Structure of DNA

DNA and RNA

Structure of DNA

• DNA from one cell would stretch 3 metres if it was unwound.
• DNA can fit into a nucleus as it can coil and fold.
• Proteins hold the DNA in its folded state.

There are four bases used in DNA:

A =

T =

G =

C =

The four bases can only join (bond) with one other base:

Diagram of structure of DNA

A DNA molecule has the sequence TAGCAT, what is the sequence of DNA on a partner strand:

The side strands will form a spiral or helix so the DNA forms a double helix shape

NOTE:

DNA in the longest human chromosome (number 1) contains about 300 million base pairs.

Genes

• A gene is made of DNA (sequence of bases).
• The sequence of bases is called the genetic code.
• A gene works or is expressed (gene expression) when its code is sent into the cells cytoplasm to form a protein.

Genetic code

• A gene is a section of DNA that is required for the production of a particular protein.

How many amino acids are used in the production of proteins?

This means that a gene must carry different codes to control the formation of the 20 different amino acids.

These genetic codes (DNA codes) for each amino acid have a sequence of three consecutive bases. This is known as a triplet (codon).

Examples:

• A DNA triplet CAA is the code for the amino acid called valine
• A DNA triplet CGA is the triplet for an amino acid called alanine.

Comparing the genetic code with the English language

Replication of DNA

• At the end of mitosis every new cell has a single stranded chromosome
• This single stranded chromosome must produce an exact copy of itself to become a double stranded chromosome so it can divide again.
• This process is known as DNA Replication.

DNA Replication

1)Double helix unwinds

2)An enzyme breaks the bond between the two base pairs.

3)New base pairs pass from the cytoplasm into the nucleus. These new base pairs join with old DNA bases.

4)Two new strands of DNA form ()

Significance of DNA replication:

• DNA can produce exact copies of itself which means it can pass on from generation to generation.

DNA profiling

DNA profiling (finger printing) is a method of making a unique pattern of bands from the DNA of a person, which can then be used to distinguish that DNA from other DNA.

Steps involved in DNA profiling:

1)DNA is released:

• Cells are broken down to release their DNA

2)DNA is cut into fragments :

• Restriction enzymes cut the DNA into tiny fragments

Example: By using the GAATTC restriction enzyme, a long strand of DNA will be cut into sections, each section starting with the base sequence AATTC

• The resulting sections of DNA will be of different lengths

3)The fragments are separated:

• The sections of DNA are separated by a process called gel electrophoresis (sugar based gel).
• An electric current is passed along the gel which attracts negatively charged DNA to one end of gel.
• They may add radioactive material to produce a fluorescent image of the DNA fragments.

4)Patterns are compared

• If pattern of bands from two different DNA samples are the same then the two samples must have came from the same person.

Applications of DNA profiles (DNA fingerprints)

Crime

• If the pattern of the DNA profile is compared with those of a victim and a suspect and a match is got for the suspect, then it can be said that this person was present at the crime scene.

Medical

DNA profiles can be used to determine whether a person is the parent of a child (e.g financial inheritance cases)

Genetic screening

Genetic screening means testing DNA for the presence or absence of a certain gene or an altered gene.

Sometimes DNA replication does not work properly and a gene may be incorrectly copied.

Genes can also be altered by mutations.

If genes are altered in any way then they do not have the correct code for the protein they are suppose to produce.

NOTE: genetic disorders caused by altered genes (defective genes) include albinism, cystic fibrosis or sickle cell anaemia.

Genetic screening can be carried out in two ways:

Adult screening :

Carried out on people who do not have a genetic disorder but may carry a defective gene (barriers)

Examples: carriers of sickle cell anaemia and cystic fibrosis

Helps them find out the chances of their children having the disorder.

Foetal screening:

Cells can be removed from the placenta or the fluid around the foetus.

Child can be tested for any disorders

Ethics of genetic screening

If genetic tests become public then insurance companies may be reluctant to get involved with them.

Would it also encourage a termination of a pregnancy.

Detailed structure of DNA can be considered under the following:

Nucleotides, base pairs and the double helix

Nucleotides

A nucleotide consists of three parts: phosphate group, a sugar and a nitrogen base

• The sugar in DNA is deoxyribose (5 carbon sugar)
• The phosphate group is known as P

There are four nitrogen bases, two are known as purines and two as pyrimidines.

• Two purine bases are (double ringed molecules) : adenine (A) and guanine (G)
• Two pyrimidine bases are (single ringed molecules) : thymine (T) and cytosine (C)

Diagrams of the four nucleotides:

Base pairs

• Adenine and thymine form two weak hydrogen bonds with each other.
• Guanine and cytosine form three hydrogen bonds.
• The bases have a complementary shape to each other.

NOTE: Each base pair has a purine and a pyrimidine.

• The forces holding the bases together are hydrogen bonds.

Double helix

Crick and Watson discovered that DNA consisted of a spiral chains of polynucleotides.

• The outside strands of the double helix are made of dioxyribose and phosphate.
• The Rungs of the DNA are the base pairs on the inside.