Genetics Second Level LMD Past Paper PDF 2024-2025

Genetics Module: Genetics Second Level LMD Academic year 2024-2025 Course n°6: Structure of nucleic acids Cours n°7: Replication of DNA  Structure of nucleic acids: Structure of DNA Nucleoside and Nucleotide...

Genetics Module: Genetics Second Level LMD Academic year 2024-2025 Course n°6: Structure of nucleic acids Cours n°7: Replication of DNA  Structure of nucleic acids: Structure of DNA Nucleoside and Nucleotide In the 1950s, the Americans James Watson and Francis Crick determined the fine structure of the molecule that makes up genes, DNA, and thus help to understand the molecular mechanisms of heredity. Deoxyribo Nucleic Acid DNA is formed by a sequence of nucleotides. The basic constituents of the nucleotide are: ✓ a phosphoric acid ✓ a deoxyribose ✓ a nitrogenous base. (A) A nucleotide (guanosine triphosphate). The nitrogenous base (guanine in this example) is linked to the 1′ carbon of the deoxyribose and the phosphate groups are linked to the 5′ carbon. A nucleoside is a base linked to a sugar. A nucleotide is a nucleoside with one or more phosphate groups. (B) A DNA strand containing four nucleotides with the nitrogenous bases thymine (T), cytosine (C), adenine (A) and guanine (G) respectively. The 3′ carbon of one nucleotide is linked to the 5′ carbon of the next via a phosphodiester bond. The 5′ end is at the top and the 3′ end at the bottom. There are 4 bases that go into the structure of DNA: Adenine, guanine, cytosine, and thymine. A nucleotide with adenine as a base is called Adenosine... P- D- A ADENOSINE P-D- T THYMIDINE P- D- C Cytidine P- D- G Guanosine Primary structure of DNA: Nucleotides can bind to each other by their sugar (deoxyribose) and phosphate group: strong phosphodiester bond. Secondary structure: The DNA molecule is formed by two chains of the same structure linked together by the complementarity effect of nitrogenous bases. The bases are connected to each other by hydrogen bridges. Hydrogen bond-like interaction Erwinn Chargaff (1947): If we separate a DNA molecule into nucleotides, we always get: A = T, C = G and A + G = T + C A+T / G+C = constant of species. The total length of the DNA; For a virus: 0.5 to 50 μm For one bacterium: 1 mm For a fruit flies: 0.13 m For a dog: 1.89 m. In men 2-2.36 m. Deoxyribonucleic acid (DNA) The DNA double helix is composed of two intertwined chains of nucleotides. Nucleotide pairing Nucleotide chains are antiparallel. RNA structure The structure of RNAs is close to that of DNA, however, there are several important differences; -In RNA, ribose replaces deoxyribose. -In RNA, thymine is replaced by uracil, which is also complementary to adenine. Differences between DNA and RNA Characteristics DNA RNA General structure Two strands twisted into the shape of a A bit straight double helix Sugar Deoxyribose Ribose Nitrogenous bases Adénine,Thymine,Guanine and Cytosine Adénine, Uracil, Guanine and Cytosine Function The genetic material that forms genes Helps make proteins Rental in the cell In the core Starts in the nucleus but can move into the cytoplasm Course 7: DNA Replication DNA replication This is the process by which a cell copies its DNA. DNA is copied by enzymes called DNA polymerases. They work on single-stranded DNA by synthesizing a new strand that complements the original strand. DNA synthesis always takes place in the 5" → 3" direction Replication is said to be semi-conservative; this means that each copied DNA molecule contains one strand derived from the parent molecule and one neosynthesized strand. The replication fork The unwinding of the double helix begins at a precise position called the origin of replication and gradually progresses along the molecule, in a bidirectional manner. Replication origins typically contain sequences rich in A′T pairing this region is called (replication fork). Figure : Réplication de l'ADN : origines et fourches de réplication. A. Petit ADN circulaire procaryote. B. Très long ADN eucaryote S phase of the cell cycle: DNA replication. SYNTHESIS 1 mol. DNA is replicated or duplicated in 2 mol. identical DNA during the S phase of the cell cycle = DNA replication. Is done in a semi-conservative way (each daughter molecule contains 1 strand of the parent molecule) and respecting the complementarity of the A, T and C G bases. DNA polymerase is an enzyme that allows the synthesis of new strands. Takes place before any mitosis or meiosis (division of gametes).