Nucleic Acids & DNA replication Revision

Nucleic acids Polymers of monomers called nucleotides. pentose sugar phosphate nitrogenous base

DNA nucleotides 2 types of bases pyrimidines - single ring structures and smaller than; purines - double ring structure Pyrimidines - big name, smaller molecule C cYtosine T thYmine Purines - small name big molecule G guanine A adenine.

polynucleotides nucleotides join together by bond formation between sugar of one and phosphate of the next to form a polynucleotide bond formed by condensation is a sugar-phosphate bond. sugar-phosphate bond

DNA double helix DNA molecules exist as a double helix 2 antiparallel polynucleotide strands held together by hydrogen bonds between complementary base pairs The four DNA bases A C T and G form base pairs such that; A can pair with T A=T C can pair with G C≡G i.e a purine with a pyrimidine.

DNA double helix A G C T A G C T complementary base pair twisted to form a double helix

RNA 3 types rRNA ribosomal tRNA transfer - used in translation to transport specific amino acids mRNA messenger – formed by transcription.

RNA nucleotides contain ribose not deoxyribose has Uracil (U) not Thymine (U is a pyrimidine) molecules are single not double stranded. ribose PiPi A U C or G

DNA replication Before a cell divides by mitosis or meiosis, the DNA in the nucleus is copied or replicated Before the mechanism of DNA replication was known in full, two hypotheses were put forward.

DNA replication Conservative replication This would involve a completely new double stranded copy of DNA being made and the original or parental DNA staying the same (being conserved).

KEY Parental DNA Newly synthesised DNA DNA double helix copied as a whole 1 new and 1 parental DNA molecule Conservative replication

DNA replication Semi-conservative replication This would involve separation of the DNA strands followed by replication of each strand separately to produce DNA half parental and half new.

KEY Parental DNA Newly synthesised DNA DNA strands unwind new strands are synthesised 2 hybrid DNA molecules produced Semi-conservative replication

Experimental evidence Meselson & Stahl designed an experiment to determine the mechanism of DNA replication. They grew E.coli on a medium containing the heavy isotope of nitrogen 15 N for many generations, so ‘all’ the DNA was ‘labelled’ with heavy N. The bacteria was transferred to a medium containing the lighter isotope of nitrogen 14 N for 1 or 2 or 3 generations.

Experimental evidence After 1, 2 or 3 generations, the cells were killed, the DNA extracted and subjected to density gradient centrifugation. DNA samples sink to a point in the centrifuge tube equal to their own density. 15 N DNA will sink further than 14 N DNA The samples are made visible under UV light. 14 N 15 N DNA sample

Results 14 N 15 N 14 N 15 N 14 N 15 N after 1 generation after 2 generations after 3 generations

KEY Parental DNA ( 15 N) Newly synthesised DNA ( 14 N) P Meselson & Stahl’s experiments demonstrate that DNA replication is semi-conservative.

DNA replication DNA double helix unzips as hydrogen bonds between complementary bases break and the two polynucleotide strands separate A G C T A G C T

DNA replication With each strand acting as a template, free DNA nucleotides complementary base pair to the exposed bases on each strand DNA polymerase forms sugar-phosphate bonds between nucleotides The molecules rewind to form double helices A G C T A G C T AGCT AGCT

DNA replication Now there are 2 DNA molecules, each one consists of one strand from the original parental molecule and one newly synthesised strand. They are formed by semi-conservative replication A G C T A G C T A G C T A G C T