DNA, genes and chromosomes CHAPTER 13 DNA THE CODE FOR LIFE - DNA is short for deoxyribonucleic acid found in cells of organisms. The DNA is embedded within the nucleus of a cell. - DNA is a long stranded molecule which contains the genetic information of cells that determines the cell structure and many other important functions and activities of the cell. - There are about 46 DNA molecules in a cell. DNA molecules are quite long so how do they fit in a nucleus? - They fit because they are bound to proteins called histones. The histones have a spherical shape. - The DNA molecules are coiled around the histones. The DNA molecules are coiled around histones so that they take up a small space due to being longer. - When the cell is not dividing the histones and DNA molecules forms a tangled network that are called chromatin. - When a cell divides exact copies of DNA must be distributed between the the 2 daughter cells. This procudure is possible as the mass of the tangled network of histones and DNA molecules (chromation) becomes more tightly coiled. - This super coiled structure is called chromosomes. - The genes are found within the chromosomes. Genes are sections of the DNA molecule or chromosomes. They have the genetic code. Structure of DNA - In a DNA molecules present in the nucleus consists of two strands of alternating sugar and phosphates with pairs of nitrogenous bases forming cross links in between the sugar molecules in the two strands. - The structure is twisted into a spiral shape called a spriral shape called a double helix. - Each phosphate group and with a sugar molecule attached with a nitrogen base is called a neucleotide. - Nucleotidesare units making up the DNA molecule. 4 types: adenine (A) that pairs with thymine (T) and cytosine (C) that pairs with guanine (G). - The nitrogen bases are bonded by weak hydrogen bonds and with chemical bonds with the sugar and phosphate group.
- The order in which the bases pair up is what determines the genetic code. Replication of DNA - When cells divides, during interphase, the DNA molecules of the cell undergoes replication. It is the process of forming the exact replicas of themselves. - The 2 linked DNA chain breaks which is possible due to the weak hydrogen bonds between the bases. - Each separated section contain half the original genetic information. - These then serve as a template for the nucleotides that will form the new half.
Role of DNA in the Cell - The gentic code in DNA provides the instructions for protein synthesis. - These proteins that have been produced are used for various functoning of the cell. Protein Synthesis - Enzymes, hormones, antibodies and other structural materials of cells are all proteins. - For example: haemoglobin the oxygen carrying molecule in red blood cells, actin and myosin which are the proteins involved in muscle contraction and insulin a hormone that regulates concentration of glucose in blood. The Genetic Code - The types of protein produced is determined by the genes, which forms parts of the DNA molecule. - The DNA molecule contains thousands of genes that holds the information for protein synthesis. - Each body cell contains identical DNA but dufferent genes are activated in different cells. Cells except sperm have no identical DNA - For example, production of insulin is activated in the pancreatic cells not in the bone cells or liver. There is a specific location. - The order in which the 4 nitrogen bases joins determines the genetic code. - A sequence of 3 bases is the code for a particular type of amino acid for example: CAG = valine, TTA = leucine, CCC = proline. - If all the bases combines and forms CAG TTA CCC then the amino acid proline, leucine and valine would be assembled. Transcription - DNA molecules are too small to leave the nucleus so the code for each amino acid is taken from the DNA to the ribosomes in the cystoplam of the cell by the ribonucleic acid (RNA). - RNA only includes a single strand of sugar and phosphate group and the bases alsooccur singly. - The RNA molecule that takes the code from the DNA in the nucleus to the ribosomes are messenger RNA (mrna). They are small enough to pass through the nuclear pores. - Transcription is the process by which the mrna is formed using the code in a DNA molecule. - The genetic instructions are transcribed from DNA to the mrna molecule. - The transcription is triggered by chemical messengers entering the nucleus. This causes the enzyme RNA polymerase to begin the process of making mrna.
- Enzymes like helicases, allows the double stranded DNA molecule to come apart. ( 17 base pairs at a time). - RNA polymerase then copies or trancribes the bases on one strand of the DNA to make a complementary molecule of mrna. - There is a cytosine on the DNA, a guanine will be added to mrna. - RNA has uracil (U) base than thymine. Adenine combines with uracil. - At the end of the gene, a sequence of bases tells the RNA polymerase to stop copying and as a result the mrna molecule is released. - Other RNA polymerase would follow the 1 st one to produce copies of same mrna. - One DNA molecule is copied during transcription. The copied strand is the template strand. It is the template from which mrna is made. - The other starnd is the coding strand. The bases always form complementary pairs, the order of bases on the coding strand will be the same as the mrna molecule. (except uracil will be in place of thymine). Translation - It is the process by which proteins are produced using the information that has been coded in the mrna molecule. - The ribosomes attahes to one end of the mrna molecule. There is a particular sequence of bases that are a binding site for the ribosomes.
- The ribosome moves along the mrna 3 bases at a time. It pulls the mrna like a ribbon and reading the bases. - Each group of the 3 bases is a code for a particular amino acid called codon. - The AUG is a start codon. As the ribosome reaches this codon it starts making proteins. - Small molecules of RNA, transfer RNA brings the individual amino acids to the ribosomes to be joined together forming the proteins. - Each trna molecule has a section that binds to its corresponding amino acid. - Halfway the chain of nucleotides forming the protein forms a tight loop which contain 3 nitrogen bases called antocodon. - These anticodon can bind with the complementary bases of the codon on the mrna molecule. - This anticodon is what determines what determines the type of amino acid carried by the trna. - As the ribosomes reads codon on mrna, trna molecules with the matching anti codon are brought in. - The amino acids on trna are joined so the protein assembled is in correct sequence. For the formation of the bonds of the amino acid ATP is needed. - When the trna has done its work delivering the amino acid, it detaches from the ribosome and picks up another amino acid. - The process of copying information from DNA on to messenger RNA and then translating the messenge into a series of amino acids is called gene expression. - The process of gene expression is complicated but it happens very quicky. Mitochondrial DNA - Aerobic respiration takes place here for the release of energy within the cells. Most of the DNA is in the nucleus but some are embedded in the mitochondria.
- In the nucleus the DNA is coiled around histones and remain in long strands but in the mitocondria they are in a small circular molecules not bounded to proteins. ( about 5-10 of these molecules) - There are 37 genes in the mitochondrial Dna. 24 contain code for the production of trna. 13 other contain instructions for making enzymes needed for reactions of cellular respiration. Gene Expression - The process of copying information from DNA on to messenger RNA and then translating the messenge into a series of amino acids is called gene expression. - These genes have the instructions for producing the mrna but at a given time the cell is making the mrna from only a fraction of its genes. - The genes that are used to make mrna are called switched on, a gene not used is switched off. - Factors influencing the gene expression meaning if is turned on or off by: the age of cell, time of the day, signals from other cells, the environment of the cells, and whether or not the cell is dividing. Epigenetics - Some of the factors that makes genes more or less likely to be expressed may be inherited and these factors are called epigenetics.
- One way in which the genes are regulated epigenetically is through the changes in chromatin. Chromatin - The DNA molecule inside the nucleus is about 2-3 meters long. Cells are microscopic so for this long DNA to fit, the DNA has to be tightly coiled around histones. - When the cell is not dividing it forms a tangled network. The Dna and the histone proteins associated with it are called chromatin. - The gene expression changes if the way in which the DNA is wrapped around the histone changes. - Eg: if the shape of amino acids in histone changes, the modified histone shape will be copied every time a new DNA molecule is formed. This modified histone will ensure that a stem cell that differentiated into a liver cell would remain a liver cell and not turn in to a stem cell. - Another histone modofication is acyteleration: which is the addition of an acetyl group to the histone. This enhances gene expression. - Chromatin remodelling can be done by adding a methyl group to the DNA which is called methylation. This inhibits gene expression. Environment and Epigenome - Environmental changes that cause epigenetic changes include: severe stress, nutitional factors, toxins and drugs. Thse do not change the Dna but interfere with transcription and translation.