Edexcel Biology Gcse - Reproduction, the Genome And Gene Expression Part 3

Each strand of DNA is made of chemicals called bases. Note that these are different to bases in relation to acids and alkalis in chemistry. There are four different bases in DNA:

thymine, T

adenine, A

guanine, G

cytosine, C

There are chemical bonds between the two strands in DNA, formed by pairs of bases. They always pair up in a particular way, called complementary base pairing:

thymine pairs with adenine (T–A)

guanine pairs with cytosine (G–C).

Each gene acts as a code, or set of instructions, for making a particular protein. Some of these proteins control the cell's internal chemistry. They tell the cell what to do, give the organism its characteristics, and determine the way its body works.

To enable genes to code for proteins, the bases A, T, G and C get together - not in pairs - but in triplets. This is how it works:

1) Each triplet of bases codes for one particular amino acid.

2) Amino acids are made in the number and order dictated by the number and order of base triplets.

3) The amino acid molecules join together in a long chain to make a protein molecule. The number and sequence of amino acids determines which protein is produced.

The DNA code for the protein remains in the nucleus, but a copy, called mRNA, moves from the nucleus to the ribosomes where proteins are synthesised in the cytoplasm. The protein produced depends on the template used, and if this sequence changes a different protein will be made.

Carrier molecules bring specific amino acids to add to the growing protein in the correct order. There are only about 20 different naturally-occurring amino acids.

Each protein molecule has hundreds, or even thousands, of amino acids joined together in a unique sequence. It is then folded into the correct unique shape. This is very important, as it allows the protein to do its job. Some proteins are enzymes, others are hormones and others form structures within the body, such as collagen. Each of these proteins needs a different shape.

Cells express their genes by converting the genetic message into protein. This process of protein synthesis occurs in two stages - transcription and translation.

When a gene is to be expressed, the base sequence of DNA is copied or transcribed into mRNA (messenger RNA). This process takes place in the nucleus and occurs in a series of stages:

1) The two strands of the DNA helix are unzipped by breaking of the weak Hydrogen bonds between base pairs. This unwinding of the helix is caused by an enzyme (helicase enzyme).

2) The enzyme RNA polymerase attaches to the DNA in a non-coding region just before the gene.

3) RNA polymerase moves along the DNA strand. Free RNA nucleotides form hydrogen bonds with the exposed DNA strand nucleotides by complementary base pairing to form a strand of mRNA:

Note - RNA nucleotides contain the same bases as DNA, except that T is replaced by U. U base pairs with A.

Because the opposite base bonds with the exposed DNA bases, the strand of mRNA is an opposite copy of the DNA strand (except that U replaces T). We call this a complementary copy.

4) The newly formed strand of mRNA is now ready to leave the nucleus and travel to the ribosome.

1) The mRNA strand travels through the cytoplasm and attaches to the ribosome. The strand passes though the ribosome.

2) For every three mRNA bases the ribosome lines up one complementary molecule of tRNA. We call every three bases a codon.

3) tRNA molecules transport specific amino acids to the ribosome which they leave behind shortly after lining up opposite the DNA. Because there are three mRNA bases for each tRNA molecule, we call this the triplet code.

4) Used tRNA molecules exit the ribosome and collect another specific amino acid.

5) A chain of several hundred amino acids in the correct order according to the original DNA is then made. This is called a polypeptide.

After translation, the polypeptide is finally folded into the correct shape and becomes a protein. Peptide bonds form between the adjacent amino acids to finalise the structure.

Not all parts of the DNA code for proteins

Parts of our DNA which don't make proteins.

it is believed that a human is made from only about 20,000 genes.

However, there are sections of non-coding DNA which can switch genes on and off. Not all the genes you need to survive are needed throughout your life. Some regions of these non-coding DNA are not as good as binding to RNA polymerase. This means the enzyme is less likely to bind and so less protein is produced. If less protein is produce this can affect the phenotype of the organism.

In different cells around the body, certain genes will be switched on and others will be switched off. This will vary depending on which cells you examine.

Every sperm and egg cell contains half of the genetic information needed for an individual. Each sex cell is known as haploid, which has half the normal number of chromosomes. When the chromosomes fuse during fertilisation, a new cell is formed, which is known as a zygote. It has all the genetic information needed for an individual, which is known as diploid and has the full number of chromosomes.

blood group

skin colour

eye colour.

Whether you have lobed or lobeless ears is due to genetic causes.

Sex is also an inherited variation - whether you are male or female is a result of genes you inherited from your parent.

Some examples of variation are not caused by the inheritance of genetics. Whether or not you have a scar or tattoo was not determined when the sperm fertilised the egg to begin your life. This variation is often caused by the environment we live in and so is called environmental variation.

Many kinds of variation are influenced by both environmental and genetic factors, because although our genes decide what characteristics we inherit, our environment affects how these inherited characteristics develop. For example:

a person might inherit a tendency to be tall, but a poor diet during childhood will cause poor growth

plants may have the potential for strong growth, but if they do not receive sufficient mineral resources from the soil, they may hardly grow at all

Identical twins are a good example of the interaction between inheritance and environment, because such twins are genetically the same. Any differences you may see between them – for example in personality, tastes and particular aptitudes – are due to differences in their experience or environment.

Continuous variation and Discontinuous variation

Continuous variation is variation that has no limit on the value that can occur within a population. A line graph is used to represent continuous variation.

Discontinuous variation is variation that has distinct groups for organisms to belong to. A bar graph is used to represent discontinuous variation.

height

weight

heart rate

hand span

leaf length

blood group

eye colour

tongue rolling.

Mutations are changes that can occur in genes. These changes are random and can be caused by background radiation and chemicals that we come into contact with, eg the chemicals in cigarette smoke. The change causes an alteration to the base sequence in the genetic code.

Sometimes these changes can be so severe that the cell dies, sometimes the cell can divide uncontrollably and become cancerous, and sometimes the changes are small and the cell survives. Occasionally, the changes may even be beneficial to us and produce new and useful characteristics.

If these changes occur in normal body cells, the changes are lost when we die. But if the changes occur in our sex cells such as sperm and ova, there is the possibility that the changes in the gene will be passed onto the next generation.

It is when these changes are passed on to the next generation that natural selection can either ensure that they are selected if they are useful, or disappear from the gene pool if they are not.

The are over three billion base pairs in the human genome.

Thymine