Genetic information, variation and relationship between organisms

A gene is a section of DNA that contains coded information for making polypeptides and functional RNA. It is located on the DNA on a locus.

The coded info is in the form of a specific sequene of bases along the DNA molecule. Polypeptides make up proteins so genes determine the proteins of an organism.

The gene is a base sequence of DNA that codes for AA sequence of a polypeptide or functional RNA including ribosomal RNA and transfer RNA's.

Adenine, guanine, cytosine and thymine are present in DNA. Using a pair of bases (4^2 ) only 16 codes are possible so there is three bases (4^3) so theres 64 different codes.

As the code has 3 bases on each AA, each one is called a triplet.

Most AA are coded for by between 2 - 6 triplets. The code is known as degenerate code. The start of DNA sequence that codes for a polypeptide is always the same triplet. This codes for the AA methionine. 3 Triplets do not code for any AA and these are called stop codes.

The code is non-overlapping and is universal.

Eukaryotic cells do not code for polypeptides. Between genes there are non coding sequences made up of multiple repeats of the same base sequences. Even within genes, only certain sequences code for AA. These coding sequences are called exons. Within the genes, these exons are seperated by further non-coding seuqences called introns. Some genes code for ribosomal RNA and transfer RNA's. The DNA are longer and form a line and are associated with proteins called histones to form structures called chromosomes. The mitochondria and chloroplasts of eukaryotic cells also contain DNA which is the same as prokaryotic.

In prokaryotic cells, DNA is shorter and form a circle and are not associated with protein molecules. They therefore do not have chromosomes.

Chromosomes are visible when cells dividing. At the start of cell division, chromosomes appear as two chromatids. The double helix is wound around histones to fix it. This DNA -Histone complex is then coiled. It is then coiled again to form a chromosome.

Homologous pairs - sexually produced the number is the diploid number (46 humans). During meiosis, the number of chromosomes is halfed .

An allele is one of a number of alternative forms of a gene.

Each gene exsists in 2 and each of these forms is called an allele.

Each individual inherits one allele from each of its parents. These 2 alleles could be the same or they are different.

When they are different, each allele has a different base sequence, therefore a different AA sequence so a different polypeptide is produced.

Any changes in a base sequence of a gene produces a new allele of that gene (mutation) and results in a different sequence of AA being coded for.

The different AA sequence will lead to the production of a different polypeptide and therefore a different protein. This protein could be denatured.

Sequence of nucleotide bases in DNA determins the sequence of AA in the proteins. In eukaryotic cells, DNA is largely in the nucleus but during synthesis of proteins(which happens in the cytoplasm) Sections of the DNA are transcribed onto a single-stranded molecule called ribonucleic acid (RNA)

There are a number of types of RNA. mRNA transfers the DNA code from the nucleus to the cytoplasm. The mRNA is small enough to leave through the nuclear pores.

Codon is the sequence of 3 bases on mRNA that codes for a single AA. Genome - the complete set of genes in a cell (including mitochondria and chloroplasts)    

Proteome - Full range of proteins produced by genome. This is sometimes called the complete proteome in which the term proteome refers to the proteins produced by a given type of cell under a certain set of conditions.

RNA is a polymer. Each nucleotide is made of pentose sugar ribose, an organic bas A,G,C,U and a phosphate group.

mRNA - consists of thousands of mononucleotides. mRNA is a long strand that is arranged in a single helix. The base sequence of mRNA is determined by the sequence of bases on a length of DNA in a process called transcription.

Once formed mRNA leaves the nucleus via pores in the nuclear envelope and enters the cytoplasm, where it associates with the ribosomes. There it acts as a template for protein synthesis. It possess the form of codons. The sequence of codons determins the AA sequence of a specific polypeptide that will be made.

tRNA - relatively small molecule that is made up of 80 nucleotides. It is a single stranded chain folded into a clover leaf shape with one end of the chain extended beyond the other. This is the part of the tRNA molecule were an AA can easily attach.

Many types of trNA binds to a specific AA. At the opposite end of the tRNA there is a sequence of three other organic bases known as the anticodon. Given the genetic code is degenerate there must be as many tRNA molecules as there are coding triplets. However each tRNA is specific to 1 AA and has an anticodon that is specific to that AA

DNA provides the instructions in the form of a long sequence of bases.

A complementary section of part of this sequence is made in the form of a molecule called pre-mRNA - a process called transcription.

The pre-mRNA is spliced to form mRNA.

mRNA is used as a template where complementary tRNA molecules attach and the AA they carry are linked to form a polypeptide- process called translation.

Transcription is the process of making pre-mRNA using DNA as a template. An enzyme acts as a specific region of the DNA causing 2 strands to separate and expose the nucleotide bases. The nucleotide bases on 1 or 2 DNA strands, known as the template strand, pair with their complementary nucleotides from the pool which is present in the nucleus.

The enzyme RNA polymerase moves along the strand and joins the nucleotides together to from a pre mRNA molecule. An exposed guannine base on  the DNA binds to the cytosine base of a free nucleotide. As the RNA polymerase adds the nucleotides one at a time to build a strand of pre-mRNA, the DNA strands rejoin behind - this results in only 12 base pairs on the DNA exposed at one time.

In prokaryotic cells, transcription resultss directly iin the production of mRNA and DNA.

In eukaryotic cells, transcription results in the production of pre-mRNA which is spliced to form mRNA.

The DNA of a gene eukaryotic cells is made up of sections called exons that code for proteins and sections called introns that do not.

these intervening introns would prevent the synthesis of a polypepide. In the pre-mRNA of eukaryotic cells. The base sequences correspond to the introns are removed and the functional exons are joined via a process called splicing.

As most prokaryotic cells do not have introns, splicing of their DNA is unnecessary.

The mRNA molecules are too large to diffuse out of the nucleus so once they have been spliced, they leave via a nuclear pore.

Outside the nucleus, the mRNA is attracted to the ribosomes to which it becomes attached so its ready for translation.

A particular tRNA has a specific anticodeon and attaches to a specific AA. Each AA has one or more tRNA molecules, with its own anticodon of bases.

Once mRNA has passed out of a nuclear pore, it determines the synthesis of a polypeptide. A ribosome becomes attached to the starting codon at one end of the mRNA molecule.

The tRNA with a complementary anticodon sequence, moves to the ribosome and pairs up with the codon on the mRNA. This tRNA carries a specific AA. This continues multiple times. The ribosomes moves along the mRNA, bringing together 2 tRNA molecules at 1 time, pairing up with the corresponding two codons on the mRNA. The two AA on the tRNA are joined by a peptide bond using an enzyme and ATP which is hydrolysed. The ribosome moves on to the 3rd codon in the sequence of mRNA, linking the AA on the 2nd/3rd tRNA molecules. As this happens the first tRNA is released from its AA and is free to collect another AA from the pool.

The process continues with up to 15 AA being added a second until a polypeptide chain is formed. Up to 50 ribosomes can pass behind the 1st so many identical polypeptides can be assembled simultaneously. The synthesis of a polypeptide continues until a ribosome reaches a stop codon. At this point the ribosome, mRNA and the last tRNA separate and the chain is complete.