Topic 1B - More Biological Molecules - complete

  • Created by: scarlett
  • Created on: 08-07-20 18:09


- DNA and RNA are both types of nucleic acid
- they're found in all living cells and they carry important information
- DNA is used to store genetic information which is all the instructions an organism needs to grow and develop from a fertilised egg to a fully grown adult
- RNA is similar in structure to DNA
- one of its main functions is to transfer genetic information from the DNA to the ribosomes
- ribosomes are the body's protein factories
- they read the RNA to make polypeptides in a process called translation
- ribosomes themselves are made from RNA and proteins

1 of 18

DNA and RNA 2

- nucleotides are important and are the monomers that make up DNA and RNA
- a nucleotide is a type of biological molecule
- theyre made from: a pentose sugar, a nitrogen containing organic base and a phosphate group

- the pentose sugar in a DNA nucleotide is called deoxyribose
- each DNA nucleotide has the same sugar and a phosphate group
- the base on each nucleotide can vary
- there are 4 possible bases - adenine, guanine, cytosine and thymine

- the sugar in RNA is called ribose
- like DNA, an RNA nucleotide also has a phosphate group and one of four different bases
- in RNA uracil replaces thymine as a base

2 of 18


- a polynucleotide is a polymer of nucleotides
- both DNA and RNA nucleotides form polynucleotides
- the nucleotides join up via a condensation reaxtion between the phosphate group of one nucleotide and the sugar of another
- 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

3 of 18

Composition of DNA

- two DNA polynucleotide strands join together by hydrogen bonding between the bases
- each base can only pair with one particular partner (complementary base pairing)
- A always pairs with T and C always pairs with G
- this means that there are always equal amounts of A and T bases and always equal amounts of C and G bases in a DNA molecule
- two hydrogen bonds form between A and T, while 3 bonds form between C and G
- two antiparallel (running in opposite directions) polynucleotide strands twist to form the DNA 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 the genetic information must be carried by proteins which are more chemically varied
- by 1953, experiments had shown that DNA was the carrier of the genetic code
- 1953 was also the year in which the double-helix structure, which helps DNA to carry out its function, was determined by Watson and Crick

4 of 18

Composition of RNA

- RNA is made from a single polynucleotide chain 
- it is much shorted than most DNA polynucleotides

5 of 18

DNA 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 DNA molecule are from the original DNA molecule
- this means that theres genetic continuity between generations of 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 strand acts as a template for a new strand. Complementary base pairing means that free-floating DNA nucleotides are attracted to the complementary exposed bases on each original template strand
-3- condensation reactions join the nucleotides of the new strands together - catalysed 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

6 of 18

DNA Polymerase

- each end of a DNA strand is slightly different in its structure
- one end is called the 3' (three prime) end and one end is called the 5' (five prime) end
- in a DNA helix, the strands run in opposite directions (they're antiparallel)
- the active site of DNA polymerase is only complementary to the 3' end of the newly forming DNA strand so the enzyme can only add nucleotides to the new strand at the 3' end
- this means that the new strand is made in a 5' to 3' direction and that DNA polymerase moves down the template strand in a 3' to 5' direction
- because the strands in the double-helix are antiparallel, the DNA polymerase working on one of the template strands moves in the opposite direction to the DNA polymerase working on the other template strand

7 of 18

Meselson and Stahl & Semi-Conservative Replication

- it wasnt until Meselson and Stahl's experiment a few years late that Watson and Crick's theory was validated
- before then, people were unsure whether DNA replication was semi-conservative or conservative
- if it was conservative, the original DNA strands would stay together and the new DNA molecules would contain two new strands
- meselson and stahl showed that DNA is replicated using the semi-conservative method
- their experiment used two isotopes of nitrogen, heavy nitrogen (15N) and light nitrogen (14N)

8 of 18

Meselson and Stahl Experiment

- 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
- a sample of DNA was taken from each batch of bacteria, and spun in a centrifuge
- the DNA from the heavy nitrogen bacteria settled lower down the centrifuge tube than the DNA from the light nitrogen bacteria because its heavier
- then the bacteria grown in the heavy nitrogen broth were taken out and put in a broth only containing 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
- if replication was conservative, the orignal heavy DNA, which would still be together, would settle at the bottom and the new light DNA would settle at the top
- 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
- as it turned out, the DNA settled out in the middle, showing that the DNA contained a mix of light and heavy nitrogen (semi-conservative replication)

9 of 18


- water makes up about 80% of a cell's contents and it has lots of important functions
- water is a metabolite in loads of important metabolic reactions, including condensation and hydrolysis reactions
- a metabolic reaction is a chemical reaction that happens in a living organism to keep it alive
- water is a solvent, which means some substances dissolve in it
- most metabolic reactions take place in a solution (e.g. in the cytoplasm of eukaryotic and prokaryotic cells), making water essential
- water helps with temperature control because it has a high latent heat of vaporisation and a high specific heat capacity
- water molecules are very cohesive (they stick together), which helps water transport in plants as well as transport in other organisms

10 of 18

Structure of Water Molecules

- a molecule of water is one atom of oxygen joined to two atoms of hydrogen by shared electrons
- because the shared negative hydrogen electrons are pulled towards the oxygen atom, the other side of each hydrogen atom is left with a slight positive charge
- the unshared negative electrons on the oxygen atom give it a slight negative charge
- this makes water a polar molecule - it has a partial negative charge on one side and a partial positive charge on the other 
- the slightly negatively-charged oxygen atoms attract the slightly positively-charged hydrogen atoms of other water molecules
- this attraction is called hydrogen bonding and it gives water some of its useful properties

11 of 18

Properties of Water

- many metabolic reactions involve a condensation or hydrolysis reaction
- a hydrolysis reaction requires a molecule of water to break a bond
- a condensation reaction releases a molecule of water as a new bond is formed
- for example, amino acids are joined together to make polypeptides (proteins) by condensation reactions, and energy from ATP is released through a hydrolysis reaction
High Latent Heat of Vaporisation
- it takes a lot of energy (heat) to break the hydrogen bonds between water molecules
- so water has a high latent heat of vaporisation - a lot of energy is used up when water evaporates
- this is useful for living organisms because it means they can use water loss through evaporation to cool down (e.g. humans sweat to cool down) without losing too much water
Buffer Changes in Temperature
- the hydrogen bonds between water molecules can absorb a lot of energy
- so water has a high specific heat capacity - it takes a lot of energy to heat it up
- this is useful for living organisms because it means that water doesnt experience rapid temperature changes which makes water a good habitat because the temperature under water is likely to be more stable than on land
- the water inside organisms also remains at a fairly stable temperature

12 of 18

Properties of Water 2

- a lot of important substances in metabolic reactions are ionic (like salt, for example)
- this meands they're made from one positively charged atom or molecule and one negatively charged atom or molecule (e.g. salt is made from a positive sodium ion and a negative chloride ion)
- because water is polar, the positive end of a water molecule will be attracted to the negative ion, and the negative end of a water molecule will be attracted to the posiitve ion
- this means the ions will get totally surrounded by water molecules (they'll dissolve)
- waters polarity makes it a useful solvent
- cohesion is the attraction between molecules of the same type (e.g. two water molecules)
- water molecules are very cohesive (they tend to stick) because they're polar
- strong cohesion helps water to flow, making it great for transporting substances
- for example, its how water travels in columns up the xylem (tube-like transport cells) in plants
- strong cohesion also means that water has a high surface tension when it comes into contact with air
- this is the reason why sweat forms droplets, which evaporate from the skin to cool an organsism down. its also the reason that pond skaters, and other insects, can 'walk' on the surface of a pond

13 of 18

ATP Energy in a Cell

- plant and animal cells release energy from glucose - this process is called respiration
- a cell cant get its energy directly from glucose
- so, in respiration, the energy released from glucose is used to make ATP (adenosine triphosphate)
- ATP is made from the nucleotide base adenine, combined with a ribose sugar and three phosphate groups
- its known as a nucleotide derivative because its a modified form of a nucleotide
- once made, ATP diffuses to the part of the cell that needs energy
- the energy in ATP is stored in high energy bonds between the phosphate groups and its released via hydrolysis reactions

14 of 18

Using and Making ATP

- when energy is needed by a cell, ATP is broken down into ADP (adenosine diphosphate) and Pi (inorganic phosphate)
- this is a hydrolysis reaction
- a phosphate bond is broken down and energy is released
- the reaction is catalysed by the enzyme ATP hydrolase
- ATP hydrolysis can be 'coupled' to other energy-requiring reactions in the cell - this means the energy can be used directly to make the coupled reaction happen, rather than being lost as heat 
- the released inorganic phosphate can also be put to use - it can be added to another compound (this is known as phosphorylation), which often makes the compound more reactive
- ATP can be re-synthesised in a condensation reaction between ADP and Pi
- this happens during both respiration and photosynthesis, and is catalysed by the enzyme ATP synthase

15 of 18

Inorganic Ions

- an ion is an atom (or group of atoms) that has an electric charge
- an ion with a positive charge is called a cation
- an ion with a negative charge is called an anion
- an inorganic ion is one which doesn't contain carbon (although there are a few exceptions to this rule)
- there are inorganic ions, in solution, in the cyotoplasms of cells and in the body fluids of organisms
- each ion has a specific role, depending on its properties
- an ion's role determines whether it is found in high or low concentrations

16 of 18

Iron and Hydrogen Ions

- haemoglobin is a large protein that carries oxygen around the body, in the red blood cells
- its made up of four different polypeptide chains, each with an iron ion (Fe2+) in the centre
- its the Fe2+ that actually binds to the oxygen in haemoglobin so its a key component
- when oxygen is bound, the Fe2+ ion temporarily becomes an Fe3+ ioin, until oxygen is released

- pH is calculated based on the concentration of hydrogen ions (H+) in the environment
- the more H+ present, the lower the pH (and the more acidic the environment)
- enzyme controlled reactions are all affected by pH

17 of 18

Sodium and Phosphate Ions

- glucose and amino acids need a bit of help crossing cell membranes
- a molecule of glucose or an amino acid can be transported into a cell (across the cell-surface membrane) alongside sodium ions (Na+)
- this is known as co-transport

- when a phosphate ion (PO43-) is attached to another molecule, its known as a phosphate group
- DNA, RNA and ATP all contain phosphate groups
- its the bonds between phosphate groups that store energy in ATP
- the phosphate groups in DNA and RNA allow nucleotides to join up to form the polynucleotides

18 of 18


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Biological molecules, organic chemistry and biochemistry resources »