DNA – Deoxyribonucleic acid

Importance of DNA DNA is the nucleic acid molecule that governs the

processes of heredity of all plants and animal cells. RNA (ribonucleic acid) is a nucleic acid that plays a

role in gene expression and protein synthesis Both DNA and RNA share a similar structure – the

sugar component of DNA is a 5-carbon sugar (deoxyribose) and RNA has a 3-carbon sugar (ribose)

Structure of DNA DNA is made up of subunits which scientists called nucleotides. Each nucleotide is made up of a sugar, a phosphate and a base. There are 4 different bases in a DNA molecule:

adenine (a purine) cytosine (a pyrimidine) guanine (a purine) thymine (a pyrimidine)

The number of purine bases equals the number of pyrimidine bases The number of adenine (A) bases equals the number of thymine (T) bases The number of guanine (G) bases equals the number of cytosine (C) bases The basic structure of the DNA molecule is helical, with the bases being stacked on

top of each other A-T and G-C are called complementary base pairs which are held together by

hydrogen bonds. You can always tell the base sequence on one strand as the strands are antiparallel,

that is, the phosphate bridges run in opposite directions in the two strands. Each end of a double-stranded DNA molecule contains the 5’ end of one strand and the 3’ end of the complementary strand.

Chargaff’s Rule•The amount of adenine and thymine were the same.

•The amount of guanine and cytosine were the same.

•The constant relationship is known as the Chargaff’s Rule

REPLICATION OF DNA

DNA replication Is the process of creating an exact copy of a

molecule of DNA This replication occurs during the S phase of

interphase of the cell cycle Each of the daughter cells requires a

complete set of genetic information that can only be made from the original DNA molecule

Semi-Conservative Replication

*Each strand, after unwinding, serves as a template for the creation of a complementary strand.

*Each new molecule of DNA contains one strand of the original complementary DNA molecule and one new parent strand.

*Thus, each new DNA molecule conserves half of the original molecule.

DNA polymerase functions Used as a starting point for DNA replication –

leading strand and nucleotide addition Used in nucleotide addition on lagging strand Proofreads each nucleotide addition

If there is a mismatch between bases, can excise the incorrect base and add the correct base using the parent strand as a template.

HELICASES: bind to the DNA at the replication origin, where they unravel the DNA into two Y-shaped areas.

A short RNA segment, called a PRIMER, serves as the starting point for the attachment of new nucleotides.

PRIMASE constructs an RNA primer which is used on the lagging strand to create a starting point for DNA replication.

The segments are spliced together by an enzyme called DNA LIGASE.

OTHER USEFUL ENZYMES

Strand differentiation On one strand, DNA polymerase begins to add nucleotides in

the 5’ to 3’ end – on the leading strand. The other strand, the lagging strand, uses DNA polymerase as

well, yet, is replicated in short segments called Okazaki fragments.

Once the primer is in place, DNA polymerase extends each fragment by adding new nucleotides.

Termination of DNA replication The replication machine, which includes

polymerase, primase, ligase and helicase and the coordinated efforts of enzymes, continues until the entire strand of DNA is replicated.

The completion of the new DNA strand and the dismantling of the replication machine is called termination.