Biodiversity

Thursday, April 27, 2006

THE STRUCTURE AND SIGNIFICANCE OF DNA TO LIFE AS WE KNOW IT

DNA is a chemical known as deoxyribonucleic acid. It is the key to life, the master molecule that contains the recipes for producing proteins that regulate all functions in an organism, from the activities and repair of individual cells to the development and function of organs systems, even to reproduction of the organism itself.

In eukaryotes (1), DNA is found in the cell nucleus where it is spiralled up in its typical double helix form. Most eukaryotes also have mitochondria, which contains its own DNA. In plants the DNA is also stored in chloroplasts. In prokaryotes (2) (mostly bacteria), cells without a nucleus, the DNA is stored in a single circular chromosome in the region of the nucleoid structure, but smaller circular pieces of DNA can also be spread throughout the cytoplasm.

DNA contains the genetic code or recipe to produce proteins (made up of amino acids), which regulate the development and growth of any living organism. DNA resembles a twisted ladder (double helix), its sides made up of linked phosphate and deoxyribose (sugar) molecules. The rungs of the “ladder” are provided by nucleotides that link the two sugar chains. There are 4 nucleotides namely adenine (A), cytosine (C), guanine (G) and thymine (T) and they combine in fixed combinations to form base pairs. “A” can only pair up with “T” and “C” can only pair up with “G”. Three base pairs together form a codon that codes for a specific amino acid, and amino acids are the basic building blocks of proteins. Each DNA strand has directionality and therefore the sequence of the nucleotides matters. “T+A” is therefore not the same as “A+T”. In this way DNA contains the master code that regulates the production of every single protein produced by an organism.

A DNA strands contains genes that code for a specific protein, regulatory areas that control the function of these genes, and areas where the function is not yet known. Genes exist as pairs or alleles, such that there are two doses for each genetic determinant, one from the mother and one from the father. Alleles code for specific hereditary traits and have many viable combinations, for example the allele that determines the colour of a flower petal has different possibilities, giving rise to different coloured flowers in the same species. In this way DNA provides the basis for transferring specific genetic traits to the next generation through inheritance.

Living cells usually divide on a regular basis, to achieve organism growth and to replace damaged cells. This type of cell division is called mitosis, but before mitosis can occur, the DNA must be replicated. Replication occurs when the DNA molecule unzips itself along its length and separates. The two single strands then act as templates for the formation of two new complementary strains. As a result, two identical copies of the original DNA molecule are produced. Once the DNA (chromosomes) has been replicated, each of the individual chromosomes line up on the equatorial plain. Their doubled structure is clearly visible as two identical chromatids. Cell division occurs when microtubules pull each of these chromatids to an opposite pole of the cell, before division of the cytoplasm takes place. In this way two identical daughter cells are formed that contain a full set of chromosomes.

Sperm cells and egg cells are produced by a different mechanism called meiosis, during which chromosomes align themselves in pairs (each consisting of two chromatids) on the equatorial plain. At this time, random genetic exchange occurs between the chromatids of the different chromosome pairs. After this process of genetic exchange, members of each chromosome pair are pulled to opposite poles of the cell before the cytoplasm divides. After the first meiotic division, each daughter cell contains only half the complete (diploid) number of chromosomes. The second meiotic division is similar to mitosis, but the end result (due to the genetic exchange that occurred) is that 4 daughter cells are formed with only half the chromosome number (haploid), but with unique genetic rearrangements.

During the process of DNA replication, when a DNA strand unzips itself to be duplicated and form two new strands, mistakes (mutations) may occur (mutations may also occur at other times). These mutations generate genetic variability, giving rise to new traits and a multitude of new possibilities. Therefore the genetic variability in organisms, which is essential for evolution to occur, results from genetic exchange during sexual reproduction and from random DNA mutations that may occur at any time.

These two processes, genetic exchange that occurs mainly during sexual reproduction (genetic exchange may also occur without sexual reproduction as happens between bacteria) and mutations, create the genetic diversity that underlies phenotypic variability. The creation of variability is an essential ingredient of natural selection, when a strong selective force constantly identifies the fittest individuals.

DNA is thus truly the recipe book for all living oranisms.

References:

1. Wikipedia contributors. Eukaryote [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 25, 16:15 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=Eukaryote&oldid=50104670.
2. Wikipedia contributors. Prokaryote [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 17, 10:15 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=Prokaryote&oldid=48824695.

I used the following site as my general source:

Wikipedia contributors. DNA [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 26, 11:37 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=DNA&oldid=50241434

Karen Marais
BCB Hons NISL student
University of the Western Cape
Private Bag X17
Bellville

E-mail 2657211@uwc.ac.za
Web http://brit-journal.com/karen2006bcbnisl/

2 Comments:

  • DNA are the most vital thing in our lives why because one mistake(mutation) the person will be somehow be deformed. even the HIV virus attacks the DNA

    By Blogger lethabo, at May 15, 2006 10:27 AM  

  • References should be noted throughout the text and all references should be included in the reference list.
    I found the paragraph on meiosis ambiguous.
    A minor point, mitochondria is the plural for mitochondrion.

    By Blogger Gwen, at May 16, 2006 9:48 AM  

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