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Molecular Genetics

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The importance of Genetics

Alexis's hemophilia illustrates the important role that genetics plays in a person's life. A difference in one gene, of the 20,000 to 25,000 genes that each human possesses, changed Alexis's life, affected his family, and perhaps even altered the course of history. We all possess genes that influence our lives. They affect our height, weight, hair color, and skin pigmentation. They influence our susceptibility to many diseases and disorders and even contribute to our intelligence and personality. Genes are fundamental to who and what we are.

The Role of Genetics in Biology Genetics provides one of biology's unifying principles: all organisms use the same genetic system. Genetics also undergirds the study of many other biological disciplines. Evolution s genetic change taking place through times; so the study of evolution requires an understanding of basic genetics.

Genetic Diversity and Evolution Life of Earth exists in tremendous array of forms and features that occupy almost every conceivable environment. Life is also characterized by adaptation: many organisms are exquisitely suited to the environment in which they are found. The history of life is a chronicle of new forms of life emerging, old forms disappearing, and exiting forms changing. Despite their tremendous diversity, living organisms have an important feature in common: all use the same genetic system. A complete set of genetic instructions for any organism is its genome, and all genomes are encoded in nucleic acids-either DNA or RNA.

Divisions of Genetics Traditionally, the study of genetics has been divided into three major subdisciplines: transmission genetics, molecular genetics, and population genetics. Transmission genetics encompasses the basic principles of genetics and how traits are passed from one generation to the next. Molecular genetics concerns the chemical nature of the gene itself: how genetic information is encoded, replicated, and expressed. It includes the cellular processes of replication, transcription, and translation-by which genetic information is transferred from one molecule to another- and gene regulation -the processes that control the expression of genetic information. Population genetics explores the genetic composition of groups of individual members of the same species and how that composition changes over time and geographic space.

Genomes

In biology the genome of an organism is the whole hereditary information of an organism that is encoded in the DNA (or, for some viruses, RNA). This includes both the genes and the non-coding sequences. The term was coined in 1920 by Hans Winkler, Professor of Botany at the University of Hamburg, Germany, as a portmanteau of the words gene and chromosome. More precisely, the genome of an organism is a complete DNA sequence of one set of chromosomes; for example, one of the two sets that a diploid individual carries in every somatic cell. The term genome can be applied specifically to mean the complete set of nuclear DNA (i.e., the nuclear genome) but can also be applied to organelles that contain their own DNA, as with the mitochondrial genome or the chloroplast genome. When people say that the genome of a sexually reproducing species has been "sequenced," typically they are referring to a determination of the sequences of one set of autosomes and one of each type of sex chromosome, which together represent both of the possible sexes. Even in species that exist in only one sex, what is described as "a genome sequence" may be a composite from the chromosomes of various individuals. In general use, the phrase genetic makeup is sometimes used conversationally to mean the genome of a particular individual or organism. The study of the global properties of genomes of related organisms is usually referred to as genomics, which distinguishes it from genetics which generally studies the properties of single genes or groups of genes.

Chromosomes

Chromosomes are packages of DNA formed in eukaryotes to organize the genetic information. They protect the DNA and are passed down from parents to offspring. Chromosomes are organized into pairs of homologs that contain alleles of the same gene.

The DNA Molecule

DNA stands for deoxyribose nucleic acid, or deoxyribonucleic acid. Its structure was discovered by James D. Watson and Francis H.C. Crick in 1953 with the assistance of Rosalind Franklin, but the knowledge of DNA was first discovered in 1871. The DNA molecule has a polymer backbone of deoxyribose molecules (Ribose in RNA), a five carbon sugar, connected together by a phosphate group (see phosphorylation). The sugar also connects to a nucleobase (or simply called "base", for short). There are 5 different bases used for coding, 4 of which are used in DNA (the other, uracil, is exclusive to RNA). The four bases are adenine, guanine, cytosine and thymine. These are represented by the letters A, G, C, & T and carry all information found in the DNA (see nucleic acid nomenclature). The structure of DNA is a double helix. This means that there are two strands coiled around each other. The molecule is bonded together by the bases with hydrogen bonds. Guanine pairs with Cytosine by three hydrogen bonds while Adenine bonds with Thymine by two hydrogen bonds (see base pair). Watson and Crick's insight into the double helical structure of the DNA molecule was based upon Erwin Chargaff noting that these pairs of bases were always in the same concentration.

The Discovery of the DNA Molecule

The DNA molecule was discovered by Friedrich Miescher in 1869. He accomplished this by mixing alcohol and pig's stomach juice over bandages containing pus (these were given to Miescher from the local hospital). However, people paid little attention to the molecule until 1952, when its biological role was discovered. In this year, Linus Pauling attempted to determine the structure of the DNA molecule, but was unable to find a structure that could exist. The next year, in 1953, the scientists James Watson and Francis Crick discovered a plausible double-helix configuration using X-ray crystallography.

Structure of the DNA Molecule

DNA is generally found as a double helix, composed of two chains, or strands, of nucleotides held together by hydrogen bonds. A good analogy to this would be a spiral staircase, with the sides of the staircase being the strands, and the steps being the hydrogen bonds.

Nucleotides

As was said above, DNA is composed of a double helix structure with nucleotides in between. Each nucleotide consists of deoxyribose (a 5-carbon sugar), which is bonded to a phosphate group and one of four nitrogenous bases. The sugar and the phosphate make what is usually referred to as the "sugar-phosphate" backbones of the DNA molecule by binding to the sugar and phosphate groups of other nucleotides. The nitrogenous base, on the inside of the double helix, makes hydrogen bonds with the nitrogenous base on the opposite strand.

Base Pairing

The four nitrogenous bases found in DNA are Guanine, Cytosine, Thymine and Adenine, abbreviated as G, C, T and A, respectively. Adenine and Guanine, being composed of two rings, are known as purines, while Cytosine and Thymine, being composed as one ring, are known as pyrimidines.
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