Structure and Function. Nucleic Acids Structure and Function.

Importance Determine the order of amino acids in proteins. Two types of nucleic acid DNA RNA

DNA Contains the 5 carbon sugar deoxyribose Four bases: A; G; C; T Purines: A and G Pyramidines: T and C A phosphate group Double helix shape. Bases held together by weak H-bonds

The 5 Carbon Sugar

The Purines

Pyramidines

DNA The 2 nucloetide strands of DNA are anti-parallel. The nucleotides are joined with a phosphodiester bond between the 3’ on one and the 5’ of another.

DNA

Functions Of DNA Carries the genetic code for the proper functioning or organisms. Able to self replicate to continue life Very long – consists of thousands of genes at set places called loci. Each gene codes for a polypeptide chain One or more polypeptides make up proteins.

Functions Of DNA DNA controls the cell by controlling protein synthesis. DNA RNA Polypeptide transcription translation

Packaging of DNA Prokaryote DNA is formed into a ring, and the nucleoid it forms is so small that it can only be seen under an electron microscope. Eukaryote DNA is so long that it has to be packaged. (estimated that each chromosome has about 2 x 108 nucleotide pairs.)

DNA Replication Zygotes have to be able to divide to form multicellular beings, each cell must carry a copy of the original blueprint of the chromosome. For this reason DNA must be able to replicate. It does this during Interphase.

DNA Replication The enzyme Helicase unwinds and unzips the DNA. Both sides of the DNA act as a template to make a new strand. The nucleotides are assembled in the 5’ to 3’ direction using DNA Polymerase. The leading strand is synthesized continuously.

DNA Replication The lagging strand is synthesized in fragments which are later joined. DNA synthesis is “Semi-conservative”. This means that each new DNA is made up of an old strand and a new strand. In cell division these 2 new DNA strands appear as sister chromatids, joined at a centromere.

Semi-conservative Replication

Okazaki Fragments The sides of the DNA molecule are anti-parallel. DNA polymerase can only join new nucleotides to the 3’ end – the leading strand. On the lagging strand small sections of complimentary DNA called Okazaki fragments are synthesized and assembled by DNA polymerase.

Okazaki Fragments These okazaki fragments are joined together into a single DNA strand by the enzyme Ligase.

DNA Replication

Telomeres 10 – 25% of a chromosome is made up of (typically) five to ten short sequences of repeated nucleotides, repeated in series thousands or even millions of times. Much of this repeated DNA is found in the centromere and the tips of the chromosomes.

Telomeres The tip repeat sequences are called Telomeres. These prevent the loss of DNA at the tips during replication. Remember! Okazaki fragments have to fit onto the DNA in sections and can’t do this on the lagging strands when they get to the ends.

Telomeres The telomeres of human chromosomes have 250 – 1500 repeats of the base sequence TTAGGG or AATCCC on the complimentary strand. RNA complex telomerase repairs these end sequences periodically. Not yet clear whether telomeres perform any other function.

Telomeres If they are removed from a chromosome the chromosome disintegrates. Think of telomeres as the protective tips on the end of a shoelace.

DNA Repair Think about mistakes in replication There can be an error rate of 1 in 10 000 base pairs. Need a system of enzymes and proteins that can proof read against the template and repair the DNA if necessary.