DNA Structure and Chromosome Organisation PDF

College of medicine MGD Module Session 4 Lectures8 DNA Structure andChromosome Organisation 9/3/2023 Dr. Ghazwan Fawzi Lectures 8 DNA, Chromosomes and DNA Replication Biological Molecules MGD Intended learning outcomes At the end of this lecture you should be able to: 1. Explain how eukar...

College of medicine MGD Module Session 4 Lectures8 DNA Structure andChromosome Organisation 9/3/2023 Dr. Ghazwan Fawzi Lectures 8 DNA, Chromosomes and DNA Replication Biological Molecules MGD Intended learning outcomes At the end of this lecture you should be able to: 1. Explain how eukaryotic DNA is condensed in nucleosomes and relate this to the structure of chromosomes. (LO 4.6) 2. Describe the process and role of DNA replication. (LO 4.7) 3. Explain the role of DNA polymerase and other enzymes in DNA replication. (LO4.8) 4. Describe the process and the role of the cell cycle. (LO 4.9) MGD If we want to study how eukaryotic DNA is condensed in nucleosomes we should know many terms : • • • • • • DNA andNucleotides Eukaryotic DNA Histone Nucleosomes Chromatin Chromosomes MGD 4 (LO4.6) DNA and Nucleotides (explain previously) A cell's genetic information, in the form of DNA, is stored in the nucleus. The space inside the nucleus is limited and has to contain billions of nucleotides that compose the cell's DNA. Therefore, the DNAhas to be highly organized or condensed. Ø Eukaryotic DNA - Which is located in anucleus. - Eukaryotic DNA is linear, compacted into chromosomes by histones. Watson and Crick hinted that the replication of the double helix could take place as the DNA unwinds, so that each strand would form a new double helix by acting MGD as a template for a newly synthesized strand. Ø Chromatin and Histone (LO4.6) • Nucleus contain many thread like coiled structures suspend in nucleoplasm known as chromatin. • Chromatin is the complex combination of DNA and the proteins that make up the chromosomes. • Major protein involved in chromatin arehistone. • The function of chromatin is to efficiently package DNA into a small volume to fit into the nucleus of a cell and protect the DNA structure and sequence. Packaging DNA into chromatin allows for mitosis and meiosis, prevents chromosome breakage and controls gene expression and DNA replication. MGD X Chromatin and Condensed Ø (LO4.6) Ø Nucleosomes and Chromosomes Every 200 nucleotide pairs, the DNA coils twice around a group of 8 histone proteins (a pair of each of four types), forming a beads-on-astring-like structure. The bead part is called a nucleosome. MGD • The repeating structural unit within eukaryotic chromatin isthe Nucleosome. It is composed of double-stranded DNA wrapped around an octamer of histoneproteins. MGD Play a role in the organization and compacting chromosome MGD (LO4.6) 1 2 3 MGD Levels of Chromatin Packing Condensing Solenoid model MGD Zigzag model Chromosomes in eukaryotes n n n n n n n Chromosome is an organized structure of DNA and protein. Composed of repeating 200 bp unit, which were called Nucleosome. Found in the nucleus. Carrier of genes or unit of hereditary. Condensed and visible during cell division. At the beginning of mitosis they can be seen to consist of two threads (sister chromatids) joined by a centromere. Chromosome must be protected at the ends by telomere and it must have “something” that mediated migration during mitosis and meiosis by centromere. MGD • Eukaryotic cells contain different types of chromosomes with different genes. The location of each gene on the chromosome is the gene locus. Ø Humans have 23 pairsdifferent types of chromosomes differ in length and position of the centromere. • 22 pairs of Autosomes(22) • 1 pair of SexChromosomes (23) MGD Do Notbe Found in Human MGD (Lo.4.7) DND Replication Where and when its occur? What we need to achieve this Process ? Ø DNA replication occurs in nucleus during S phase of the cell cycle. MGD 1. 2. 3. 4. Helicase: Zipper enzyme Primase : Director enzyme DNA polymerase : Builder enzyme Ligase : Gluer enzyme Ø(LO.4.8) ØDNA Replication is a semiconservative process that results in a doublestranded molecule that synthesizes to produce two new double stranded molecules such that each original single strand is paired with one newly made single strand. 1- Replication begins at specific sites on DNA molecule called "origins of replication " mammalian DNA have many origins MGD 2-Replication fork is a structure that forms within the nucleus during DNA replication. It is created by helicases, which break the hydrogen bonds holding the two DNA strands together. Replication fork is where the parental DNA strands hasn't untwist. 3-Replication bubbles allow DNA replication to speed up therefore the untwisted DNA would not be attacked by enzymes while replicating. 3 5 3 MGD 5 LO.4.10 The cell cycle • The continuity of life depends on cells growing, replicating their genetic material, and then dividing, a process called the cell cycle. Although cells usually divide when they have doubled in volume, the control of this process is very complex and precise. Not only do all the steps have to occur in sequence, but the cell must also “know”MGwD hDer. nAlyatqoubi proceed and when to wait. Ø All multicellular organisms use cell division for growth and the maintenance and repair ofcells and tissues. Ø Single-celled organisms use cell division as their method reproduction. of Ø Somatic cells divide regularly; all human cells (except for the cells that produce eggsand sperm) are somatic cells. Ø Somatic cells contain two copies of each of their chromosomes (one copy from eachparent). Ø The cell cycle has two major phases: interphase and the mitotic phase. Ø During interphase, the cell grows and DNA is replicated; during the mitotic phase, the replicated DNA and cytoplasmic contents are separated and the celldivides. MGD The phases of cell cycle The period from one cell division to the next was divided into two phases 1 Interphase 2 Mitosis Interphase was the name given to the interval when the chromosomes were not evident. It was incorrectly assumed that interphase was a RESTIN PHASE. Mitosis included the part of the cell cycle in which the chromosomes were visible and distributed equally to newly formed daughter nuclei, followed immediately by cell division. MGD Phases of cell cycle Interphase G1 - primary growth S - genome replicated G2 - secondary growth M - mitosis C - cytokinesis The durations of the phases of a cell cycle G1 …………………………………… 11h S ……………………………………….8 h G2 …………………………………… 4h M phases …………………………1 h