The genetic The genetic code is code is used as a used as a blueprint to blueprint to make make proteins.proteins.

We have looked at DNA. But how is We have looked at DNA. But how is this genetic code actually used for this genetic code actually used for anything?anything?

Lets Lets See!See!

Proteins are widely used in cells Proteins are widely used in cells to serve diverse functions. Some to serve diverse functions. Some proteins provide the structural proteins provide the structural support for cells while others support for cells while others act as enzymes to catalyze act as enzymes to catalyze certain reactions.certain reactions.

But, where do But, where do proteins come proteins come from?from?

Since the Since the beginning of beginning of evolution, cells evolution, cells have developed have developed the ability to the ability to synthesize synthesize proteins.proteins.

They can produce new proteins They can produce new proteins either for reproduction or to either for reproduction or to simply replace a degraded one.simply replace a degraded one.

To manufacture proteins, cells To manufacture proteins, cells follow a very systematic procedure follow a very systematic procedure that first that first transcribes DNA into mRNA transcribes DNA into mRNA and then translates the and then translates the mRNA mRNA into chains of amino acids.into chains of amino acids. The amino acid The amino acid

chain then chain then folds folds into into specific specific proteins.proteins.

has only a single strand of has only a single strand of nucleotides instead of two nucleotides instead of two strands strands

contains a different sugar contains a different sugar (ribose instead of (ribose instead of deoxyribose) deoxyribose)

contains the nitrogen base contains the nitrogen base uracil (U) instead of thymine uracil (U) instead of thymine (T) (T)

(Transcribed is a fancy way of (Transcribed is a fancy way of saying “copied.”)saying “copied.”)

RNA is very similar to DNA except RNA is very similar to DNA except that it:that it:

• Messenger RNA (mRNA)Messenger RNA (mRNA) copies DNA’s code & carries copies DNA’s code & carries the genetic information to the the genetic information to the ribosomesribosomes

• Ribosomal RNA (rRNA),Ribosomal RNA (rRNA), along along with protein, makes up the with protein, makes up the ribosomesribosomes

• Transfer RNA (tRNA)Transfer RNA (tRNA) transfers transfers amino acids to the ribosomes amino acids to the ribosomes where proteins are where proteins are synthesizedsynthesized

The “m” in “mRNA” The “m” in “mRNA” stands for stands for “messenger,” “messenger,” because mRNA because mRNA copies genetic copies genetic information from information from DNA (which is found DNA (which is found in the nucleus) and in the nucleus) and carries it to another carries it to another part of the cell (the part of the cell (the ribosomes).ribosomes).

Codon Codon Codon Codon Codon Codon

O.K., we’ve gone

O.K., we’ve gone from DNA to

from DNA to mRNA. How do

mRNA. How do proteins fit into all

proteins fit into all this?this?

Think of the Think of the genetic code the genetic code the mRNA is carrying mRNA is carrying as a series of as a series of three-letter three-letter “words.” Each of “words.” Each of these three-letter these three-letter words is called a words is called a codoncodon..

Different codons Different codons code for different code for different amino acids. For amino acids. For example, the codon example, the codon for the amino acid for the amino acid methionine is AUG methionine is AUG (adenine, uracil, (adenine, uracil, guanine). The codon guanine). The codon for the amino acid for the amino acid serine is UCA (uracil, serine is UCA (uracil, cytosine, adenine).cytosine, adenine).

Another type of RNA, Another type of RNA, called tRNA (“t” stands called tRNA (“t” stands for “transfer”) matches for “transfer”) matches the codons in mRNA to the codons in mRNA to the correct amino acids.the correct amino acids.

As the mRNA strand moves along As the mRNA strand moves along the ribosome, the amino the ribosome, the amino acids are joined in the acids are joined in the correct sequence to correct sequence to form a protein. This form a protein. This process is called process is called translationtranslation..

C C U

Proin

e

U C G

Ser

ine

U C C

Tryp

toph

an

G G A

Pro

ine

Codon Codon Codon Codon CodonCodon

U C G

Ser

ine

G U U

Valin

e

G

The diagram below shows a codon chart. A The diagram below shows a codon chart. A codon chart shows which codons code for codon chart shows which codons code for which amino acids.which amino acids.

Suppose a DNA Suppose a DNA mutation led to mutation led to a change in a a change in a single mRNA single mRNA codon. Now codon. Now suppose this suppose this codon changed codon changed from GCC to from GCC to GCG.GCG.

By looking at the codon chart, you can see By looking at the codon chart, you can see that both of these codons code for the that both of these codons code for the amino acid alanine. So even though the amino acid alanine. So even though the DNA and mRNA have changed, there is no DNA and mRNA have changed, there is no change in the protein!change in the protein!

This chart shows the amino acids coded for This chart shows the amino acids coded for by each of the 64 possible mRNA codons. To by each of the 64 possible mRNA codons. To find which amino acid the codon CAA codes find which amino acid the codon CAA codes for, follow these steps. (1) Look on the left for, follow these steps. (1) Look on the left side of the chart to find the large row of side of the chart to find the large row of codons that begin with C. (2) Move across codons that begin with C. (2) Move across this row until you get to the column of this row until you get to the column of codons whose second codons whose second base is A. (3) Move base is A. (3) Move down this column until down this column until you get to the row of you get to the row of codons whose third codons whose third base is A. The codon base is A. The codon CAA codes for the CAA codes for the amino acid glutamine.amino acid glutamine.