AS Biology - Nucleic acids

AS Biology - Nucleic acids
Covering:
Nucleotides
DNA
DNA replication
Protein synthesis

Transcript

1. Nucleic Acids
2. Nucleotides Basic units which make up most of the important chemicals in all organisms
3. Nucleotide structure Three constant parts: • 1 pentose sugar • 1 nitrogenous base • 1 phosphate group DNA pentose sugar – Deoxyribose RNA pentose sugar – Ribose 5 bases: • Adenine • Guanine • Cytosine • Thymine • Uracil (Instead of thymine in RNA) Base types: • Purines (Adenine and Guanine) • Pyrimidines (Cytosine and thymine/Uracil)
4. Polynucleotides • A chain of nucleotides joined together by a condensation reaction between the pentose sugar of one nucleotide and the phosphate group of another nucleotide • The bond formed by this reaction is called a phosphodiester bond • Sugar and phosphate groups create the chains backbone • Nitrogenous bases ‘stick’ out • DNA has 2 polynucleotide strands • RNA has 1 polynucleotide strand
5. DNA structure • Two polynucleotides joined by condensation reactions between complementary base pairs • Adenine and thymine • Cytosine and Guanine • The chains coil to create a double helix
6. DNA adaptations • Very stable and able to pass from generation to generation without change • Joined by weak hydrogen bonds so it can separate during DNA replication and protein synthesis • Long molecule but coiled tight so more genetic information can fit into a cells nucleus • Has the ability to self replicate • Sugar-phosphate backbone protects the bases from external chemical and physical forces
7. DNA – Genetic coding • Genes are sections of DNA that code for a specific sequence of amino acids that form a particular protein • Each sequence contains 3 bases (base triplets) • 3 bases = 1 amino acid • Different sequences code for different amino acids • 20 different amino acids in human biology however there are 64 possible base triplet sequences • Spare triplets code for the same amino acids as another sequence or become START or STOP codons
8. • Each triplet is read separately from the one before it • Bases are read in frames of 3 • This is called non-overlapping • Genetic coding is called degenerative • There are more triplet codes than amino acids • Introns are sections of DNA that do not code for amino acids • Exons are parts of DNA that do form part of the genetic code
9. DNA replication
10. • DNA double helix unwinds and unzips because weak hydrogen bonds are broken (under the influence of DNA helicase) between complementary base pairs • The 2 strands separate each becoming a template • Free DNA nucleotides are attracted to and attach to complementary base pairs • The complementary pairs join together by a condensation reaction and action by the enzyme DNA polymerase • This forms 2 identical strands of DNA because each strand contains half of the original genetic material • Called a semi-conservative method
12. Meselson and Stahl Biologists who discovered DNA replication in their experiment using E.coli (bacteria)
13. The experiment • Bacteria was grown in (heavy) Nitrogen for many generations (All DNA is heavy) • This was called generation 0 • Generation 0 was transferred to (light) Nitrogen 14N and allowed to replicate • The result was generation 1, each DNA molecule had one “heavy” and one “light” strand • Generation 1 was allowed to replicate again in 14N • The result was generation 2, there were 2 “light” molecules and 2 “intermediate” molecules • Proves DNA replication is a semi-conservative method
14. Pink = Heavy nitrogen strand Blue = Light nitrogen strand Pink + blue = Intermediate molecule
15. Protein synthesis
16. Transcription (DNA – mRNA) • Occurs in the nucleus • Double helix unwinds and unzips (under the influence of DNA helicase) as weak hydrogen bonds break • Free RNA nucleotides attach to complementary base pairs • Cytosine + Guanine, Adenine + Uracil • RNA polymerase catalyses the formation of the new RNA molecule • Forms a phosphodiester bond • Newly synthesised mRNA (messenger RNA) leaves the nucleus via a nuclear pore
17. Translation (mRNA – Polypeptide) • mRNA attaches to a ribosome • tRNA (translation RNA) brings a specific amino acid to the ribosome • Complementary bases pair between the mRNA codon and the tRNA anticodon • Ribosome brings 2 amino acids into close proximity • Condensation reactions occur between amino acids and form a polypeptide chain • Ribosome moves to the next codon until a STOP codon is reached • Post translation processing of the polypeptide occurs