Aso
SAA
codon = 2 4 Alucleotide
8 codon - 8 Amino Acids
Homework ?
2) If mRNA. contain total 198 Nucleotides
One Stop codon at ath position
then how many
Amino acids well be
found in Resulting Protein
0
198
STO
cod
&codon
(8 Amino
crid)
3
UA,
Answers
Answer:
Genes that provide instructions for proteins are expressed in a two-step process.
In transcription, the DNA sequence of a gene is "rewritten" in RNA. In eukaryotes, the RNA must go through additional processing steps to become a messenger RNA, or mRNA.
In translation, the sequence of nucleotides in the mRNA is "translated" into a sequence of amino acids in a polypeptide (protein chain).
If this is a new concept for you, you may want to learn more by watching Sal's video on transcription and translation.
Codons
Cells decode mRNAs by reading their nucleotides in groups of three, called codons. Here are some features of codons:
Most codons specify an amino acid
Three "stop" codons mark the end of a protein
One "start" codon, AUG, marks the beginning of a protein and also encodes the amino acid methionine
Codons in an mRNA are read during translation, beginning with a start codon and continuing until a stop codon is reached. mRNA codons are read from 5' to 3' , and they specify the order of amino acids in a protein from N-terminus (methionine) to C-terminus.
What do 5' and 3' mean?What are the N- and C-terminus?
The genetic code table
The full set of relationships between codons and amino acids (or stop signals) is called the genetic code. The genetic code is often summarized in a table. How do you read the codon table?
Image credit: "The genetic code," by OpenStax College, Biology (CC BY 3.0).
Notice that many amino acids are represented in the table by more than one codon. For instance, there are six different ways to "write" leucine in the language of mRNA (see if you can find all six).
An important point about the genetic code is that it's universal. That is, with minor exceptions, virtually all species (from bacteria to you!) use the genetic code shown above for protein synthesis.
Reading frame
To reliably get from an mRNA to a protein, we need one more concept: that of reading frame. Reading frame determines how the mRNA sequence is divided up into codons during translation.
That's a pretty abstract concept, so let's look at an example to understand it better. The mRNA below can encode three totally different proteins, depending on the frame in which it's read:
So, how does a cell know which of these protein to make? The start codon is the key signal. Because translation begins at the start codon and continues in successive groups of three, the position of the start codon ensures that the mRNA is read in the correct frame (in the example above, in Frame 3).
Mutations (changes in DNA) that insert or delete one or two nucleotides can change the reading frame, causing an incorrect protein to be produced "downstream" of the mutation site:
_Image credit; "The genetic code: Figure 3," by OpenStax College, Biology, CC BY 4.0._