DNA & RNA

Genes:

• The genetic information in all organisms is found in the DNA (deoxyribonucleic acid)

• A gene is a selection of DNA that codes for a particular protein

• An allele is an alternative form of a gene

• Just because an organism has a gene in its cells does not mean it will be used properly 

• Gene expression is the process of converting the information in a gene into a protein

• DNA can also be found outside of genes, for example in mitochondria or chloroplasts

Chromosomes:

• The DNA in an organism is found in strands

• These strands are tightly wound around a protein scaffold (histone)

• These structures are called chromosomes and are made of chromatin (protein and DNA)

• Each chromosome comprises a large number of genes

• Most cells normally contain two sets of chromosomes

• As a result each cell has two copies of each gene

• These interact to produce the traits visible in an organism

• A cell is diploid (2n) when it has two copies of each chromosome

• In sexual reproduction, offspring gets information from each parent 

• Special cells, or gametes contain only one copy of each chromosome and thus each a gene

• A cell is haploid (n) when it only has one copy of each chromosome or gene

• Fertilisation occurs when the nuclei of two gametes fuse and the resultant cell (zygote) has two copies of each gene

DNA structure:

• DNA is a very long, coiled molecule called a double helix

• It consists of two strands made up of phosphate and sugars

• One strand runs up the helix and one strand runs down it

• These two stands are linked together by bases

• The bases are paired: one on each strand

• They form a ladder-like structure that is then twisted into the characteristic double helix of DNA

• DNA has four bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C

• A is always paired with T. C is always paired with G

• These pairs of bases are called complementary base pairs

• Adenine and Guanine are (double ring) purine bases

• Cytosine and Thymine are (single ring) pyramidine bases

• A basic unit is called a nucleotide

• A nucleotide is composed of three basic chemicals:

• A five carbon sugar

• A phosphate group 

• A nitrogenous base

• Each base is attached to the sugar deoxyribose 

• Due to their structure, these bases are able to form bonds called hydrogen bonds

Coding and non-coding DNA:

• Coding DNA contains information for the production of protein in the cell

• These coding sequences are present within genes

• Non-coding structures are the parts of DNA that do not contain critical information for the synthesis of protein

• Non-coding sequences are found between genes and within genes

• Non-coding DNA makes up 95% of human DNA

• Non-coding DNA within genes are called introns

DNA replication:

• The messages that the DNA carries are vital for the survival of the cell

• When the cell replicates, it produces and identical copy of itself

• Repplication: the process of an identical copy of DNA

• Enzymes (Helicase and DNA Polymerase) control the process

• DNA replication takes place during the interphase of the cell cycle

• The process requires enzymes and energy (ATP) as well as free nucleotides

1. Enzymes (Helicase) unwind the double helix and ‘unzip’ the two strands of DNA 

2. The enzyme DNA Polymerase attaches to both strands and reads the exposed bases (5’ 3’)

3. The exposed bases pair with their complementary base pairs from a pool of free nucleotides

4. Two identical strands of DNA are produced from the original strand, although one is backwards

5. Each now twists to form a helix

6. One side of each new DNA molecule comes from the original and one is new

7. The genes on each chromosome are the same

RNA - Ribonucleic acid:

• RNA is a single nucleic acid produced from the DNA template

• It differs from DNA in a number of ways:

• RNA has the base Uracil (U) instead of Thymine

• RNA contains sugar ribose instead of deoxyribose

• RNA is a single stand (no hydrogen bonds or double helix)

• RNA is found in the nucleus, cytoplasm, and ribosomes

Types of RNA:

• There are three different types of RNA (messenger, ribosomal, transfer) and all are involved in protein synthesis

• mRNA: copies the information from the DNA and moves to a ribosome

• rRNA: each ribosome is composed of roughly equal parts RNA and protein

• tRNA: carries amino acids to the mRNA in contact with the ribosome

DNA profiling:

• DNA profiling is the process of making a pattern of bands from a person’s DNA to compare with other DNA patterns

• In 1984 Alex Jefferys noted that sections of people’s non-coding DNA varies enormously 

• He used these variable regions to produce a DNA fingerprint or DNA profile

• Genetically different individuals produce different profiles

• The closer the genetic relationship between individuals the more similar their profiles

Making a genetic profile:

1. Take a sample of call containing material (cheek cell)

2. Extract the DNA from the cells

3. Treat the extracted DNA with special enzymes (DNases)

4. This produces DNA fragments

5. The DNA fragments are placed at one end of a gel

6. An electric charge is passed through the gel and the fragments move (gel electrophoresis)

7. The distance travelled by the pieces of DNA is seen by putting a stain onto the gel

8. The result is a series of bands

Applications of DNA profiling:

• Establishing parenthood

• To identify criminals from blood, semen or other tissue left at the scene of the crime

• To identify species, especially plants 

• To look for genetic matches in organs transplants

• Archaeological: check relationship between human remains in archaeological sites and people alive today 

Genetic screening:

• Genetic screening is a test to determine if an individual carries an abnormal gene for a particular trait

• Genetic screening can also tell if the individual is homozygous normal, homozygous abnormal or heterozygous 

• Special genetic probes or DNA profiling may be used in genetic screening