The Role Of The Nucleus
Every cell contains a nucleus, except short-lived cells such as red blood cells and primitive types, Eg bacteria. Each nucleus holds all the information needed to make every cell in the body. This enables it to pass this information on, cell to cell as the body grows; or passes on this information from one generation to the next. This information(needed to make a new individual) is called hereditary information. Whether we are talking about a plant, cow or human; this information is passed on in the same way.
Structure Of The Nucleus
Electron microscopes have shown that nucleus have a sort of cytoplasm, called nucleoplasm; which is enclosed in a nuclear envelope. This envelope is made of two cell membranes separated by a thin perinuclear space. The nuclear envelope has many small pores on it, so the nucleoplasm is in contact with the normal cytoplasm. There is DNA in the nucleus which make up the Chromatin network. This network can't be seen as it is the thickness of a single molecule, although very long. As a cell is about to divide the DNA molecules, coil up like a spring and become coated with certain proteins - making them much shorter and fatter so they can be seen. In this coiled up position we call them chromosomes. Dense structures called nucleoli are also in the nucleus, they control the manufacturing of a substance called ribosome RNA. The nucleoli leave the nucleus and joins proteins from the cytoplasm, to make ribosome;
used for protein synthesis. More than 50% of each ribosome is ribosome RNA, though scientists do not agree about what it does.
DNA stands for Deoxyribose Nucleic Acid, and its important jobs are, cell control and heredity. DNA is said to be one of the more fantastic discoveries of the twentieth century. These molecules can hold all the information about the structure and characteristics of any living animal. DNA was discovered by James Watson and Francis Clark in 1953, and won the Nobel Prize in 1962 for their work.
DNA functions :
1)DNA controls the working of the cell
The metabolism(working) of the cell is the result of making proteins. Proteins and enzymes take up a large portion of the normal protoplasm. DNA molecules determine what proteins are made in each cell and how they are formed. Therefore by controlling the proteins being made, the DNA controls both the structure and working of the cell. A gene is the part of the DNA molecule which controls the making of a particular protein. RNA stands for Ribose Nucleic Acid, which is concerned which making the cell's proteins. It carries the information from the DNA to the ribosomes where the proteins are actually made, so they act as messengers.
2)DNA holds the hereditary information
DNA remains unchanged, and has the remarkable ability of make exact
copies of itself. When an organism is created it receives a
set of DNA molecules(which also means a complete set of genes), in order to work properly all the cells must have the same protoplasm and enzymes, life wouldn't be possible if DNA information were not constant.
Structure Of The DNA Molecule
DNA is made up of units called nucleotides, all the different DNA molecules that make up all the living organisms in the world are made from only four different nucleotides. Each nucleotide is made of:
1)A molecule of a sugar-like substance called deoxyribose.
2)A phosphate ion
3)A nitrogenous base
The four different nucleotides are the same, they only differ in which nitrogenous bases they contain. These bases are : adenine, cytosine, thymine and guanine. All of them contain nitrogen which is why they are called 'nitrogenous'. Adenine and guanine are larger than the other two and are called purines, while cytosine and thymine are called pyrimidines. Each DNA molecule contains millions of nucleotides.
*Each nucleotide is joined to its neighbour in the same way. The phosphate part of one nucleotide is joined to the saugar of the next by a strong sugar-phosphate bond. This always leaves the nitrogenous base standing out on the same side so that a short length of only four nucleotides, one of each type, would be arranged in a chain.
The DNA molecule is made up of two chains of nucleotides, lying side by side which nitrogenous bases linked together by weak hydrogen bonds. These two chains can't lie side by side in any pattern. To make an ordinary molecule with two lines of phosphate/sugar evenly spaced, each nitrogenous base must be joined to a partner of different length. The DNA molecule doesn't exist as straight chains, each of the chains is coiled round the other in a shape known as a helix - so the DNA molecule is a double helix shape. Even this complex double helix itself is wound up as a helix. The helices of the DNA molecule are coiled up around bead-like proteins called histone. It is thought that the histones control which parts of the DNA molecule are active and which are not working in any particular cell. Each strand of chromatin network in a nucleus actually contains one DNA molecule(double helix).
Before a cell divides, the DNA of its chromatin network makes two copies of itself, to have one copy for each cell; with the same information about that individual - complete set of genes. This is replication. The two cells are known as 'daughter' cells.
Replication of a DNA molecule involves taking nucleotides from the surrounding nucleoplasm and joining them together in exactly the same order, as they appear in the DNA molecule which is to be copied. Each of the four types of nucleotides has a partner(its complement) and this is the only one it can join up with(eg cytosine-guanine).
The DNA helix first unwinds and the two strands separate as enzymes work on the weak hydrogen bonds that join across the 'rungs' of the
ladder. This exposes the bases and the complementary nucleotides from the surrounding nucleoplasm become attached, to make two identical double strands. These two double strands each then coil up to give two identical DNA molecules both of which are also identical to the original DNA strand. The process works along the molecule quickly so that only a few nucleotides are exposed at any one time and most of the molecule remains coiled up - this effect has been called 'growing fork'.
Gene mutations sometimes occur (estimated every hundred thousand replication), and a 'new' DNA molecule with slightly different characteristics to its original occurs. Usually this results in the cell dying or not functioning properly, so it doesn't affect the body. If a mutant DNA molecule becomes a zygote cell of a new life, it will affect the new individual a great deal.
REMEMBER - Genes is a short length of a DNA molecule.
A common result of mutation is an inheritable disease called sickle-cell anaemia, which causes a person to suffer all his life and can kill him conditions of oxygen shortage.
The Role Of DNA In Protein Synthesis
DNA are important in the making of protein (which is needed by all living cells) because :
1) Protein makes up a large proportion of the protoplasm, which is called the cytosol.
2) Protein is an important part of the cell membrane.
3) Enzymes are proteins.
4) Many body structures such as hair, nails and feathers are proteins.
5) Many hormones are proteins.
DNA controls the making of proteins and therefore the cells and body of every living thing. A protein is a very large molecule which always contains the elements carbon, hydrogen, oxygen and nitrogen and may contain sulphur. Each type of protein is made of units called amino acids. Although there are only twenty different amino acids, they can combine together to make an infinite number of diferent patterns, each of which is a different protein. Each person has a unique collection of proteins wich have never existed on the earth.