Nucleic Acids and Protein Synthesis

What are nucleic acids?

Nucleic acids are made up of…1. Nitrogen-containing "base"

There are two type of nitrogenous base present in nucleotides, pyrimidine (one ring) or purine (two rings) which are quite differ from each other in there structures.

(a) PurineThere are two purine base commonly found in nucleic acid, Adenine (A) and guanine (G). Both purine bases bonded with sugar through the N9 of base with C1 of the sugar

(b) Pyrimidine

There are three pyrimidine base, Thymine (T) and cytosine(C) present in DNA and Uracil (U)

2. Five carbon sugar

Two pentose sugars are present in nucleic acids, ribose or deoxyribose sugar. DNA contains deoxyribose sugar while RNA contains ribose sugar. Both sugars are differing only in the presence of one oxygen atom at C2 position.

In any nucleotide, the combination of these two components, a base and sugar is known as a nucleoside. The bonding between nucleosides and phosphoric acid molecules results the formation of nucleotides.

3. A phosphate group

A phosphate group acts as a linking chain between two sugar molecules to make a complete strand of nucleic acid. The oxygen part of phosphate group linked with carbon atom of sugar to form nucleotide. A Nucleotide can also exist in activated forms with two or three phosphates, known as nucleotide diphosphates or triphosphates.

On the basis of sugar, they also termed as deoxyribonucleotide and ribonucleotide

DNA replication• Replication is the process of making a new

DNA molecule from an existing DNA molecule that is identical to the original molecule.

• This process takes place during INTERPHASE

• This process is necessary to produce another molecule exactly the same as itself to ensure that the genetic code is passed on to each new daughter cell formed during cell division.

How does replication take place?

• The process is catalysed by the enzyme DNA polymerase

• The double helix unwinds• The weak hydrogen bonds holding the base pairs

together break, allowing the two strands to part• Each single chain of bases is exposed• Free nucleotides in the cytoplasm become attached to

their matching, exposed base partners• The two daughter DNA molecules each twist to form a

double helix which then winds itself around the histones (proteins), forming a chromosome.

Why are complementary bases important?

• The fact that A will only bond with T and C only with G, makes sure that the sequence of bases in the daughter DNA is exactly the same as in the parent DNA. One DNA double helix therefore becomes two identical double helices.

RNA• Different to DNA in that:– Consists of a single strand– Is much shorter– Sugar is ribose, not deoxyribose– Contains uracil, not thymine

• RNA carries instructions from DNA in the nucleus to the ribosomes in the cytoplasm, where it controls the synthesis of proteins from amino acids

Types of RNA

• Messenger RNA (mRNA)• Transfer RNA (tRNA)• Ribosomal RNA (rRNA)

Protein Synthesis

TRANSCRIPTION1. A small piece of DNA (gene) unwinds and the two strands

separate2. New nucleotides pair up with their complementary bases on

one of the DNA strand (template / pattern strand)3. Nucleotides join up to form a strand of mRNA (sequence is

determined by DNA nucleotides) – Ie – DNA transcribes its genetic code to mRNA– NOTE – Uracil, NOT thymine, will pair with adenine

4. A completed strand of mRNA breaks away form the DNA, which then re-zips.

5. The mRNA moves through the pores of the nuclear membrane and carries the genetic code to the ribosomes (the site of protein synthesis)

Translation

1. The genetic code in DNA is carried as a sequence of ‘code-words’ on the mRNA.

2. Each codeword is made up of three bases called a codon and each codon codes for a single amino acid.

3. Each amino acid, therefore, has a different code of three letters.

4. The order of the codons in mRNA will therefore determine the sequence of the amino acids which, in turn, determines which protein is formed.– A gene is made up of a group of codons that code for the

synthesis of one protein. (Smallest protein = 50 amino acids)

Translation cont…

5. A tRNA picks up a specific amino acid from the cytoplasm and transfers it to a ribosome.

6. The anticodon bases link up to their complementary bases of the codon. THIS IS CALLED TRANSLATION.

7. Catalysed by enzymes, the amino acids link together by means of peptide bonds and the tRNA molecule is released to carry moe of its specific amino acids to the ribosome.

8. In this way the amino acids are linked up to form a specific protein in an order corresponding to the sequence of codons in the mRNA, which, in turn, was coded by the DNA.

What about rRNA?• rRNA is the most common form of RNA in the cell and it,

together with proteins, makes up the ribosomes. • The rRNA moves from codon to codon along the mRNA,

reading the code. rRNA therefore plays an important role in controlling the process of protein synthesis.

DNA Fingerprinting

https://www.youtube.com/watch?v=DbR9xMXuK7c

PCR (polymerase chain reaction)

https://www.youtube.com/watch?v=iQsu3Kz9NYo