide bond
- the first tRNA releases it’s a.a. and leaves
- the ribosome moves down by one codon
- new tRNA attaches…
- keeps going until a STOP codon is reached
One gene, one polypeptide?
Latest theory says one gene creates one mRNA but the
mRNA could be modified to create different
polypeptides.