Variation
Interspecific variation: When one species differs from another species.
Intraspecific variation: When members of the same species differ from each other.
Sampling: This involves taking measurements of individual, selected from the population being investigated. If these individuals are representative then the measurements can be relied upon.
Sampling bias: The selection process may be biased. The investigators may make unrepresentative choices.
Chance: The individuals chosen may be not representative.
The best way to prevent sampling bias is to eliminate human involvement in choosing the samples. This can be achieved by random sampling.
1) Divide study area into a grid of numbered lines. 2) Using random numbers obtain a series of coordinates. 3) Take samples at the intersection of each pair of coordinates.
We cannot remove chance completely but can minimise its effect by:
- Using a large sample size: The more individuals selected the smaller the probability that chance will influence the result. The greater the sample size the more reliable the data.
- Analysis of the data collected: The data can be analysed using statistical tests to determine the extent chance has influenced the data.
Variation is the result of two main factors: genetic differences and environmental influences.
Genetic differences
These are due to the different genes that each individual organism possessed. These change from generation to generation. Genetic variation occurs as a result of:
Mutations: These sudden changes to genes and chromosomes may be passed to the next generation.
Meiosis: This form of nuclear division forms the gametes. This mixes the genetic material before it is passed to the gametes.
Fusion of gametes: In sexual reproduction, offspring inherit some characteristics of each parent. Which gamete fused with which a random process adds to variety.
O.B
D.S
Nucleotide structure:
- A sugar called deoxyribose.
- A phosphate group.
- An organic base belonging to one of two groups: (a) Single-ring: cytosine and thymine. (b) Double-ring: adenine and guanine.
The deoxyribose sugar, phosphate group and organic bases are a result of condensation reactions to give a mononucleotide.
Two nucleotides combine to form a dinucleotide.
The continuous linking of monucleotides forms a long chain known as a polynucleotide.
DNA structure:
In 1953, Watson and Crick worked out the structure of DNA.
DNA is made of two strings of nucleotides each of which is extremely long. They are joined by hydrogen bonds formed between bases.
Pairing of bases
The organic bases contain nitrogen and are of two types.
Those with a double-ring structure have longer molecules than those with a single-ring structure.
- Adenine always pairs with thymine by two hydrogen bonds.
- Guanine always pairs with cytosine by three hydrogen bonds.
Adenine is said to be complementary to thymine and guanine is said to be complementary to cytosine.
The function of DNA
DNA is adapted to carry out its functions in a number of ways:
- It is very stable and can pass to generations without change.
- Its two separate strands are joined only with hydrogen bonds which allow them to separate during replication.
- It is a large molecule and carries an immense amount of information.
- By having the base pairs in the helical cylinder the information is protected from being corrupted.
The triplet code
What is a gene?
Genes are sections of DNA that contain the coded info for making polypeptides.
The coded information is in the form of a specific sequence of bases.
Polypeptides combine to make proteins and so genes determine the proteins of an organism.
As
