Chromosomes are structures found inside a cell that contain all the information transferred from the previous generations to the current ones. In eukaryotes, or more complex organisms, the chromosomes are seen to occur inside the nucleus of the cell and they are made up of DNA and proteins.
Chromosomes can be seen via a microscope only just before the cell divides. Prior to this the DNA has already been replicated and it coils around proteins. The DNA strands that are formed are known by the name of chromatids and are joined together by means of a disk of protein called centromere.
J. H. Watson and F. H. C. Crick are credited with constructing the DNA’s structure for the first time.
Genes – What are they?
A gene, a section of the chromosome, carries coded information in the form of a series of base pairs. Whenever the conditions are deemed appropriate, a portion of the information is decoded. This provides the recipe for the synthesis of specific proteins that are found inside the cell. These proteins generated may affect the manner in which the cell and the entire organism interact with the environment.
Thus, genes control cell function directly. When the cell reproduces for the purpose of repair or for tissue growth, it is important that the chromosomes reproduce in such a manner that every new cell formed has the entire DNA needed for the purpose of survival.
The biological role that most genes perform is to carry information pertaining to composition and synthesis of proteins. This protein composition information, together with the timing and amount of each protein, primarily determines the structure and physiology of the organism.
Structure of the DNA
The DNA consists of two polynucleotide strands that are coiled in an alpha-helix structure. The four different bases found are: cytosine(C), thymine (T), adenine (A), and guanine (G). The DNA appears as two chains held by complementary pairs of the bases: A-T and C-G. The two chains are bound by a loose hydrogen bond. The existence of base pairs that are complementary enables them to replicate. The DNA molecule is very metabolically stable enabling replication from the previous to the next generation with very little variation. The H bond can easily be broken and reformed during replication.
Relationship between DNA and Genes
The sequence of nucleotides that are found in a section of the DNA (or the gene) decide the arrangement or sequence of amino acids in a polypeptide, which is the protein being synthesized. Thus, a gene is merely a specific portion of the DNA that contains relevant information which is required to synthesize a specific protein. A single DNA molecule may contain many genes.
In the nucleus of the cell, the DNA is bound and coiled with protein histones to form chromosomes. Therefore, there are many genes in a chromosome. The genes in a chromosome stay together as a single unit during the process of cell division.
How Gene expression is affected by DNA structure and shape
The method in which the genes express themselves depends on how the DNA is “packaged” in the chromosomes. The DNA packaging can also be affected by modifications of the histone proteins around which the DNA is wound. Methylation of cytosine may cause mutations. DNA sequences can be damaged by mutagens that include oxidizing agents, electromagnetic radiation, alkylating agents, UV light, etc.