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GENETICS
The genetics is a branch of biology concerned with the study of heredity and variation , that is mean it is define and analyzes heredity and changes in vast array of physiologic functions that form properties of organisms , the unite of heredity is a gene , a segment of DNA that carries in its nucleotide sequence information for a specific biochemical or physiologic property .
Within the cell DNA is organized into long structures called chromosomes , these chromosomes are duplicated before cell divide in a process called DNA replication , gnomic DNA is tightly and orderly packed in a process called DNA condensation to fit the small available volumes of cell . DNA usually occurs as linear chromosomes in eukaryotes and circular chromosomes in prokaryotic cells . The set of chromosome in a cell makes up its genome for ex. in human genome has approximately 3 billions base pairs of DNA arranged into 46 ( 23 pairs ) . Eukaryotic organisms store most of their DNA inside of cell nucleus and some of their DNA in mitochondria or chloroplast , in contrast prokaryotic cells store their DNA only in cytoplasm .
The chemical composition of DNA and RNA
Both DNA and RNA are macromolecules , polymeric molecules made up of different monomeric units called nucleotides , each nucleotide consists of three distinct parts : 1- pentose ( 5- carbon ) sugar , 2- nitrogenous ( N2 – containing) base , 3- phosphate group , because they can be isolated from nuclei and because they are acidic , these macromolecules are called nucleic acids .
For DNA , the pentose sugar is deoxyribose and for RNA it is ribose , the two sugars differ by the chemical groups attached to the 2? carbon ; a hydrogen atom
( H) in deox
yribose and a hydroxyl group ( OH) in ribose ( the carbon atoms in pentose sugar are numbered 1? to 5? to distinguish them from the numbered carbon and nitrogen atoms in the rings of the bases ) . There are two classes of bases , the purines and pyrimidines , in DNA the purines are adenine (A) and guanine ( G) , the pyrimidines are thymine ( T) and cytosine ( C). RNA also contains adenine,guanine and cytosine but the pyrimidine uracil ( U) replaces thymine .
In DNA and RNA , bases are always attached to the 1? carbon of pentose sugar by a covelent bond . The purine bases are bonded at the 9 nitrogen , while the pyrimidines bond at the 1 nitrogen . The phosphate group (PO4 ) is attached to 5? carbon of sugar in both DNA and RNA . The nucleotide , a basic building block of DNA and RNA molecules , consist of sugar , base and a phosphate group . The sugar and base only is called a nucleoside .
Watson – Crick?s double helical model of DNA
The nucleic acids are informational molecules because their primary structure contains a code by which they can duplicate themselves and guide the synthesis of proteins. The synthesis of proteins - most of which are enzymes - ultimately governs the metabolic activities of the cell . In 1953, Watson, an American biologist, and Crick, an English biologist, proposed the double helix structure for DNA. This development set the stage for a new and continuing era of chemical and biological investigation.
Watson and Crick?s helical model of DNA has main features as fellow :
1- The DNA molecule consist of two polynucleotide chains wound around each other in a right – handed double helix , that is the two strands wind around each other in a clockwise ( right – handed ) fashion .
2- The external diameter of the helix is 2 nm .
3- The two chains are antiparallel that is , the two strands are oriented in opposite directions , with one strand oriented in the 5? to 3? way , while the other strand oriented 3? to 5? , antiparallel means that the head of one chain is against the tail of the other chain .
4- The two sugar – phosphate backbones of the double helix are not equally spaced along the helical axis , this result in grooves of unequal size between the backbones called the major ( wider ) groove and the minor ( narrower ) groove, both of these grooves are large enough to allow protein molecules to make contact with bases .
5- The bases of opposite strands are bonded together by hydrogen bonds , which are relatively weak chemical bonds . The specific pairings observed are A with T ( two hydrogen bonds) and G with C ( three hydrogen bonds ) . The hydrogen bonds make it relatively easy to separate the two strands of the DNA . The specific A – T and G – C pairs are called complementary base pairs , so the nucleotide sequence in one stand dictates the nucleotide sequence of the other , for instance , if one chain has the sequence 5?- TATTCCGA- 3? , then the opposite antiparallel chain must bear the 3?- ATAAGGCT - 5? .
6- The sugar – phosphate backbones are on the outsides of the double helix , while the bases oriented toward the central axis .
DNA replication
Replication involves the separation of the DNA strands to synthesis of a complementary copy of new DNA on each of the two parent strands . As a result of this process two DNA molecules of which half of each is derived from the parent DNA molecule and the other half is newly synthesized .
Replication begins when the hydrogen bonds that hold the purine – pyrimidine bases together break apart , the separation starts at one end and proceeds along DNA molecule each half acts as a template for reconstructing the missing half .
There is information that the DNA polymerase catalyzes DNA replication and also share in the repair of DNA in vivo . This requires the presence of all four deoxyribonucleoside triphosphates , d ATP, d GTP , d TTP , d CTP and Mg2 .
The information carried by DNA is held in the sequence of pieces of DNA is called genes , transmission of genetic information in genes is achieved via complementary base pairing in a process called transcription, when a cell uses the information in a gene, the DNA sequence is copied into a complementary RNA sequence through the attraction between the DNA and the correct RNA nucleotides
Base: a building block of DNA and RNA , there are five different bases: Adenine, Thymine, Guanine, Cytosine, and Uracil which is found only in RNA and replaces Thymine in DNA.
Amino acids: Polypeptide building blocks.
Polypeptides: chains of amino acids form the proteins that made up of several polypeptides
Proteins: the organic compounds that make up cells and organ structures that carries out reactions throughout the body, from breaking down food to fighting off disease.
Genome : all the genes contained in a single set of chromosome .
Genotype : The genetic composition of an organisms .
Phenotype : the collective structural and physiologic properties of an organism such as eye color in a human or resistance to an antibiotics in a bacterium .
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .