Assertion: Genetic code shows code in mrna not in dna
Reason: dna is present inside the nucleus and code is read from mrna inside the cell
Answers
Answer:
Have you ever written a secret message to one of your friends? If so, you may have used some kind of code to keep the message hidden. For instance, you may have replaced the letters of the word with numbers or symbols, following a particular set of rules. In order for your friend on the other end to understand the message, he or she would need to know the code and apply the same set of rules, in reverse, to figure out what you had written.
As it turns out, decoding messages is also a key step in gene expression, the process in which information from a gene is used to construct a protein (or other functional product). How are the instructions for building a protein encoded in DNA, and how are they deciphered by the cell? In this article, we'll take a closer look at the genetic code, which allows DNA and RNA nucleotide sequences to be translated into the amino acids they represent.
Overview: Gene expression and the genetic code
Genes that provide instructions for proteins are expressed in a two-step process.
In transcription, the DNA sequence of a gene is "rewritten" using RNA nucleotides. 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 or protein subunit).
Cells decode mRNAs by reading their nucleotides in groups of three, called codons. Each codon specifies a particular amino acid, or, in some cases, provides a "stop" signal that ends translation. In addition, the codon AUG has a special role, serving as the start codon where translation begins. The complete set of correspondences between codons and amino acids (or stop signals) is known as the genetic code. [Codon table]
The mRNA sequence is:
5'-AUGAUCUCGUAA-5'
Translation involves reading the mRNA nucleotides in groups of three, each of which specifies and amino acid (or provides a stop signal indicating that translation is finished).
3'-AUG AUC UCG UAA-5'
AUG $\rightarrow$ Methionine
AUC $\rightarrow$ Isoleucine
UCG $\rightarrow$ Serine
UAA $\rightarrow$ "Stop"
Polypeptide sequence: (N-terminus) Methionine-Isoleucine-Serine (C-terminus)
The mRNA sequence is:
5'-AUGAUCUCGUAA-5'
Translation involves reading the mRNA nucleotides in groups of three, each of which specifies and amino acid (or provides a stop signal indicating that translation is finished).
3'-AUG AUC UCG UAA-5'
AUG \rightarrow→right arrow Methionine AUC \rightarrow→right arrow Isoleucine UCG \rightarrow→right arrow Serine UAA \rightarrow→right arrow "Stop"
Polypeptide sequence: (N-terminus) Methionine-Isoleucine-Serine (C-terminus)
In the rest of this article, we'll more closely at the genetic code. First, we'll see how it was discovered. Then, we'll look more deeply at its properties, seeing how it can be used to predict the polypeptide encoded by an mRNA.
Code crackers: How the genetic code was discovered
To crack the genetic code, researchers needed to figure out how sequences of nucleotides in a DNA or RNA molecule could encode the sequence of amino acids in a polypeptide.
Why was this a tricky problem? In one of the simplest potential codes, each nucleotide in an DNA or RNA molecule might correspond to one amino acid in a polypeptide. However, this code cannot actually work, because there are 202020 amino acids commonly found in proteins and just 444 nucleotide bases in DNA or RNA. Thus, researchers knew that the code must involve something more complex than a one-to-one matching of nucleotides and amino acids.
Answer:
Both assertion and reason are correct and reason is the correct explanation of the assertion.
Explanation:
- A cell "reads" the information from a messenger RNA (mRNA) during translation and uses it to create a protein.
- RNA nucleotides (As, Us, Cs, and Gs) are read in groups of three in an mRNA to provide instructions for constructing a polypeptide. Codons are these three-membered groups.
- Each of the 616161 codons for amino acids can be "read" to specify one of the 202020 amino acids that are most frequently found in proteins. The amino acid methionine is designated by one codon, AUG, which also serves as a start codon to indicate the beginning of protein synthesis.
- Three further codons do not specify any other amino acids. The UAA, UAG, and UGA end codons alert the cell that a polypeptide is finished.
- Because it enables cells to "decode" an mRNA into a chain of amino acids, this collection of codon-amino acid associations is known as the genetic code.
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