DNA

Structure and Function of DNA - DNA is a polymer composed of two polynucleotide chains that form a double helix. - The two DNA strands are made up of nucleotides, which consist of a nitrogen-containing nucleobase, a sugar called deoxyribose, and a phosphate group. - The nitrogenous bases of DNA are adenine (A), thymine (T), cytosine (C), and guanine (G). - The two polynucleotide strands are held together by hydrogen bonds between the complementary base pairs (A-T and C-G). - DNA carries genetic instructions for the development, functioning, growth, and reproduction of organisms. - It stores and transmits genetic information from one generation to the next. - DNA is replicated when the two strands separate, ensuring that each daughter cell receives a complete set of chromosomes. - Only a small portion of DNA codes for proteins, while the majority is non-coding. - DNA serves as a template for the synthesis of RNA strands, which are involved in protein synthesis. Organization and Properties of DNA - Eukaryotic cells store DNA in long structures called chromosomes. - Chromosomes are duplicated during DNA replication before cell division. - Eukaryotic organisms store most of their DNA in the cell nucleus, with additional DNA in mitochondria and chloroplasts. - Prokaryotes store their DNA in the cytoplasm in circular chromosomes. - Chromatin proteins, such as histones, compact and organize DNA within eukaryotic chromosomes. - DNA is a long polymer made up of nucleotide units. - It can coil into tight loops and other shapes, making its structure dynamic. - DNA strands are held together by hydrogen bonds between the nucleobases. - The diameter of a DNA helix is approximately 20 angstroms. - The buoyant density of most DNA is 1.7g/cm. - Nucleobases are classified into two types: purines (A and G) and pyrimidines (C and T). - Uracil (U) replaces thymine in RNA and lacks a methyl group on its ring. - Adenine pairs with thymine, and cytosine pairs with guanine, forming base pairs. - Artificial nucleic acid analogues have been created for studying nucleic acid properties and biotechnology. - Modified bases, such as 5-methylcytosine, can occur in DNA and serve various functions. DNA Structure and Function in Gene Expression - Cytosine and adenine play vital roles in epigenetic control of gene expression in plants and animals. - Noncanonical bases are known to occur in DNA, including modifications of the canonical bases and uracil. - DNA major and minor grooves provide binding sites for proteins and dyes. - Proteins, such as transcription factors, usually make contact with the exposed bases in the major groove. - Unusual conformations of DNA can affect the accessibility of bases in the grooves. - Complementary base pairing allows nucleobases on one strand to bond with specific nucleobases on the other strand. - Purines form hydrogen bonds with pyrimidines, with adenine bonding to thymine and cytosine bonding to guanine. - Watson-Crick base pairs are formed by two nucleotides binding across the double helix. - GC-content affects the stability of DNA, with high GC-content DNA being more stable. - Melting temperature is the temperature at which 50% of the double-strand molecules are converted to single-strand molecules. DNA Amount and Sense/Antisense Strands - The total diploid nuclear genome per cell in humans extends for 6.37 Gigabase pairs (Gbp). - Chromosome 1 is the largest human chromosome with approximately 220 million base pairs. - Mitochondrial DNA (mtDNA) is smaller in comparison to nuclear DNA and forms closed circular molecules. - Each human mitochondrion contains approximately 5 mtDNA molecules, and each human cell contains approximately 100 mitochondria. - An egg cell can contain up to 100,000 mitochondria, corresponding to up to 1,500,000 copies of the mitochondrial genome. - A DNA sequence is called a sense sequence if it is the same as that of a messenger RNA copy that is translated into protein. - The sequence on the opposite strand is called the antisense sequence. - Antisense RNA sequences are produced in both prokaryotes and eukaryotes, but their functions are not entirely clear. - Antisense RNAs may be involved in regulating gene expression through RNA-RNA base pairing. - Overlapping genes in some DNA sequences blur the distinction between sense and antisense strands. Alternative DNA Structures and Other Topics - DNA can be twisted like a rope in a process called DNA supercoiling. - Supercoiling affects the structure and function of DNA. - Positive supercoiling occurs when the DNA is overwound, while negative supercoiling occurs when the DNA is underwound. - Supercoiling plays a role in DNA packaging and gene regulation. - DNA gyrase is an enzyme that can introduce supercoils into DNA molecules. - DNA exists in different conformations: A-DNA, B-DNA, and Z-DNA. - B-DNA and Z-DNA are observed in functional organisms. - DNA conformation depends on hydration level, DNA sequence, supercoiling, chemical modifications, metal ions, and polyamines. - A-DNA and B-DNA were first analyzed using X-ray diffraction patterns. - Watson and Crick proposed the double helix structure based on X-ray diffraction patterns. - Telomeres are specialized regions at the ends of linear chromosomes. - Telomeres contain guanine-rich sequences that form G-quadruplex structures. - G-quadruplex structures are stabilized by hydrogen bonding and metal ions. - Telomeres also form loop structures called T-loops. - T-loops have a triple-stranded