nucleic acids

• nucleic acids are large polymers formed from many nucleotides linked together.
• an individual nucleotide is made up of: pentose monosaccharide, phosphate group, and a nitrogenous base.
• nucleotides are linked together by condensation reactions to form polynucleotide. the phosphate group at the fifth carbon of the pentose sugar of one nucleotide forms a covalent bond with the hydroxyl group at the third carbon of the pentose sugar of an adjacent nucleotide. these bonds are phosphodiester.
• phosphodiester bonds form a long strong sugar phosphate backbone with a base attached to each sugar. the bonds are broken by hydrolysis, releasing individual nucleotides.
• the sugar in deoxyribonucleic acid is deoxyribose, a sugar with one fewer oxygen atoms than ribose.
• pyrimidines are the smaller bases, which contain single carbon ring structures, thymine and cytosine.
• purines are larger bases, which contain double carbon ring structure, adenine and guanine.
• the two strands of the double helix are held together by hydrogen bonds between the bases. each strand has a phosphate group at one end (5') and a hydroxyl group at the other end (3'). the parallel strands are arranged so they run in opposite directions (antiparallel).
• adenine and thymine form two hydrogen bonds, cytosine and guanine form three hydrogen bonds (complementary base pairing).
• complementary base pairing means DNA always has equal amounts of adenine and thymine and equal amounts of cytosine and guanine.
• it is the sequence of bases along a DNA strand that carries the genetic information in the form of a code.
• ribonucleic acid (RNA) has an essential role in the transfer of genetic information from DNA to proteins that make up the enzymes and tissues of the body.
• the short section of the long DNA molecule corresponding to a single gene is transcribed into a similarly short messenger RNA (mRNA). each individual mRNA is much shorter than the whole chromosome of DNA.
• RNA nucleotides are different to DNA nucleotides as the pentose sugar is ribose rather than deoxyribose and the thymine base is replaced with uracil.
• the DNA nucleotides form polymers in the same way as DNA nucleotides, by the formation of phosphodiester bonds in condensation reactions. the DNA polymers are small enough to leave the nucleus and travel to the ribosomes, where they are central in the process of protein synthesis.
• after protein synthesis the RNA molecules are degraded in the cytoplasm, the phosphodiester bonds are hydrolysed and the RNA nucleotides are released and reused.
• DNA extraction: grind sample in mortar and pestle, this breaks down cell wall. mix with detergent, breaks down membrane and releases cell contents. add salt, breaks h bonds. add protease, break down proteins associated with DNA. add layer of ethanol, causes DNA to precipitate out solution. DNA will be seen as white strands between layer of sample and ethanol.
• semi conservative replication: double helix unwinds, hydrogen bonds break, free DNA nucleotides pair with complementary bases, hydrogen bonds reform between them, and new nucleotides join to adjacent nucleotides with phosphodiester bonds.
• semi conservative replication produces two molecules, one consists of the old strand of DNA and one is new.
• the unwinding of the strands of DNA double helix is carried out by DNA helicase. it travels along DNA backbone, catalysing reactions that break hydrogen bonds between base pairs.
• DNA polymerase catalyses formation of phosphodiester bonds between the nucleotides.
• DNA polymerase moves along template strand in same direction, only binds to the 3' (OH) end, so travels 3' to 5'. as DNA unzips, polymerase replicates each strand in opposite directions. the strand that is unzipped from 3' end is called the leading strand. the other strand unzipped from 5' end so polymerase waits, this results in DNA being produced in sections (okazaki fragments) and is the lagging strand.
• a mutation is a change in sequence
• a codon is a sequence of three bases, each codon codes for an amino acid.
• a section of DNA that contains complete sequence of bases to code for an entire protein is a gene.
• there are four different bases, so there are 64 different codons possible. this includes one codon that acts as the start codon, if it is in the middle of the gene it codes for the amino acid methionine. there are also three stops codons that do not code for any amino acids.
• having a codon to start th sequence ensure the codons are read in frame, so the genetic code is non overlapping.
• there are only 20 different amino acids that regularly occur in biological proteins, and a lot more codons than amino acids, so many amino acids can be coded for by more than one codon, this code is known as a degenerate.
• transcription: DNA unwinds by helicase beginning at a start codon, this breaks hydrogen bonds. one of the two strands contains the code for protein synthesis, this is the sense strand (5' to 3'), the other strand is complimentary to sense strand and is antisense strand and acts as the template strand. RNA nucleotides pair with bases on antisense. phosphodiester bonds formed between RNA nucleotides by RNA polymerase. transcription stops and completed short strand is mRNA which detaches from DNA and leaves nucleus.
• translation: mRNA binds to specific site on ribosome, the ribosome holds it in position while it is translated.
• tRNA is necessary for translation, it is composed of RNA folded so three bases (anticodon) are at one end of molecule. it will bind to complementary codon on mRNA. when tRNA anticodons bind to complementary codons along mRNA, amino acids brought together in correct sequence to form primary sequence of protein.