Genetic Diversity

A Gene is a section of DNA that contains coded info. The info is for making polypeptides and functional RNA. The informations comes in the form of a specific base sequence along the DNA molecule.

Genes along with other environmental factors, determine the nature and development of all organisms. 

A gene is a section of DNA located at a specific point, called a locus on the DNA molecule.

The gene is a base sequence of DNA that codes for:

Scientists suggest that each amino acid is coded for by 3 bases on the DNA because:

As the code has 3 bases per amino acid, each one is called a triplet.

Some amino acids are coded for by more than one triplet.

• A few amino acids are only coded for by one triplet
• Remaining amino acids are coded for by 2-6 triplets each
• The code is known as the degenerate code as most amino acids are coded for by more than one triplet
• A triple is always read in a particular direction on the DNA strand
• The start of the DNA sequence that codes for a polypeptide is always the same - the start codon and codes for methionine
• There are three triplets that don't code of an amino acid, they code for a stop codon
• The code is universal, so triplets will code for the same amino acids in different organisms
• The code is non-overlapping, so each base is only read once

There are parts of eukaryotic DNA that are non-coding and have multiple repeats.These are introns.

The parts of the DNA that are coding are known as exons

In prokaryotic cells the DNA is musch shorter,forming a circular shape, and they're not associaed with proteins. E.g. bacteria

In eukaryotic cells the DNA molecules are longer, forming a line and they're associated with proteins called histones. The histones help the DNA form structures called chromosomes. The mitochondria and chloroplasts in the wukaryotic cell also contain DNA that is short, circular and not associated with proteins.

Chromosomes are only visible as distict structures when the cell is dividing, otherwse they're wide spread throughout the nuceus.

When they become visible, they're seen as two threads joined by a single point. Each thread is called a chromatid as DNA has already replicated giving 2 identical DNA molecules. The DNA in chromosomes is held by histones, which the DNA is wrapped around.

DNA found in each human cell is highly coiled and folded

One sperm cell and one egg cell form a complete set of chromosomes for offspring once fusion has occurred. Therefore, one of each pair comes from the mother and the other from the father. These are known as homologous pairs, and the total number of pairs is referred to as the diploid number, in humans it's 46.

A homologous pair is ALWAYS two chromosomes that carry the same genes, but not necessarily the same alleles of the genes. 

During meiosis the division of cells ensures that each daughter cell recieves one chromosome from each homologous pair. Each cells therefore recieves one gene for each characteristic of the organism. When the haploid cells combine, the diploid state is restored.

An allele is one of the number of different forms of a gene. Each gene exsits in at least 2 different forms, each form is known as a allele. Each individual inherits one allele from each of its parents, they may be the same or different. When they're different it is due to the amino acid sequence differeing, therefore producing a different polypeptide. 

Changes in the base sequenceof an allele produce a new allele fot that gene (mutation), resulting in a different sequence of amino acids being coded for therefore a different protein being made. 

If the mutation is significant enough it could cause a non-functioning protein. 

The sequence of nucleotide bases in DNA determines the sequence of amino acids in the in proteins of an organism. Protein sysnthesis takes place in the cytoplasm, but DNA is too big to leave the nucleus. Sections of the DNA are transcribed onto a single-stranded molecule, RNA.

Messenger RNA (mRNA) transfers the DNA code from the nucleus to the cytoplasm. mRNA is small enough to leave the nucleus and enter the cytoplasm via nuclear pores in the nuclear envelope. 

A codon is the sequence of 3 bases on mRNA coding for a single amino acid. 

Genome - The complete set of genes in a cell (including in chloroplasts and mitochondria)

Proteome - The full range of proteins produced by the genome

RNA is a polymer made up of repeating mononucleotide sub-units. It forms a single strand, each nucleotide if made up of:

There are two important type of RNA important for protein synthesis:

mRNA is a long strand, arranged in a single helix. Base sequences of RNA are determined by the base sequence of DNA in transcription. mRNA leaves the nucleus via pores in the nuclear envelope, enters the cytoplasm and associates with ribisomes. It acts as a template for protein synthesis. 

Its structure suits its function as it posesses information as codons. The sequence of codons determines amino acid sequence of a specific polypeptide to be made. 

tRNA is a relatively small molecule. It's a single stranded folded into a clover-leaf shape. An amino acid is easily able to attach to the side that extends slightly past the end of the other. The opposite end of the tRNA has a sequence of 3 organic bases, the anticodon.

Each tRNA is specific to one amino acid, so the anticodon presented is specific to that amino acid. The anticodon pairs with the three complemetary organic bases that make up the codon on the RNA. 

Prtoeins are made up of one or more polypeptides, every organism needs to make their own, unique proteins. Each cells has the ability to make every protein from 20 amino acids. The protein made depends on the instructions given by the DNA in the nucleus. 

The basic process is as follows:

• DNA provides the instructions to form a long sequence of bases
• A complemetary section of that part of the sequence is made as pre-mRNA in transcription
• The pre-mRNA is spliced to form mRNA
• mRNA is used as a template which complementary tRNA molecules attch to, the amino acids they carry are linked to form a polypeptide in translation

Transcription is the process of making pre-mRNA using DNA as a template. The process is as follows:

1) An enzyme acts on a specfic region of the DNA causing the two strands to separate and expose the nucleotide bases

2) Nucletide bases on one of the two DNA strands, the template strand, pair with their complementary nucleotides that are present in the nucleus. RNA polymerase then moves along the strand joining the nucleotides together to form pre-mRNA

3) As the RNA polymerase adds the nucleotides to build pre-mRNA, the DNA strand rejoins behind it. Only about 12 bases are exposed at a time.

4) RNA polymerase reaches a particular sequence on the DNA recognised as the 'stop' triplet code, it detaches, completeing pre-mRNA production.

In eukaryotic cells transcription results in pre-mRNA being produced, it's then spliced to form mRNA. The DNA of a gene is made up of exons and introns. Exons code for proteins, introns do not code for anything. The intervening introns would prevent the synthesis of a polypeptide if not removed. 

The base sequences making the introns are removed, and the functional exons are joined together during splicing. 

Pre-mRNA molecules are too large to diffuse out of the nucleus, once they've been spliced they're able to leave the nucleus via pores in the nuclear envelope. Once outside the nucleus the mRNA is attracted to ribosomes and therefore attach to ribosomes, ready for translation.

The triplet code of DNA is transcribed into codons on mRNA. Next, codons on the mRNA are translated into a sequence of amino acids making a polypeptide. 

Each particular tRNA has a specific anticodon and attaches to a specific amino acid. Each amino acid therefore has one or more tRNA molecule with its own anticodon. 

• A ribisome  becomes attached to the starting codon at one end of the mRNA
• The tRNA with the complementary anticodon moves to the ribosome and pairs up with the mRNA codon, the tRNA carries a specific amino acid
• tRNA molecule with a compelementary anticodon pairs with the next mRNA codon, it carries another amino acid
• The ribosome moves along the mRNA, bringing together 2 tRNA molecules at a time, each pairs with the corresponding two codons
• Two amino acids on the tRNA are joined by a peptide bond using an enzyme and ATP hydrolysis
• The ribosome moves aong to the 3rd codon on the mRNA, linking the amino acids on the second and third tRNA molecules
• As this happens the first tRNA is released by its amino acid to collect another one in the cell
• The process continues until a polypeptide chain is built
• The synthesis continues until a ribosome reaches a stop codon, this is when the ribosome, mRNA and tRNA molecule all separate and the polypeptide chain is complete

Sometimes a single polypeptide chain is a functional protein, but often a number of polypeptides link together to form a functional protein. What the protein's function is will depend on its structure which involves;