Definitions:
- Heredity is the passing on of characteristics/traits from one generation to the next
- A gene is a short region of a chromosome that contains a code for the production of a protein
- Gene expression is the process by which the code in DNA is used to make a protein
Genetic Code
- The code for a particular protein can be thousands of bases long
- Only approx 3% of DNA is thought to actually code for proteins (coding DNA)
- The rest (97%) is called non-coding DNA – does not code for any proteins
Chromosome Structure
- The genes are contained within a much longer piece of DNA called a chromosome
- The genes are spread out along the length of the chromosome
- There is coding DNA (DNA that codes for a specific protein) and non-coding DNA (DNA whose function is generally unknown)
- The non-coding regions of the chromosomes used to be called ‘junk DNA’
- Chromosomes are composed of 40% DNA: 60% protein
- The protein (histones) makes the DNA very stable and enable it to be supercoiled into a very small space (i.e. the nucleus) •DNA wraps around proteins called histones, which then supercoil into a chromosome
- NOTE: chromosome only exist during mitosis
- At all other times the DNA is in the form of chromatin
DNA
- DNA stands for deoxyribonucleic acid
- It is a polymer and has a helical shape.
- DNA consists of two strands (double-stranded) – made up of alternating sugar (deoxyribose) and phosphate molecules
- The two strands are attached to each other by nitrogenous bases.
- DNA contains 4 bases:
1. Adenine (A)
2. Guanine (G)
3. Thymine (T)
4. Cytosine (C)
Adenine and guanine are purines
Thymine and cytosine are pyrimidines
- The strands are twisted on themselves creating a spiral ladder
- The spiral ladder shape is called a double helix
- The bases attach the two strands together in pairs (complementary base pairing)
- The bases always attach to the sugar molecules
Complementary base pairing
- Complementary base pairing occurs between the bases in DNA:
- Adenine can only pair with thymine (A = T)
- Cytosine can only pair with guanine (C ≡ G)
- A = T: double hydrogen bond
- C ≡ G: triple hydrogen bond
- Individual hydrogen bonds are very weak but as there are so many hydrogen bonds they are collectively very strong – holding the two strands of DNA together and making DNA very stable
Nucleotide
- A nucleotide is a 3 molecule unit composed of a phosphate, sugar (deoxyribose), and base (A, T, C or G)
- It is the basic unit of the structure of DNA
DNA replication
- DNA replication occurs towards the end of interphase
- An enzyme unwinds and unzips the DNA
- Free nucleotides diffuse in from the cytosol and are placed into their complementary position by the enzyme DNA polymerase
- Once the DNA has been replicated the DNA coils and supercoils into chromosomes in preparation for mitosis/meiosis
DNA Profiling
- DNA profiling is a method of making a unique pattern of bands from a sample of DNA for identification purposes
DNA Profiling Method
1. DNA isolation:
DNA isolation: extraction/release of DNA from cells.
- DNA is released from cells by using a type of detergent that splits open cell membranes
- Even if the sample to be tested is very small (such as a hair follicle/blood smear) the amount of DNA can be increased (by DNA replication) several million-fold in a few hours!
2. Cutting:
Cutting: DNA is cut into fragments
- Restriction enzymes cut DNA at specific base sequences
- Products of this process are fragments of DNA (restriction fragments) that are different sizes
- Everyone’s DNA is different which means that restriction enzymes will cut everyone’s DNA in slightly different places
3. Separation:
Separation: restriction fragments of DNA are grouped based on size and moved apart
- Because everyone has their own unique DNA they also have their own unique set of restriction fragments after the cutting stage
- The mixture of restriction fragments can be separated out into a unique pattern of bands
- The process of separating out the different fragments is carried out by gel electrophoresis•Agarose gel is poured into specialised shallow tray and allowed to set
- The mixture of DNA is loaded into ‘wells’ at the top end (negative end) of the gel and an electric current is passed through the gel
- DNA is a negatively charged molecule and will be attracted towards the positive end
- The large restriction fragments will move more slowly than the short fragments – this creates a unique pattern of bands of fragments
4. Pattern analysis:
- An invisible pattern has been produced by the gel electrophoresis
- To make the pattern visible the whole gel is stained (e.g. ethidium bromide) and then viewed under UV light
- The patterns produced can then be compared and analysed for identification purposes
Applications of DNA Profiling
- Species identification
- Criminology: placing suspect at a crime scene
- Medical: used often in paternity testing
RNA
- RNA – ribonucleic acid
- RNA is single stranded
- RNA contains nitrogenous bases:
– Adenine (A)
– Uracil (U)
– Cytosine (C)
– Guanine (G)
- RNA contains the sugar ribose
- Nucleotides in RNA are composed of a phosphate, sugar (ribose) and a base (A, U, C, or G)
- RNA is made by the enzyme RNA polymerase using one DNA strand as a template (see picture below).
DNA versus RNA
| DNA: | RNA: |
| > Double-stranded > Sugar: deoxyribose > Nucleotides: – Adenine – Thymine – Guanine – Cytosine | > Single-stranded > Sugar: ribose > Nucleotides: – Adenine – Uracil – Guanine – Cytosine |
Experiment: to isolate DNA from onion
- Add a finely chopped small onion to 3 g salt and 100 ml distilled water
- Heat (60˚C) and stir mixture gently for 15 minutes (heat denatures enzymes that breaks DNA down)
- Cool in ice bath for 5 minutes (to prevent the DNA itself being broken down)
- Blend mixture for 3 seconds (this step breaks the cell walls)
- Filter blended mixture through coffee filter paper (not lab filter paper)
- Take 3 ml of filtrate in test tube and add 3 drops of freshly squeezed kiwi fruit juice – slowly swirl the test tube to mix (kiwi fruit juice contains proteases that digest the histones)
- Slowly add 10 ml ice cold ethanol down the side of the test tube (DNA is insoluble in ice cold ethanol)
- DNA becomes visible at the junction between the filtrate and ethanol
