General Animal Biology 109 - King Saud University PDF

‫علم األحياء‬ Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings In April 1953, James Watson and Francis Crick shook the scientific world with an elegant double-helical model ‫ النموزج الحلزوني المزدوج‬for the structure of deoxyribonucleic acid or DNA. Watson and Crick began...

‫علم األحياء‬ Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings In April 1953, James Watson and Francis Crick shook the scientific world with an elegant double-helical model ‫ النموزج الحلزوني المزدوج‬for the structure of deoxyribonucleic acid or DNA. Watson and Crick began to work on a model of DNA with two strands, the double helix ‫الحلزوني المزدوج‬.  The amino acid sequence of a polypeptide is programmed by a gene. A gene is a small region in the DNA. Nucleic acids store and transmit hereditary information ‫المعلومات الوراثية‬. There are two types of nucleic acids: ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). DNA also directs mRNA synthesis, thus, controls protein synthesis. Organisms inherit ‫ تتوارث‬DNA from their parents. – Each DNA molecule is very long and usually consists of hundreds to thousands of genes. – When a cell divides ‫تنقسم‬, its DNA is copied and passed to the next generation of cells (during Meiosis) The mRNA interacts with ribosomes to direct the synthesis of amino acids in a polypeptide (protein). 3 o5 Bases o P o o DNA Basenucleotide Adenine Phosphate group 5 CH2 o ‫نيوكليوتيدة واحدة‬ (A) Purine 4 H H 1 H Guanine 3 H 2H (G) o Deoxyribose o P o o Base Cytosine CH2 o (C) Pyrimidine H H H 3 H Thymine 5 3 H (T) Sugar-phosphate backbone  The PO43- group of one nucleotide is attached to the sugar of the next nucleotide in line ‫في صف مستقيم‬. The result is a “backbone” of alternating ‫تبادل‬ phosphates and sugars, from which the bases starts.  3 5 Hydrogen bonds Nitrogenous bases Cytosine Guanine (C) (G) Thymine Adenine (T) (A) Uracil (U) Pyrimidines Purine 5 3 Sugar-phosphate backbones  Adenine (A) would form 2 hydrogen bonds only with thymine (T) Guanine (G) would form 3 hydrogen bonds only with cytosine (C).  CH2 o CH2 o H H H H H H H H H OH Deoxyribose sugar Ribose sugar (O on C-2 is missed) (no missed O) Deoxiribo-Nucleic-Acid Ribo-Nucleic-Acid Double stranded nucleic acid Single stranded nucleic acid Bases: A, G, C, T Bases: A, G, C, U  Nucleic acids are polymers of monomers called nucleotides. Each nucleotide consists of three parts: a nitrogenous base, a pentose sugar, and a phosphate group. The nitrogenous bases (rings of carbon and nitrogen) come in two types: Purines and Pyrimidines. The pentose sugar joined to the nitrogen base is ribose in nucleotides of RNA and deoxyribose in DNA. The only difference between the sugars is the lack ‫ نقص‬of an oxygen atom on carbon-2 in the deoxyribose.  Polynucleotides are synthesized by connecting the sugars of one nucleotide to the phosphate of the next with a phosphodiester link. This creates a repeating backbone of sugar-phosphate units with the nitrogenous bases as appendages. The sequence of nitrogen bases along a DNA or mRNA polymer is unique for each gene. Genes are normally hundreds to thousands of nucleotides long. The linear order ‫ الترتيب التتابعي‬of bases in a gene specifies ‫ يُحدد‬the order of amino acids ‫( ترتيب األحماض األمينية‬the monomers of a protein).  The flow of genetic information is from DNA mRNA protein (at the ribosome). – Protein synthesis occurs by ribosomes. – In eukaryotes, DNA is located in the nucleus, but most ribosomes are in the cytoplasm with mRNA as an intermediary ‫وسيط‬.  An RNA molecule is a single polynucleotide chain (single strand). DNA molecules have two polynucleotide strands (double strand) that spiral around ‫ تدور حلزونيا‬to form a double helix ‫حلزون مزدوج‬.  The sugar-phosphate backbones of the two polynucleotides are on the outside of the helix. Pairs of nitrogenous bases (one from each strand) connect the polynucleotide chains with hydrogen bonds. Most DNA molecules have thousands to millions of base pairs ‫( زوج من القواعد‬bP).  Because of their shapes, only some bases are compatible ‫ متوافقة‬with each other. – Adenine (A) always pairs with thymine (T) and guanine (G) with cytosine (C). With these base-pairing rules, if we know the sequence of bases on one strand, we know the sequence on the opposite strand ‫الشريط المقابل‬ The two strands are complementary ‫مكملين لبعضهما‬. During preparations for cell division each of the strands serves as a template ‫ قالب نسخ‬to order nucleotides into a new complementary strand (see the next lecture). This results in two identical copies ‫ نسختين طبق األصل‬of the original double-stranded DNA molecule. – The copies are then distributed ‫ توزع‬to the daughter cells (by meiosis). This mechanism ensures that the genetic information is transmitted to the new cells. DNA A G C T A T C mRNA T U C G A T U A G