DNA and the Molecular Structure of Chromosomes PDF

Introduction DNA and the Molecular Structure of Chromosomes Dr. Madona Akhobadze 2024 1. Introduction. DNA and the Molecular Structure of Chromosomes Functions of the Genetic Material. Proof That Genetic Information Is Stored in DNA. The Structures of DNA and RNA. Chromosome Structure in Prokaryotes...

Introduction DNA and the Molecular Structure of Chromosomes Dr. Madona Akhobadze 2024 1. Introduction. DNA and the Molecular Structure of Chromosomes Functions of the Genetic Material. Proof That Genetic Information Is Stored in DNA. The Structures of DNA and RNA. Chromosome Structure in Prokaryotes and Viruses. Chromosome Structure in Eukaryotes. Reading: Ch. 9 - Principles of Genetics, by Snustad & Simmons “NUCLEIN” IN 1868, JOHANN FRIEDRICH MIESCHER A YOUNG SWISS MEDICAL STUDENT FUNCTIONS OF THE GENETIC MATERIAL THE GENETIC MATERIAL MUST REPLICATE, CONTROL THE GROWT AND DEVELOPMENT OF THE ORGANISM, AND ALLOW THE ORGANISM TO ADAPT TO CHANGES IN THE ENVIRONMENT IN 1865, MENDEL SHOWED THAT “MERKMALEN” (NOW “GENES”) TRANSMITTED GENETIC INFORMATION THREE ESSENTIAL FUNCTIONS: 1. THE GENOTYPIC FUNCTION, REPLICATION. THE GENETIC MATERIAL MUST STORE GENETIC INFORMATION AND ACCURATELY TRANSMIT THAT INFORMATION FROM PARENTS TO OFFSPRING, GENERATION AFTER GENERATION. 2. THE PHENOTYPIC FUNCTION, GENE EXPRESSION. THE GENETIC MATERIAL MUST CONTROL THE DEVELOPMENT OF THE PHENOTYPE OF THE ORGANISM. THAT IS, THE GENETIC MATERIAL MUST DICTATE THE GROWTH OF THE ORGANISM FROM THE SINGLE-CELLED ZYGOTE TO THE MATURE ADULT. THREE ESSENTIAL FUNCTIONS: 3. THE EVOLUTIONARY FUNCTION, MUTATION. THE GENETIC MATERIAL MUST UNDERGO CHANGES TO PRODUCE VARIATIONS THAT ALLOW ORGANISMS TO ADAPT TO MODIFICATIONS IN THE ENVIRONMENT SO THAT EVOLUTION CAN OCCUR. Cells and Chromosomes IN BOTH PROKARYOTIC AND EUKARYOTIC CELLS, THE GENETIC MATERIAL IS ORGANIZED INTO CHROMOSOMES ALL LIFE HAS A CELLULAR BASIS CELLS AND CHROMOSOMES EACH CELL IS A COMPLICATED ASSEMBLAGE OF MOLECULES THAT CAN ACQUIRE MATERIALS, RECRUIT AND STORE ENERGY, AND CARRY OUT DIVERSE ACTIVITIES, INCLUDING REPRODUCTION STRONG EVIDENCE THAT GENES ARE USUALLY LOCATED ON CHROMOSOMES CHROMOSOMES ARE COMPOSED OF TWO TYPES OF LARGE ORGANIC MOLECULES (MACROMOLECULES) CALLED PROTEINS AND NUCLEIC ACIDS. THE NUCLEIC ACIDS ARE OF TWO TYPES: DEOXYRIBONUCLEIC ACID (DNA) AND RIBONUCLEIC ACID (RNA). THE RESULTS OF ELEGANT EXPERIMENTS CLEARLY ESTABLISHED THAT THE GENETIC INFORMATION IS STORED IN NUCLEIC ACIDS, NOT IN PROTEINS. PROOF THAT GENETIC INFORMATION IS STORED IN DNA IN MOST ORGANISMS, THE GENETIC INFORMATION IS ENCODED IN DNA. IN SOME VIRUSES, RNA IS THE GENETIC MATERIAL. MOST OF THE DNA OF CELLS IS LOCATED IN THE CHROMOSOMES, WHEREAS RNA AND PROTEINS ARE ALSO ABUNDANT IN THE CYTOPLASM CORRELATION EXISTS BETWEEN THE AMOUNT OF DNA PER CELL AND THE NUMBER OF SETS OF CHROMOSOMES PER CELL MOST SOMATIC CELLS OF DIPLOID ORGANISMS CONTAIN TWICE THE AMOUNT OF DNA AS THE HAPLOID GERM CELLS (GAMETES) OF THE SAME SPECIES THE MOLECULAR COMPOSITION OF THE DNA IS THE SAME DNA IS MORE STABLE THAN RNA OR PROTEINS FREDERICK GRIFFITH’S DISCOVERY OF TRANSFORMATION IN STREPTOCOCCUS PNEUMONIAE SIA AND DAWSON’S DEMONSTRATION OF TRANSFORMATION IN STREPTOCOCCUS PNEUMONIAE IN VITRO OSWALD AVERY, COLIN MACLEOD, AND MACLYN MCCARTY’S DEMONSTRATION THAT THE “TRANSFORMING PRINCIPLE” Avery, MacLeod, and McCarty’s Experiment HIGHLY PURIFIED DNA FROM TYPE IIIS CELLS WAS TREATED WITH THE ENZYMES (1) DEOXYRIBONUCLEASE (DNASE), WHICH DEGRADES DNA, (2) RIBONUCLEASE (RNASE), WHICH DEGRADES RNA, (3) PROTEASES, WHICH DEGRADE PROTEINS THE STRUCTURES OF DNA AND RNA DNA IS USUALLY DOUBLE-STRANDED, WITH ADENINE PAIRED WITH THYMINE AND GUANINE PAIRED WITH CYTOSINE. RNA IS USUALLY SINGLE-STRANDED AND CONTAINS URACIL IN PLACE OF THYMINE. NUCLEIC ACIDS, THE MAJOR COMPONENTS OF MIESCHER’S NUCLEIN, ARE MACROMOLECULES COMPOSED OF REPEATING SUBUNITS CALLED NUCLEOTIDES EACH NUCLEOTIDE IS COMPOSED OF (1) A PHOSPHATE GROUP, (2) A FIVE-CARBON SUGAR, OR PENTOSE, AND (3) A CYCLIC NITROGEN-CONTAINING COMPOUND CALLED A BASE ADENINE (A), GUANINE (G), THYMINE (T), AND CYTOSINE (C), > URACIL (U) ADENINE AND GUANINE ARE DOUBLE-RING BASES CALLED PURINES; CYTOSINE, THYMINE, AND URACIL ARE SINGLE-RING BASES CALLED PYRIMIDINES. DNA STRUCTURE: THE DOUBLE HELIX 1953 JAMES WATSON AND FRANCIS CRICK - B-DNA ROSALIND FRANKLIN, MAURICE WILKINS ERWIN CHARGAFF AND COLLEAGUES – THYMINE = ADENINE, CYTOSINE = GUANINE THYMINE + CYTOSINE = ADENINE + GUANINE X-RAY DIFFRACTION PATTERNS DNA EXISTS AS A RIGHT-HANDED DOUBLE HELIX IN WHICH THE TWO POLYNUCLEOTIDE CHAINS ARE COILED ABOUT ONE ANOTHER IN A SPIRAL SEQUENCE OF NUCLEOTIDES LINKED TOGETHER BY PHOSPHODIESTER BONDS TWO POLYNUCLEOTIDE STRANDS ARE HELD TOGETHER IN THEIR HELICAL CONFIGURATION BY HYDROGEN BONDING WHEN DNA MOLECULES IN AQUEOUS SOLUTION ARE HEATED TO NEAR 100C, THESE BONDS ARE BROKEN AND THE COMPLEMENTARY STRANDS OF DNA SEPARATE - DENATURATION. IF THE COMPLEMENTARY SINGLE STRANDS OF DNA ARE COOLED SLOWLY UNDER THE RIGHT CONDITIONS, THE COMPLEMENTARY BASE SEQUENCES WILL FIND EACH OTHER AND WILL RE-FORM BASEPAIRED DOUBLE HELICES RENATURATION. THE TWO STRANDS OF A DNA DOUBLE HELIX ARE SAID TO BE COMPLEMENTARY THIS PROPERTY, THE COMPLEMENTARITY OF THE TWO STRANDS OF THE DOUBLE HELIX, MAKES DNA UNIQUELY SUITED TO STORE AND TRANSMIT GENETIC INFORMATION FROM GENERATION TO GENERATION TWO COMPLEMENTARY STRANDS ARE ANTIPARALLEL THE PLANAR SIDES OF THE BASE PAIRS ARE RELATIVELY NONPOLAR AND THUS TEND TO BE HYDROPHOBIC (WATERINSOLUBLE) - STABLE DNA STRUCTURE: ALTERNATE FORMS OF THE DOUBLE HELIX IN HIGH CONCENTRATIONS OF SALTS OR IN A PARTIALLY DEHYDRATED STATE, DNA EXISTS AS ADNA, WHICH IS A RIGHT-HANDED HELIX LIKE B-DNA, BUT WITH 11 NUCLEOTIDE PAIRS PER TURN A-DNA IS A SHORTER, THICKER DOUBLE HELIX WITH A DIAMETER OF 2.3 NM. DNA MOLECULES ALMOST CERTAINLY NEVER EXIST AS A-DNA IN VIVO. LEFT-HANDED, DOUBLE-HELICAL FORM CALLED Z-DNA ALL THE FUNCTIONAL DNA MOLECULES PRESENT IN LIVING CELLS DISPLAY ONE OTHER VERY IMPORTANT LEVEL OF ORGANIZATION—THEY ARE SUPERCOILED RIGHT-HANDED = A NEGATIVE SUPERCOIL KEY POINTS DNA USUALLY EXISTS AS A DOUBLE HELIX, WITH THE TWO STRANDS HELD TOGETHER BY HYDROGEN BONDS BETWEEN THE COMPLEMENTARY BASES: ADENINE PAIRED WITH THYMINE AND GUANINE PAIRED WITH CYTOSINE. THE COMPLEMENTARITY OF THE TWO STRANDS OF A DOUBLE HELIX MAKES DNA UNIQUELY SUITED TO STORE AND TRANSMIT GENETIC INFORMATION. THE TWO STRANDS OF A DNA DOUBLE HELIX HAVE OPPOSITE CHEMICAL POLARITY. RNA USUALLY EXISTS AS A SINGLE-STRANDED MOLECULE CONTAINING URACIL INSTEAD OF THYMINE. THE FUNCTIONAL DNA MOLECULES IN CELLS ARE NEGATIVELY SUPERCOILED. CHROMOSOME STRUCTURE IN PROKARYOTES AND VIRUSES THE DNA MOLECULES OF PROKARYOTES AND VIRUSES ARE ORGANIZED INTO NEGATIVELY SUPERCOILED DOMAINS MONOPLOID - THEY HAVE ONLY ONE SET OF GENES (ONE COPY OF THE GENOME). GENOME OF BACTERIOPHAGE MS2 CONSISTS OF 3569 NUCLEOTIDES AND CONTAINS 4 GENES. THE SMALLEST KNOWN DNA VIRUSES HAVE ONLY 9 TO 11 GENES. THE CHROMOSOMES REMAIN IN A HIGHLY CONDENSED STATE FOLDED GENOME - FUNCTIONAL STATE OF A BACTERIAL CHROMOSOME BACTERIAL CHROMOSOMES CONTAIN CIRCULAR MOLECULES OF DNA SEGREGATED INTO ABOUT 50 DOMAINS. CHROMOSOME STRUCTURE IN EUKARYOTES Eukaryotic chromosomes contain huge molecules of DNA that are highly condensed during mitosis and meiosis.  The centromeres and telomeres of eukaryotic chromosomes have unique structures.   most eukaryotes are diploid, having two complete sets of genes, one from each parent NOT ONLY DO MOST EUKARYOTES CONTAIN MANY TIMES THE AMOUNT OF DNA IN PROKARYOTES, BUT ALSO THIS DNA IS PACKAGED INTO SEVERAL CHROMOSOMES, AND EACH CHROMOSOME IS PRESENT IN TWO (DIPLOIDS) OR MORE (POLYPLOIDS) COPIES CHROMATIN - THE COMPLEX OF THE DNA, CHROMOSOMAL PROTEINS, AND OTHER CHROMOSOME CONSTITUENTS ISOLATED FROM NUCLEI PROTEINS ARE OF TWO MAJOR CLASSES: (1) BASIC (POSITIVELY CHARGED AT NEUTRAL PH) PROTEINS CALLED HISTONES (H1, H2A, H2B, H3, AND H4) FOUR OF THE FIVE TYPES OF HISTONES ARE SPECIFICALLY COMPLEXED WITH DNA TO PRODUCE THE BASIC STRUCTURAL SUBUNITS OF CHROMATIN, SMALL ELLIPSOIDAL BEADS CALLED NUCLEOSOMES. (2) A HETEROGENEOUS, LARGELY ACIDIC (NEGATIVELY CHARGED AT NEUTRAL PH) GROUP OF PROTEINS COLLECTIVELY REFERRED TO AS NONHISTONE CHROMOSOMAL PROTEINS. ONE LARGE DNA MOLECULE PER CHROMOSOME CONSIDERABLE EVIDENCE NOW INDICATES THAT EACH CHROMOSOME CONTAINS A SINGLE, GIANT MOLECULE OF DNA THAT EXTENDS FROM ONE END THROUGH THE CENTROMERE ALL THE WAY TO THE OTHER END OF THE CHROMOSOME ONE LARGE DNA MOLECULE PER CHROMOSOME THREE LEVELS OF DNA PACKAGING IN EUKARYOTIC CHROMOSOMES    1. The first level of condensation involves packaging DNA as a negative supercoil into nucleosomes, to produce the 11-nm-diameter interphase chromatin fiber. This clearly involves an octamer of histone molecules, two each of histones H2a, H2b, H3, and H4. 2. The second level of condensation involves an additional folding or supercoiling of the 11-nm nucleosome fiber, to produce the 30-nm chromatin fiber. Histone H1 is involved in this 3. Finally, nonhistone chromosomal proteins form a scaffold that is involved in condensing the 30-nm chromatin fiber into the tightly packed metaphase chromosomes. This third level of condensation appears to involve the separation of segments of the giant DNA molecules present in eukaryotic chromosomes into independently supercoiled domains or loops. The mechanism by which this third level of condensation occurs is not known. CENTROMERES AND TELOMERES THE CENTROMERE OF A METAPHASE CHROMOSOME CAN USUALLY BE RECOGNIZED AS A CONSTRICTED REGION PRESENCE OF SPECIFIC DNA SEQUENCES THAT ARE REPEATED MANY TIMES, FREQUENTLY IN LONG TANDEM ARRAYS EACH CENTROMERE OF HUMAN CHROMOSOMES, FOR EXAMPLE, CONTAINS 5000 TO 15,000 COPIES OF A 171 BASE-PAIRLONG SEQUENCE CALLED THE ALPHA (SOMETIMES “ALPHOID”) SATELLITE SEQUENCE CENTROMERES AND TELOMERES TELOMERES (FROM THE GREEK TERMS TELOS AND MEROS, MEANING “END” AND “PART”), OR ENDS OF EUKARYOTIC CHROMOSOMES, HAVE UNIQUE PROPERTIES NATURAL ENDS OF NORMAL (UNBROKEN) CHROMOSOMES ARE STABLE AND SHOW NO TENDENCY TO FUSE WITH OTHER BROKEN OR NATIVE ENDS TELOMERES MUST PROVIDE AT LEAST THREE IMPORTANT FUNCTIONS (1) prevent deoxyribonucleases from degrading the ends of the linear DNA molecules, (2) prevent fusion of the ends with other DNA molecules, and (3) facilitate replication of the ends of the linear DNA molecules without loss of material IN NORMAL (NONCANCEROUS) HUMAN SOMATIC CELLS, TELOMERES USUALLY CONTAIN 500 TO 3000 TTAGGG REPEATS AND GRADUALLY SHORTEN WITH AGE. IN CONTRAST, THE TELOMERES OF GERM-LINE CELLS AND CANCER CELLS DO NOT SHORTEN WITH AGE THE TELOMERES OF HUMANS AND A FEW OTHER SPECIES HAVE BEEN SHOWN TO FORM STRUCTURES CALLED T-LOOPS THE DNA IN THESE T-LOOPS IS PROTECTED FROM DEGRADATION AND/OR MODIFICATION BY DNA REPAIR PROCESSES BY A TELOMERE-SPECIFIC PROTEIN COMPLEX CALLED SHELTERIN THE CHROMOSOMES OF EUKARYOTES CONTAIN MANY DNA SEQUENCES THAT ARE REPEATED IN THE HAPLOID CHROMOSOME COMPLEMENT, SOMETIMES AS MANY AS A MILLION TIMES. DNA CONTAINING SUCH REPEATED SEQUENCES, CALLED REPETITIVE DNA, IS A MAJOR COMPONENT (15 TO 80 PERCENT) OF EUKARYOTIC GENOMES. KEY POINTS EACH EUKARYOTIC CHROMOSOME CONTAINS ONE GIANT MOLECULE OF DNA PACKAGED INTO 11-NM ELLIPSOIDAL BEADS CALLED NUCLEOSOMES. THE CONDENSED CHROMOSOMES THAT ARE PRESENT IN MITOSIS AND MEIOSIS AND CAREFULLY ISOLATED INTERPHASE CHROMOSOMES ARE COMPOSED OF 30-NM CHROMATIN FIBERS. AT METAPHASE, THE 30-NM FIBERS ARE SEGREGATED INTO DOMAINS BY SCAFFOLDS COMPOSED OF NONHISTONE CHROMOSOMAL PROTEINS. THE CENTROMERES (SPINDLE-FIBER-ATTACHMENT REGIONS) AND TELOMERES (TERMINI) OF CHROMOSOMES HAVE UNIQUE STRUCTURES THAT FACILITATE THEIR FUNCTIONS. EUKARYOTIC GENOMES CONTAIN REPEATED DNA SEQUENCES, WITH SOME SEQUENCES PRESENT A MILLION TIMES OR MORE. მადლობა!