NUCLEIC ACIDS

NUCLEIC ACIDS

NUCLEOTIDES

FORM THE MONOMERS OF NUCLEIC ACIDS, DNA AND RNA IN DNA THE NUCLEOTIDE PENTOSE SUGAR IS DEOXYRIBOSE IN RNA THE NUCLEOTIDE PENTOSE SUGAR IS RIBOSE THEY HELP REGULATE MANY METABOLIC PATHWAYS, FOR EXAMPLE BY ATP, ADP AND AMP THEY MAY BE THE COMPONENTS OF MANT COENZYMES

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DNA

Is a nucleic acid Found in the nuclei of all eukaryotic cells, within the cytoplasm of prokaryotic cells and is also inside some types of viruses It is the hereditary material and carries coded instructions used in the development and functioning of all known living organisms DNA is one of the important macromolecules that make up the structure of living organisms, the others being proteins, carbohydrates and lipids

STRUCTURE OF DNA

DNA is a polymer as it is made up of many repeating monomeric units called nucleotides A molecule of DNA consists of two polynucleotide strands The two strands run in opposite directions, described as anti-parallel Each DNA nucleotide consists of a phosphate group, a five carbon sugar deoxyribose, and one of four nitrogenous bases; adenine, guanine, thymine or cytosine The covalent bond between the sugar residue and the phosphate group in a nucleotide is also called a phosphodiester bond These bonds are broken when polynucleotides break down and are formed when polynucleotides are synthesised DNA molecules are long and so they can carry a lot of encoded genetic information

PURINES AND PYRIMIDINES

DNA consists of four types of nucleotide In each nucleotide the phosphate and sugar groups are the same but the nitrogenous base differs In DNA it may be  either a purine - adenine or guanine - or a pyrimidine - thymine or cytosine

HYDROGEN BONDS

The two antiparallel DNA strands are joined to each other by hydrogen bonds between the nitrogenous bonds Adenine pairs with thymine Guanine pairs with cytosine A purine pairs with a pyrimidine, giving equal size rungs on the DNA ladder These can then twist, like twisting a rope ladder around an imaginary axis, into the double helix (coil), giving the molecule stability Hydrogen bonds allow the molecule to unzip for transcription and replication

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Antiparallel Sugar-phosphate Backbones

The DNA ladder is formed by the sugar-phosphate backbones of the antiparallel polynucleotide strands The 'opposite directions' of the two strands refers to the direction that the third and fifth carbon molecules on the five carbon sugar, deoxyribose, are facing The 5' end of the molecule is where the phosphate group is attached to the fifth carbon atom of the deoxyribose sugar The 3' end is here the phosphate group is attached to the third carbon atom of the deoxyribose sugar The rungs of the ladder consist of the complementary base pairs, joined by hydrogen bonds The molecule is very stable, and the integrity of the coded information within the base sequences is protected

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HOW DNA IS ORGANISED IN CELLS

Eukaryotic cells: The majority of DNA content, or the genome, is in the nucleus Each large molecule of DNA is tightly wound around special histone proteins called chromosomes. Each chromosome is therefore 1 molecule of DNA There is also a loop of DNA, without the histone proteins, inside mitochondria and chloroplasts

Prokaryotic cells: DNA is in a loop and is within the cytoplasm, not enclosed in a nucleus It is not wound around histone proteins Is described as 'naked'

SELF-REPLICATING - DNA

DNA carries the coded instructions to make and maintain a particular organism Every time a cell divides, the DNA has to be copied so that each new daughter cell receives the full set of instructions Each molecule of DNA replicates This replication takes place during interphase In eukaryotes, this results in each chromosome having an identical copy of itself At first they are joined together, at the centromere, forming two sister chromatids The DNA within mitochondria and chloroplasts also replicates each time these organelles divide, which is just before the cell divides

SEMI-CONSERVATIVE REPLICATION

To make a new copy of itself, each DNA molecule; unwinds - the double helix is untwisted, catalysed by a gyrase enzyme unzips - hydrogen bonds between the nucleotide bases are broken, catalysed by DNA helicase, and results in two single strands of DNA with exposed nucleotide bases Next; free phosphorylated nucleotides are bonded to the exposed bases, following complementary base pairing rules the enzyme DNA polymerase catalyses the addition of the new nucleotide bases to the single strands of DNA, it uses each single strand of unzipped DNA as a template hydrolysis of the activated nucleotides, to release the extra phosphate groups, supplies the energy to make phosphodiester bonds between the sugar residue of one nucleotide and the phosphate group of the next nucleotide Product; two DNA molecules, identical to each other and the parent Each of these molecules contains one old strand and one new strand - semi conservative replication

MUTATIONS

During DNA replication, errors may occur and the wrong nucleotide may be inserted This could change the genetic code, and is an example of a point mutation During the replication process there are enzymes that can proofread and edit out such incorrect nucleotides, reducing the rate that mutations are produced However many genes have changes to their nucleotide sequence Different version of a particular gene are called alleles or gene variants Not all mutations are harmful

RNA

The sugar molecule in each nucleotide is ribose The pyrimidine base uracil replaces thymine The polynucleotide chain is usually single-stranded The polynucleotide chain is shorter There are 3 forms of RNA; messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA)

GENES AND THE GENETIC CODE

On each chromosome there are specific lengths of DNA called genes. Each gene contains a code that determines the sequence of amino acids in a particular polypeptide of protein Within each gene there is a sequence of DNA base triplets that determines the amino acid sequence, or primary structure, of a polypeptide As long as this primary structure of a polypeptide is correct, it will then fold correctly and be held in its tertiary structure or shape, enabling it to carry out its function Genes are inside the cell nucleus but proteins are made in the cytoplasm, at ribosomes As the instructions inside the genes, on chromosomes, cannot pass out of the nucleus, a copy of each gene has to be transcribed into a length of mRNA In this form, the sequence of bas triplets, now called codons, can pass out of the nucleus to the ribosome, ensuring that the coded instructions are translated and the protein is assembled correctly from amino acids

THE NATURE OF THE GENETIC CODE

The genetic code is near universal, because in almost all living organisms the same triplet of DNA bases codes for the same amino acid The genetic code is described as degenerate, because, for all amino acids, except methionine and tryptophan, there is more than one base triplet. This may reduce the effect of point mutations, as a change in one base of the triplet could produce another base triplet that still codes for the same amino acid The genetic code is also non-overlapping, and is read starting from a fixed point in groups of three bases. If a base is added or deleted then it causes a frame shift, as every base triplet after that, and hence every amino acid being coded for, is changed

TRANSCRIPTION

A gene unwinds and unzips Hydrogen bonds between complementary nucleotide bases break The enzyme RNA polymerase catalyses the formation of temporary hydrogen bonds between RNA nucleotides and their complimentary unpaired DNA bases A bonds with T; C with G; G with C; and U with A, on one strand of the unwound DNA This DNA strand is called a template strand A length of RNA that is complementary to the template strand of the gene is produced. It is therefore a copy of the other DNA strand - the coding strand The mRNA now passes out of the nucleus, through the nuclear envelope, and attaches to a ribosome Ribosomes are made in the nucleolus, in two smaller sub units. These pass separately out of the nucleus through pores in the nuclear envelope, and then come together to form the ribosome. Ribosomes are made of ribosomal RNA and protein

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TRANSLATION

Transfer RNA molecules are made in the nucleolus and pass out of the nucleus into the cytoplasm They're single-stranded polynucleotides, but can twist into a hairpin shape At one end is a trio of nucleotide bases that recognises and attaches to a specific amino acid At the loop of the hairpin is another triplet of bases, called an anticodon, that is complementary to a specific codon of bases on the mRNA Ribosomes catalyse the synthesis of polypeptides Transfer RNA molecules bring the amino acids and find their place when the anticodon binds by temporary hydrogen bonds to the complementary codon on the mRNA molecule As the ribosome moves along the length of mRNA, it reads the code, and when two amino acids are adjacent to each other a peptide bond forms between them Energy, in the form of ATP, is needed for polypeptide synthesis The amino acid sequence for the polypeptide is therefore ultimately determined by the sequence of triplets of nucleotide bases on the length of DNA - the gene After the polypeptide has been assembled, the mRNA breaks down. Its component molecules can be recycled into new lengths of mRNA, with different codon sequences The newly synthesised polypeptide is helped, by chaperone proteins in the cell, to fold correctly into its 3D shape or tertiary structure, in order to carry out its function