Biology Topic 1B

• DNA and RNA carry important information

  D N A (deoxyribonucleic acid) is used to store genetic information — that's all the instructions an organism needs to grow and develop.
• RNA

  RNA (ribonucleic acid) is similar in structure to D N A. O n e of its main functions is to transfer genetic information from the D N A to the ribosomes. Ribosomes are the body's 'protein factories' — they read the RN A to make polypeptides (proteins) in a process called translation. Ribosomes themselves are made from RNA and proteins.
• DNA and RNA are polymers of nucleotides


  A nucleotide is a biological molecule made from: • a pentose sugar (that's a sugar with 5 carbon atoms), • a nitrogen-containing organic base, • a phosphate group.
• Nucleotides are really important:

  They are the monomers that make up DNA or RNA.
• 1. The sugar in DNA is called Deoxyribose. The sugar in RNA is called Ribose
• DNA nucleotide:

  1. The pentose sugar in a DNA nucleotide is called deoxyribose. 2) Each DNA nucleotide has the same sugar and a phosphate group. The base on each nucleotide can vary though. 3) There are four possible bases — adenine (A), thymine (T), cytosine (C), and guanine (G).
• RNA nucleotide:

  1. RNA contains nucleotides with a ribose sugar (not deoxyribose). 2) Like DNA, an RNA nucleotide also has a phosphate group and one of four different bases. 3) In RNA though, uracil (U) replaces thymine as a base.
• Nucleotides join together to form polynucleotides.


  A polynucleotide is a polymer of nucleotides. Both DNA and RNA nucleotides form polynucleotides
• The nucleotides join up via a condensation reaction between the phosphate group of one nucleotide and the sugar of another. 3) This forms a phosphodiester bond (consisting of the phosphate group and two ester bonds).
• The chain of sugars and phosphates is known as the sugar-phosphate backbone.
• DNA is made of two polynucleotide chain in a double helix structure
• Two DNA polynucleotide strands join together by hydrogen bonding between the bases.
• Each base can only join with one particular partner this is called complementary base pairing.
• Adenine always pairs with thymine (A -T) and cytosine always pairs with guanine (C - G). This means that there are always equal amounts of adenine and thymine in a DNA molecule and equal amounts of cytosine and guanine.
• Two hydrogen bonds form between A and T, and three hydrogen bonds form between C and G
• Two antiparallel (running in opposite directions) polynucleotide strands twist to form the D N A double-helix
• DNA was first observed in the 1800s, but lots of scientists at the time doubted that it could carry the genetic code because it has a relatively simple chemical composition. Some argued that genetic information must be carried by proteins — which are much more chemically varied.
• By 1953, experiments had shown that DNA was the carrier of the genetic code. This was also the year in which the double-helix structure, which helps D N A to carry out its function, was determined by Watson and Crick.
• RNA is made from a single polynucleotide chain (not a double one). It's much shorter than most D N A polynucleotides.
• DNA Replicates by Semi-Conservative Replication
• DNA copies itself before cell division so that each new cell has the full amount of DNA. The method is called semi-conservative replication because half of the strands in each new D N A molecule are from the original D N A molecule. This means that there's genetic continuity between generations of cells (i.e . the cells produced by cell division inherit their genes from their parent cells).
• 1. The enzyme DNA helicase breaks the hydrogen bonds between bases on the two polynucleotide DNA strands. This makes the helix unwind to form two single strands.
• 2. Each original single strand acts as a template for a new strand. Complementary base pairing means that free-floating DNA nucleotides are attracted to their complementary exposed bases on each original template strand — A with T and C with G.
• 3. Condensation reactions join the nucleotides of the new strands together — catalyse d by the enzyme DNA polymerase. Hydrogen bonds form between the bases on the original and new strands
• 4. Each new DNA molecule contains one strand from the original DNA molecule and one new strand.
• Meselson and Stahl Provided Evidence for Semi-Conservative Replication
• Watson and Crick determined the structure of DNA. They also came up with the theory of semi-conservative DNA replication.
• It wasn't until Meselson and Stahl's experiment a few years later that this theory was validated. Before that, people were unsure whether DNA replication was semi-conservative or conservative.
• Meselson and Stahl showed that DNA is replicated using the semi-conservative method, Their experiment used two isotopes of nitrogen - Heavy nitrogen (N15) and light nitrogen (N14)
• Two samples of bacteria were grown — one in a nutrient broth containing light nitrogen, and one in a broth with heavy nitrogen. As the bacteria reproduced, they took up nitrogen from the broth to help make nucleotides for new DNA. So the nitrogen gradually became part of the bacteria's DNA.
• 2. A sample of DNA was taken from each batch of bacteria and spun in a centrifuge. The DNA from the heavy nitrogen bacteria settled lowered own the centrifuge tube than the DNA from the light nitrogen bacteria — because it's heavier.
• 3. Then the bacteria grown in the heavy nitrogen broth were taken out and put in a broth containing only light nitrogen. The bacteria were left for one round of DNA replication, and then another DNA sample was taken out and spun in the centrifuge.
• 4. If replication was conservative, the original heavy DNA, which would still be together, would settle at the bottom and the new light D N A would settle at the top
• 5. If replication was semi-conservative, the new bacterial DNA molecules would contain one strand of the old DNA containing heavy nitrogen and one strand of new DNA containing light nitrogen. So the DNA would settle out between where the light nitrogen DNA settled out and where the heavy nitrogen DNA settled out.
• 6. As it turned out, the DNA settled out in the middle, showing that the DNA molecules contained a mixture of heavy and light nitrogen. The bacterial DNA had replicate d semi-conservatively in the light nitrogen.
• Once Meselson and Stahl had confirmed that DNA replication in bacteria was semi-conservative, other scientists carried out experiments to show that it was the universal method for DNA replication in all living things.
• Water is Vital to Living Organisms. Water makes up about 80% of a cell's contents.
• 1. Water is a metabolite in loads of important metabolic reactions, including condensation and hydrolysis reactions.
• 2. Water is a solvent, which means some substances dissolve in it. Most metabolic reactions take place in solution (e.g. in the cytoplasm of eukaryotic and prokaryotic cells).
• 3. Water helps with temperature control because it has a high latent heat of vaporisation and a high specific heat capacity.