Nucleic acids and protein synthesis

Nucleic acids and protein synthesisYou should be able to: describe the structures of nucleotides and nucleic acids describe the semi-conservative replication of DNA explain how the sequence of nucleotides in DNA codes for the sequence of amino acids in a polypeptide explain the roles of DNA and RNA in transcription and translation discuss the effect of gene mutation

GM organisms

http://www.bbc.co.uk/schools/gcsebitesize/science/aqa/geneticvariation/reproductionrev5.shtml

Genetic moleculeGenetic molecule- would have to be the ability to carry instructions. It is a Blueprint for the construction and behavior of cells and the way in which they grow together to form a complete living organism.It should have the ability to be copied perfectly, over and over again, so that whenever the nucleus of a cell divides it can pass on an exact copy of each genetic molecule to the nuclei of each of its daughter cells.

The structure of DNA and RNADNA stands for Deoxyribonucleic acid. RNA is Ribonucleic acid.They are macro molecules. The smaller molecules from which DNA and RNA molecules are made are Nucleotides.DNA and RNA are therefore polynucleotides.

NucleotidesNucleotides are made up of three smaller components. These are: a nitrogen-containing base. a pentose sugar. a phosphate group.

Nucleotides

PhosphatePentoseSugarNitrogenousBase

NucleotidesThe phosphate and sugar form the backbone of the DNA molecule, whereas the bases form the rungs.

There are four types of nitrogenous bases.

Nucleotides

AAdenine

TThymine G

Guanine

CCytosine

NucleotidesEach base will only bond with one other specific base.

Adenine (A)Thymine (T)

Cytosine (C)Guanine (G)

Form a base pair.Form a base pair.

DNA StructureBecause of this complementary base pairing, the order of the bases in one strand determines the order of the bases in the other strand.

G

G

A

T

T

A

A

C

TG

C

A

T

C

DNA StructureTo crack the genetic code found in DNA we need to look at the sequence of bases.

The bases are arranged in triplets called codons.

A G G - C T C - A A G - T C C - T A GT C C - G A G - T T C - A G G - A T C

RNAAn RNA molecule also contains four bases, but never the base thymine.Instead, RNA molecules contain a base called uracil.

PolynucleotidesTo form the polynucleotides DNA and RNA, many nucleotides are linked together into a long chain. This takes place inside the nucleus, during interphase of the cell cycle.

The covalent sugarphosphate bonds (phosphodiester bonds) link the 5-carbon of one sugar molecule and the 3-carbon of the next. The polynucleotide strand is said to have 3 and 5 ends.

DNA molecules are made of two polynucleotide strands lying side by side, running in opposite directions. The strands are said to be antiparallel. The two strands are held together by hydrogen bonds between the bases.

Purines and Pyrimidines

Adenine always pairs with thymine.cytosine always pairs with guanine A with T C with G.The hydrogen bonding of Adenine with Thymine or Uracial and of Cyosine with Guanine in nucleic acids is called complementary base pairing.

DNA is often described as a double helix.

The hydrogen bonds linking the bases, and therefore holding the two strands together,can be broken relatively easily. This happens during DNA replication (DNA copying) and also during proteinsynthesis (protein manufacture).

The breaking of the hydrogen bonds is a very important feature of the DNA molecule that enables it to perform its role in the cell.RNA molecules, unlike DNA, remain as single strands of polynucleotide and can form very different three dimensional structures.

DNA replicationGenetic molecule would have to be the ability to be copied perfectly many times over.

Watson and Crick suggested that the two strands of the DNA molecule could split apart.

New nucleotides could then line up along each strand, opposite their appropriate partners, and join up to form complementary strands along each half of the original molecule. The new DNA molecules would be just like the old ones, because each base would only pair with its complementary one. Each pair of strands could then wind up again into a double helix, exactly like the original one.

semi-conservative replicationDuring DNA replication, a double stranded DNA molecule separate, and each strand is used as a Template for the synthesis of a new strand. This results in the formation of two identical copies of the original double stranded molecule. This is called semiconservative replication. Each round of DNA replication produces hybrid molecules each of which contains one old strand and one newly synthesized strand.

DNA replicationThe first step in DNA replication is to unzip the double helix structure of the DNA molecule.Replication of DNA starts at specific locations known as the origin of replication.

DNA Polymerase - This enzyme complex travels down the DNA strand adding nucleotide bases to each strand.DNA ligase - The enzyme DNA ligase seals up the fragments into continuous strands.

In the 1950s, no-one knew exactly how DNA replicated, Three possibilities were suggestedConservative replication, in which one completely new double helix would be made from the old one.

Semi-conservative replication, in which each new molecule would contain one old strand and one new one.

Dispersive replication, in which each new molecule would be made of old bits and new bits scattered randomly through the molecules.Material in the two parental strands is distributed more or less randomly between two daughter molecules.

Matthew Meselson and Franklin Stahl experiment Proof of semi conservative model They used the bacterium Escherichia coli, a bacterium which lives in the human alimentary canal.

They grew populations of the bacterium in a food source that contained ammonium chloride as a source of nitrogen.The experiment relied on the variation in structure of Nitrogen atoms. All nitrogen atoms contain seven protons but the number of neutrons can vary. Most nitrogen atoms have seven neutrons, so their relative atomic mass (the total mass of all the protons and neutrons in an atom) is 14.Some nitrogen atoms have eight neutrons, so their relative atomic mass is 15. These two forms are said to be isotopes of nitrogen.

Genes and MutationsThe bacterium E. coli has just one DNA molecule, which is four million base pairs long.

Plasmid of E coli

A part of a DNA molecule, where the nucleotide sequence codes for just one polypeptide, is called a gene.

one DNA molecule contains many genes. A change in the nucleotide sequence of a gene, which may then result in an altered polypeptide, is called a mutation.Most genes have several different variants called alleles. Allele is one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.

How can a single type of molecule like DNA control all the activities of a cell? All chemical reactions in cells, and therefore all the cells activities, are controlled by enzymes. Enzymes are proteins. DNA is a code for proteins, controlling which proteins are made.Thus, DNA controls the cells activities.Protein molecules are made up of strings of amino acids. The shape and behavior of a protein molecule depends on the exact sequence of these amino acids.DNA controls protein structure by determining the exact order in which the amino acids join together when proteins are made in a cell.

The triplet codeThe sequence of nucleotide bases in a DNA molecule is a code for the sequence of amino acids in a polypeptide.

The code is a three-letter code. Each sequence of three bases stands for one amino acid. The sequence is always read in the same direction and from only one of the two strands of the DNA molecule which is called the sense strand.The complementary strand is referred to as the anti-sense strand.

An example of mutation: sickle cellanemiaOne mutation that has a significant effect is the one involved in the inherited blood disorder sickle cell anemia.Hemoglobin is the red pigment in red blood cells which carries oxygen around the body.

Hemoglobin molecule is made up of four polypeptide chains, each with one iron-containing haem group in the center. Two of these polypeptide chains are called chains, and the other two chains.

The gene which codes for the amino acid sequence in the polypeptides is not the same in everyone. In most people, the polypeptides begin with the amino acid sequence:Val-His-Leu-Thr-Pro-Glu-Glu-Lys- This is coded from the HbA (normal) allele of the gene.

But in some people, the base sequence CTT is replaced by CAT, and the amino acid sequence becomes:Val-His-Leu-Thr-Pro-Val-Glu-Lys- This is coded from the HbS (sickle cell) allele of the gene.

This type of mutation is called a substitution. In this case, the small difference in the amino acid sequence results in the genetic disease sickle cell anemia in individuals with two copies of the HbS allele.

Protein synthesisThe code on the DNA molecule is used to determine the sequence of amino acids in the polypeptide.

Transcription

Transcription is the first stage of protein synthesis and this is a process by which the information in DNA is copied into messenger RNA (mRNA) for protein production.mRNA carries the code from DNA to the ribosomes where translation into a protein occurs.

Transcription = DNA RNATranslation = RNA pProten

In the nucleus, part of a DNA molecule unwinds and unzips as the hydrogen bonds between the bases break.Free activated RNA nucleotides pair up with the exposed bases of one strand only, in a similar way to that shown.

As the RNA nucleotides pair up with their complementary ones, their sugarphosphate groups are bonded together by RNA polymerase to form a sugarphosphate backbone. The new single-stranded molecule which has formed is called messenger RNA. It leaves the nucleus via a pore in the nuclear envelope.

Anticodon and codon In the production of proteins, codons refer to the three-base segments in messenger ribonucleic acid or mRNA, while anticodons are the three-base segments in transfer RNA or tRNA.

In the cytoplasm, there are free amino acids and transfer RNA (tRNA) molecules. At one end of each tRNA molecule is a site to which an amino acid can bind. At the other end are three unpaired bases.

The base triplet is called an anticodon. Each tRNA molecule bonds with a particular amino acid, under the control of a specific enzyme and with energy from ATP.

Anti codon

The first three exposed bases, or codon, are always AUG. A tRNA molecule with the complementary anticodon, UAC, forms hydrogen bonds with this codon. This tRNA molecule has the amino acid methionineattached to it.

A second tRNA molecule bonds with the next three exposed bases. This one brings a different amino acid. The two amino acids are held closely together, and a peptidebond is formed between them. This reaction is catalyzed by the enzyme peptidyl transferase,which is found in the small subunit of the ribosome.