A-Level Biology Nucleic Acids (Kerboodle Course Guide)

- Nucleic acids are important biological molecules made up of nucleotides. They are used by all organisms to encode and store their genetic information. The molecule that provides energy for all life process is a nucleotide.

- Before you start, remind yourself of what you already know from GCSE and chapter 1:

• nucleic acids are polymers built from nucleotides
• there are 4 nitrogenous bases in DNA: adenine, guanine, cytosine and thymine. these bases encode for all the information needed by the cell
• the information encoded by the DNA is used to build proteins from amino acids
• ATP is used as energy in the cell

- Nucleic acids are a group of important molecules. RNA and DNA are the best known of the nucleic acids

- Both RNA and DNA are involved in the storing of genetic information in cells and the creation of proteins from this information

- A nucleotide has three parts which are:

• a pentose sugar
• a phosphate
• a nitrogenous organic base: adenine, cytosine, guanine, thymine and uracil

- Mononucleotides are joined as a result of a condensation reaction by a phosphodiester bond to make a polynucleotide. RNA and DNA are polynucleotides

- Ribonucleic acid is a polymer of nucleotides and is a single, relatively short strand. The sugar in RNA is ribose. The 4 bases found in RNA are adenine, cytosine, guanine and uracil

- These are 3 types of RNA:

• messenger RNA: this transfers genetic information from the nucleus to ribosomes
• ribosomal RNA: ribosomes are made of another type of RNA
• transfer RNA: involved in protein synthesis

-  Deoxyribose nucleic acid is a polymer of nucleotides. The sugar in DNA is deoxyribose. The 4 bases in DNA are adenine, cytosine, guanine and thymine.

- The structure of DNA is formed from two strands of polynucleotides joined by hydrogen bonds between complementary base pairs. Adenine always pairs with thymine and cytosine always pairs with guanine.

- This means that the quantities of adenine and thymine in a DNA molecule are always the same, and the quantity of cytosine and guanine are always the same. 

- DNA is a very stable molecule because of its structure which makes it a good molecule to carry the genetic information required for all cells

- As a result of complemtary base pairing, it is possible to calculate the frequencies of all of the bases if you know the frequency of one

- Cell division is an important process for creating new cells. Before a cell divides, the DNA must duplicate so that the daughter cells all contain DNA

- In this topic, you will learn how DNA replication takes place

- Procss of DNA replication:

• a) a representative portion of DNA, which is about to undergo replication
• b) an enzyme, DNA helicase, causes the 2 strands of the DNA to seperate by breaking the hydrogen bonds that join the complementary bases together
• c) DNA helicase completes the seperation of the strand. Meanwhile, free nucleotides that have been activated, bind specifically to their complementary bases
• d)  once the activated nucleotides are bound, they are joined together by DNA polymerase which makes phosphodiester bonds. The remaining unpaired bases continue to attract their complementary nucleotides
• e) finally, all the nucleotides are joined to form a complete polynucleotide chain using DNA polymerase. In this way, two identical molecules of DNA are formed. As each molecule retains half of the original DNA material, this method or replication is called the semi-conserative method

- There are diagrams on Kerboodle

- The process of DNA replication is described as semi-conservative as one of the original strands of DNA is conserved in each of the new DNA molecules

- Semi-conservative replication reduces errors in the production of the new DNA as there is always one strand to act as a template. Each nitrogenous base can only form hydrogen bonds with one other base, so the complementary strand is formed

- All organisms require energy to fuel the processes they need to survive

- Energy initially comes from the sun but plants are able to convert this energy by photosynthesis into complex organic molecules that can be stored.Both plants and animals then oxidise these complex organic molecules to produce adenosine triphosphate (ATP), which is the main source of energy for processes within cells

- Adenosine triphosphate  (ATP) is a nucleotide with three types:

• adenine: a nitrogen-containing base
• ribose: a pentose sugar
• phosphate: a chain of three phosphates

- ATP stores energy using the phosphate group. The bonds between them are unstable and have a low activiton energy ehich means the bonds can easily be broken. When the bonds are broken they release a large amount of energy

- This is a hydrolysis reaction. It is catalysed by the enzyme ATP hydrolase

- The hydrolysis reaction on the previous page is reversible. ATP molecules are made from ADP by adding an inorganic phosphate. This is a condensation reaction as water is produced. The reaction is catalysed by ATP synthase

- This reaction occurs by:

• photophosphorylation: during photosynthesis in chlorophyll containing plant cells
• oxidative phoshorylation: during respiration in animal and plant cells
• substrate-level phosphorylation: when phosphate groups are transferred from donor molecules to ADP in animal and plant cells

- ATP is a good immediate source of energy but it is not used for long term stores of energy because it is so unstable. Once used, it can be rapidly reformed in mitochondria.Cells that require a lot of ATP have large numbers of mitochondria

- ATP is used in the following processes:

• metabolic processes
• movement
• active site
• secretion
• activation of molecule

- Water is the most abundant liquid on Earth and it is found in all cells

- Water is made of 2 hydrogen atoms and 1 oxygen atom. The oxygen molecule has a slight negative charge and the hydrogen molecules have slight positive charges making the molecular dipolar and, therefore, capable of forming hydrogen bonds

- The structure of water gives it unusual properties and these properties make it useful in all cells

- Specific heat capacity: water molecules stick together with hydrogen bonds so more energy is required to seperate them than would be needed if they did not bond in this way. This gives water an unusually high boiling point and it takes more energy to heat a given mass of water. Water has a high specific heat capacity and so can act as a buffer against sudden temperature changes.

- Latent heat of vaporisation: it takes a lot of energy to evaporate one gram of water due to the hydrogen bonding between the molecules. This is the latent heat of vaporisation. It makes water effective at cooling surfaces, including body surfaces.

- Cohesion and surface tension: the tendency of water molecules to stick together is called cohesion. The cohesive force allows water to be drawn up through tubes, such as the xylem vessels in plants, against the force of gravity ], and it gives a body of water a stable and strong surface that ca support small organisms

- Water is an important component of all living organisms

- Some of the uses of water in organisms include:

• the breakdown of complex molecules in hydrolysis reaction
• as a reactant in photosynthesis
• as a solvent for gases, enzymes, ions, waste substances, and small hydrophylic molecules like ATP
• as support since it is incompressible
• for temperature control

- Inorganic ions are found dissolved in body fluids at a range of concerntrations from very low to high

- These ions are involved in a large number of processes in organisms, for examples, iron ions are involved in binding oxygen molcules to haemoglobin and phosphate ions are involved in storing energy in ATP

- In this chapter, you have learnt more about RNA and DNA and how genetic information is stored and transferred in the cell

- You have also learned about ATP which is the immediate source of energy in the cell. The inorganic phosphate ions in ATP are some of many important inorganic ions used in cell processes