Dental Biochemistry 1- (11)

Genetic code and Protein

biosynthesis

1

Ribonucleic acid (RNA)

• RNA is also a polymer of purine and pyrimidine nucleotides linked by

phosphodiester bonds

2

Differences between RNA and DNA DNA RNA

Mostly inside nucleus Mainly seen in cytoplasm

Millions of base pairs Usually 100-5000 bases

Double stranded Generally single stranded

Sugar is deoxyribose Sugar is ribose

Purins: Adenine, Guanine Pyrimidines: Cytosine, Thymine

Purins: Adenine, Guanine Pyrimidines: Cytosine, Uracil

Guanine content is equal to cytosine and adenine is equal to thymine

Guanine content is not equal to cytosine and adenine is not equal to uracil

Alkali resistant Easily destroyed by alkali 3

4

Types of RNA

• Messenger RNA (mRNA).

• Transfer RNA (tRNA) or (sRNA).

• Ribosomal RNA (rRNA).

5

Messenger RNA or mRNA

• It acts as a messenger of the information in the gene in DNA to the protein synthesizing machinery in cytoplasm. It carries the message to be translated to a protein.

• The template strand of DNA is transcribed into a single stranded mRNA. The is accomplished by the DNA dependent RNA polymerase.

• The mRNA is a complementary copy of the template strand of the DNA.

• However, thymine is not present in RNA; instead uracil will be incorporated.

6

Transfer RNA (tRNA) or (sRNA)

• They transfer amino acids from cytoplasm to the ribosomal protein synthesizing machinery; hence the name transfer RNA.

• Since they are easily soluble, they are also referred to as soluble RNA or sRNA.

• They are RNA molecules present in the cytoplasm.

• Each molecule is only 73-93 nucleotides in length; much shorter than mRNA molecules.

7

Transfer RNA

8

Ribosomal RNA (rRNA)

• Ribosomes provide necessary infrastructure for the mRNA, tRNA and amino acids to interact with each other for the translation process.

• Thus, ribosomal assembly is the protein synthesizing machinery.

9

Central Dogma of Molecular Biology: The information available in the DNA is passed to messenger RNA, which is then used for synthesis of a particular protein.

10

Steps of protein synthesis

1. Transcription: • It is taking place all the time. Only certain

areas of the DNA are copied (selected region on the sense strand).

• The genetic information (code) of DNA is transcribed (copied) to the messenger RNA (mRNA).

• During transcription, the message from the DNA is copied in the language of nucleotides

11

2. Translation:

• The mRNA then reaches the cytoplasm where it is translated into functional proteins.

• During translation, the nucleotide sequence is translated to the language of amino acid sequence.

12

13

Inhibitors of RNA synthesis

• Actinomycin D and Mitomycin intercalate with DNA strands, thus blocking transcription. They are used as anticancer drugs.

• Rifampicin is widely used in the treatment of tuberculosis and leprosy.

14

Genetic code

A triplet sequence of nucleotides on the mRNA is the codon for

each amino acid.

15

Salient features of genetic code

• 1. Triplet codons: Each codon is a consecutive sequence of three bases

on the mRNA,

e.g. UUU codes for phenylalanine.

• 2. Non overlapping: The codes are consecutive. Therefore, the starting

points is extremely important. The codes are read one after another in a continuous manner, e.g. AUG, CAU,CAU, GCA, etc.

16

• 3. Non punctuated: There is no punctuation between the codons. It is

consecutive or continuous.

• 4. Degenerate: When an amino acid has more than codon, this called

degeneracy of the code.

E.g. serine has 6 codons while glycine has 4 codons.

• 5. Unambiguous: Through the codons are degenerate, they are

unambiguous: or without any doubtful meaning.

That is, one codon codes only one amino acid. 17

• 6. Universal: The codons are the same for the same amino acid in all

species; the same for “Elephant and E.coli”.

The genetic code has been highly preserved during evolution.

• 7. Terminator codons: There are three codons which do not code for any

particular amino acids. They are “nonsense codons”, more correctly termed as punctuator codons or terminator codons. They put “full stop” to the protein synthesis. These three codons are UAA, UAG, and UGA.

• 8. Initiator codon: • In most of the cases, AUG acts as the initiator codon.

18

Inhibitors of protein synthesis

• The modern medical practice is heavily dependent on the use of antibiotics.

• They generally act only on bacteria and are nontoxic to human beings.

• Some antibiotics act as irreversible inhibitors to bacteria:

• E.g. Streptomycin which causes misreading of mRNA.

• Others act as reversible inhibitors to bacteria

• E.g. Erythromycin prevents translation process 19