Polynucleotides: DNA and RNA Structure and function http://vega.org.uk/video/programme/80

State that DNA is a polynucleotide, usually double-stranded, made up of nucleotides containing the bases adenine, thymine, cytosine and guanine. State that RNA is a polynucleotide, usually single-stranded, made up of nucleotides containing the bases adenine, uracil, cytosine and guanine.

Structure diagrams of phosphate, ribose and deoxyribose sugars attached to nitrogenous bases

Structure of DNA and RNA Basic Unit = NUCLEOTIDE A nucleotide consists of:- PENTOSE sugar molecule In DNA this is DEOXYRIBOSE In RNA the pentose sugar is RIBOSE PHOSPHATE Nitrogen-containing BASE: a purine or pyrimidine

There are FOUR different bases in DNA Making four different types of NUCLEOTIDE :

Purine and pyrimidine molecules

POLYNUCLEOTIDES Within a DNA or RNA Molecule Nucleotides join together, to form polymers, called POLYNUCLEOTIDES Bases ‘sticking out’ to one side Sugar-phosphate backbone

Nitrogenous bases in DNA and RNA Purines Pyrimidines Number of rings Names of bases Found in DNA? Found in RNA?

State that DNA is a double-stranded polynucleotide. Describe how a DNA molecule is formed by hydrogen bonding between complementary base pairs on two antiparallel DNA strands. Explain how twisting of the DNA molecule produces the double helix shape.

TWO Polynucleotide strands Each DNA Molecule actually contains TWO Polynucleotide strands NOTE : ‘Base-pairing’ rule A - T G - C Hydrogen bonds Strands are ‘anti-parallel’

So, a section of a DNA molecule

Furthermore, The two strands are wound into a HELIX The famous ‘DOUBLE HELIX’

Outline how DNA replicates semi-conservatively, with reference to the role of DNA polymerase.

DNA REPLICATION DNA-HELICASE Step 1 Double helix unwinds Hydrogen bonds break and strands separate Step 2 Under control of the enzyme : DNA-HELICASE

Nucleotides in the cell are activated, by combining with TWO PHOSPHATES : Step 3

Activated nucleotides attach to ‘spare’ bases on each strand of DNA - according to the ‘base-pairing’ rule. Step 4 Under the control of the enzyme : DNA-POLYMERASE Result : TWO DNA molecules identical to each other and the original.

SEMI-CONSERVATIVE What does this mean? This type of replication is described as : SEMI-CONSERVATIVE What does this mean?

Would act as a template to make a complete copy If replication was CONSERVATIVE The original molecule Would act as a template to make a complete copy

However, SEMI-CONSERVATIVE Means only HALF the original molecule appears in each of the new molecules >>>>>>>>>>>

State that a gene is a sequence of DNA nucleotides that codes for a polypeptide. Outline the roles of DNA and RNA in the cells of living organisms.