Nucleic Acids.pdf

Nucleic Acids located in the nucleus carry hereditary(genetic) information nucleic acids contain instructions on the formation of all proteins two types: Ribonucleic acid (RNA) De...

Nucleic Acids located in the nucleus carry hereditary(genetic) information nucleic acids contain instructions on the formation of all proteins two types: Ribonucleic acid (RNA) Deoxyribonucleic acid (DNA) nucleotides bind to form polynucleotides by continued condensation Components of nucleotides three components: nitrogenous base Purines (double rings) Adenine Guanine Pyrimidines (single ring) Thymine Cytosine Uracil Chargaff's first rule - states that in a double-stranded DNA molecule, the amount of adenine (A) is equal to the amount of thymine (T), and the amount of guanine (G) is equal to the amount of cytosine (C). Chargaff’s second rule - states that while the proportions of adenine, thymine, guanine, and cytosine may vary between different species, they remain consistent within a particular species. Nucleic Acids 1  pentose sugar a 5-C sugar either ribose or deoxyribose phosphate group gives nucleotide acidic properties Nucleic Acids 2  Formation of a nucleotide nucleoside - pentose sugar + base joined by a beta glycosidic bond by condensation polymerisation nucleotide - nucleoside + phosphate group joined by an ester bond by condensation polymerisation Nucleic Acids 3  The carbon 5’ phosphate of an incoming nucleotide attaches to the carbon 3’ hydroxyl of a pentose sugar on a growing chain to form a 3’-5’ phosphodiester linkage (bridge). this is how the phosphate backbone for DNA and RNA is formed Nucleic Acids 4  Reading a polynucleotide Structure the sequence is read from the 5’ end to the 3’ end this sequence reads: 5’-A-C-G-T-3’ Nucleic Acids 5  DNA contains genetic information written in a triplet code a sequence of three bases each triple makes up a gene bases making DNA: Adenine Guanine Thymine Cytosine Pentose sugar - deoxyribose double-stranded helix structure A-T C-G RNA three types: mRNA tRNA rRNA participate in the assembling of amino acids into different proteins bases making DNA: Adenine Guanine Uracil Cytosine Nucleic Acids 6  pentose sugar - ribose single stranded DNA replication the duplication of DNA during interphase stage of mitosis allows DNA to be conserved in it’s original form stages: DNA unwound to separate the two strands the hydrogen bonds between the bases are broken the single strands acts as a template to synthesise the new strand bases are added one at a time until two duplicate strands form Semi-Conservative replication DNA replication is semiconservative Nucleic Acids 7  the parent DNA strand separates into two each complementary strand is used as a template to create a new one the new strand binds with the template by hydrogen bonds one strand is the parent strand and the other is the new part of the DNA is conserved Meselson-Stahl experiment proved this and disproved the conservative and dispersive methods Conservative hypothesis - proposed that the entire DNA molecule acted as a template for the synthesis of an entirely new one. The original copy of DNA is left unchanged following synthesis of its new daughter duplicate. Dispersive hypothesis - suggested that new DNA molecules were synthesised by breaking the DNA in short pieces alternating from one strand to the other, resulting in two new daughter DNA molecules. Mechanism of DNA replication Nucleic Acids 8  Topoisomerase unknots and uncoils the DNA molecule Helicase breaks the hydrogen bonds between the bases Nucleic Acids 9  this forms a replication fork DNA polymerase extends both the leading and lagging strands by adding complementary nucleotide one by one in the 5’ to 3’ direction the leading strand template is in the 3’ to 5’ direction the lagging strand template is in the 5’ to 3’ direction DNA polymerase III needs primase on the leading strand to synthesis an initial RNA primer On the lagging strand primase synthesise RNA primers. The DNA polymerase I replaces the RNA primers with DNA nucleotides in a classic 5’ to 3’ away from the replication fork. DNA polymerase I forms short pieces called okazaki fragments. The Okazaki fragments are joined by enzyme DNA ligase which catalyzes the formation of the phosphodiester bond between pieces of DNA. protein synthesis occurs in 2 major steps: transcription translation The Meselson-Stahl experiment was conducted in 1958 by Matthew Meselson and Franklin Stahl. It provided strong evidence supporting Watson and Crick’s hypothesis that DNA replication is semiconservative. Here’s how it unfolded: Three Models of DNA Replication: Before the experiment, scientists proposed three models for DNA replication: Semi-conservative: Each DNA strand serves as a template for a new complementary strand, resulting in two DNA molecules with one old and one new strand. Conservative: DNA replication produces one molecule with both original strands and another with two new strands. Dispersive: DNA replication results in hybrid molecules with patches of both parental and daughter DNA. The semi-conservative model seemed most likely based on DNA’s structure. The Experiment: Meselson and Stahl used E. coli bacteria and isotopes of nitrogen to label DNA. They grew bacteria in a medium containing heavy nitrogen (15N), which incorporated into their DNA. Then, they transferred the bacteria to a medium with light nitrogen (14N). After several generations, they extracted DNA samples and analyzed their density using density gradient centrifugation. Results and Confirmation: If DNA replication were semiconservative, the DNA would gradually shift from heavy to light nitrogen. The experiment’s results showed exactly that: the DNA became lighter over generations. This confirmed that DNA replication is indeed semiconservative, with each new DNA molecule containing one old and one new strand. In summary, the Meselson-Stahl experiment elegantly demonstrated the fundamental mechanism of DNA replication Nucleic Acids 10