RNA Structure and Function

Ribonucleic acid (RNA) also consists of nucleotides, but these nucleotides contain the sugar ribose.

 

RNA has A,C,G and U. 

f11-08_structure_of_rna_c.jpg

 

Although both RNA and DNA are nucleic acids, there are key differences in the structure and function of RNA and DNA. 

 

There are three types of RNA and each is involved in protein synthesis.

Protein synthesis is the process in which the correct amino acids are connected together in the order that is written on the gene.

 

1. Messenger RNA

 

Messenger RNA (mRNA) is produced in the nucleus by a process called transcription.

 

Messenger RNA carries genetic information from DNA to the cytoplasm where the amino acids will be connected together.

 

2. Transfer RNA

Transfer RNA (tRNA) is a carrier molecule for amino acids, delivering them to the site of protein synthesis.

 

 f11-13a_trna_structure__c.jpg

As there are 20 different amino acids, there are also 20 different types of tRNA.

 

3. Ribosomal RNA

The two ribosomal RNA (rRNA) subunits join with proteins in the cytoplasm to form the subunits of ribosomes.

 f11-13c_trna_structure__c.jpg

Ribosomes help to build proteins

11.2 Gene Expression

DNA and RNA are involved in the synthesis of proteins. 

 

The genes in DNA contain the instructions for the amino acid sequence of a protein. 

 

Since enzymes are proteins, an error in the gene for that enzyme could render the enzyme non-functional. 

 

If an enzyme is defective, then the cell may not be able to perform a chemical reaction. 

 

This can create inborn errors in metabolism that lead to metabolic diseases.

 

Structure and Function of Proteins

Proteins differ in the number and sequence of amino acids. 

 

This sequence of amino acids gives each protein a unique shape and function.

 

From DNA to RNA to Protein

In order to synthesize a protein, the genetic information in the DNA must be converted to an amino acid sequence.  Its similar to the way that someone needs to read a recipe and mix the correct ingredients together.

 

Step 1- Transcription involves the synthesis of mRNA from template DNA. Think of this step as making a RNA photocopy of a recipe from your DNA cookbook

 

Step 2- During translation, the correct amino acids are dropped off by tRNA's according to the instructions written on the mRNA.

 

summary11-from_dna_to_r_c.jpg

 

The Genetic Code

Within a gene, information for the amino acid sequence of a protein is encoded in a triplet code. The triplet code means that three bases are read at a time and each three bases stand for one of the twenty amino acids.

Imagine if you had a recipe that used 3 letter abbreviations for the ingredients water, sugar and flour. It might look like this: WATSUGFLO 

 

This triplet code is transcribed into the codons in mRNA. Each codon specifies one of the twenty different amino acids.

 

These codons provide redundant sequences for the placement of amino acids in a protein. 

f11-10_messenger_rna_co_c.jpg

Transcription

During transcription, a strand of mRNA is formed that is complementary to the sequence within the DNA. 

 

Transcription begins when RNA polymerase binds to the DNA promoter for a gene. 

 

The DNA helix is unwound and the primary mRNA strand is made by RNA polymerase.

 

The primary mRNA strand is then processed to remove introns.

 

The remaining sequence of genetic information, the exons, are retained in the mature mRNA for protein synthesis. 

Exons contain the information to build a protein, introns are just extra DNA- like advertisements in a cooking magazine. 

 

 

f11-11_transcription_to_c.jpg

Translation: An Overview

The translation of a mature mRNA into proteins requires several enzymes, tRNA, and rRNA.

 

The tRNA is a single-stranded RNA molecule with an amino acid bound to one end and an anticodon on the other end.

 

The anticodon is complementary to the corresponding mRNA codon and can temporarily match up to it. This is how the tRNA knows when to drop off its amino acid when a protein is being made.

 

As a mature mRNA moves to a ribosome, the sequence of codons in the mRNA determines the sequence of anticodons that can match up.

 

The order of the anticodons of the tRNA molecules determines the order of the amino acids of the protein that is being made. 

 

 

 

 

 Toggle open/close quiz question

Which of the following lists are needed for a protein to be made?
    a.DNA polymerase, mRNA, mutation
    b.codon, ribosome, DNA polymerase
    c.tRNA, gene, ligase
    d.a gene, RNA polymerase, tRNA, ribosome