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DNA and Protein Synthesis Notes
We have already referred to DNA as a code or blueprint for living things. More specifically it is a
code for making Proteins.
The Hypothesis, “1 Gene= 1 Polypeptide” illustrates the fact that every gene located on a
chromosome “codes” for 1 polypeptide or protein.
If polypeptides are strings of proteins and proteins are strings of amino acids, then DNA
determines the sequence (or order) of the amino acids.
Protein Synthesis – Simply the manufacturing of proteins.
We know that the assembly of proteins occurs in the Ribosomes, but if the blueprint for proteins
is inside the nucleus on DNA, then how do the ribosomes know what to do?
The actual transfer and assembly of proteins is carried out by the nucleic acid RNA. RNA
(Ribonucleic Acid) is similar to DNA but instead of being double stranded, it is
only single stranded.
It consists of a Pentose sugar (ribose), a Phosphate, and a Nitrogenous Base. The bases
offer one of the biggest differences.
Purines are the same with RNA = Adenine and Guanine
Pyrimidines are different – Uracil(instead of thymine) and cytosine.
A binds with T
C binds with G
The process of protein synthesis:
mRNA (messenger RNA) carries the instructions from the DNA to the ribosomes.
tRNA ( transfer RNA) transports the amino acids to ribosomes in their correct order.
rRNA ( ribosomalRNA) helps to make up the structure of the ribosome.
The process of transferring information from DNA to mRNA is called Transcription.
During the process of transcription, the following happens:
1. A particular portion of a DNA strand begins to unwind.
2. The enzyme RNA Polymerase begins to match up base pairs.
mRNA
A binds with T
U binds with A
G binds with C
C binds with G
Exposed parent
strand of DNA
G
U
A
G
A
T
C
3.
4.
5.
6.
These new pairs begin to form a strand of mRNA.
The mRNA leaves the nucleus through the Nuclear Pore
The original DNA winds back up.
The mRNA goes to a ribosome and binds to it. (the organelle that actually synthesizes
the protein)
7. The codons on the mRNA bind with Anti-codon found on tRNA.
8. tRNA then goes and finds the corresponding Amino Acid and brings them to the
ribosome to be synthesized into protein.
9. The process of converting codons into anticodons and then amino acids is called
Translation.
More specifically, the mRNA creates codes for the proteins from DNA. These codes
occur in sequences of 3 . For example, 3 U’s in a row codes as UUU and equals the
amino acid Penylalanine. The letter code is called a Codon.
With the letters available – U, A, G, and C; there are 64 possible combinations.
There are only 20 amino acids that occur in nature. This means that some amino acids
may have more than 1 code.
AUG = the amino acid Methionine = the universal start codon. This tells a ribosome
when to start making proteins.
There are also STOP Codons that instruct a ribosome to stop protein synthesis.
These include: UGA, UAA & UAG
The corresponding letters on the tRNA are called Anti-codons.