DNA structure and replication TA (original) (raw)

DNA, RNA, and the Flow of Genetic Information 4

C H A P T E R D NA and RNA are long linear polymers, called nucleic acids, that carry information in a form that can be passed from one generation to the next. These macromolecules consist of a large number of linked nucleotides, each composed of a sugar, a phosphate, and a base. Sugars linked by phosphates form a common backbone that plays a structural role, whereas the sequence of bases along a nucleic acid strand carries genetic information. The DNA molecule has the form of a double helix, a helical structure consisting of two complementary nucleic acid strands. Each strand serves as the template for the other in DNA replication. The genes of all cells and many viruses are made of DNA. Genes specify the kinds of proteins that are made by cells, but DNA is not the direct template for protein synthesis. Rather, a DNA strand is copied into a class of RNA molecules called messenger RNA (mRNA), the information-carrying intermediates in protein synthesis. This process of transcription is followed by translation, the synthesis of proteins according to instructions given by mRNA templates. Information processing in all cells is quite complex. The scheme that underlies information processing at the level of gene expression was first proposed by Francis Crick in 1958. DNA ¬ ¬¬¡ Transcription RNA ¬ ¬¬¡ Translation Protein Crick called this scheme the central dogma. The basic tenets of this dogma are true, but, as we will see later, this scheme is not as simple as depicted. Replication

Understanding biochemistry: structure and function of nucleic acids

Essays in Biochemistry, 2019

Nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), carry genetic information which is read in cells to make the RNA and proteins by which living things function. The well-known structure of the DNA double helix allows this information to be copied and passed on to the next generation. In this article we summarise the structure and function of nucleic acids. The article includes a historical perspective and summarises some of the early work which led to our understanding of this important molecule and how it functions; many of these pioneering scientists were awarded Nobel Prizes for their work. We explain the structure of the DNA molecule, how it is packaged into chromosomes and how it is replicated prior to cell division. We look at how the concept of the gene has developed since the term was first coined and how DNA is copied into RNA (transcription) and translated into protein (translation).

BASIC MOLECULAR BIOLOGY AND MECHANISMS

▲ FIGURE 4-2 Alternative representations of a nucleic acid strand illustrating its chemical directionality. Shown here is a single strand of DNA containing only three bases: cytosine (C), adenine (A), and guanine (G). (a) The chemical structure shows a hydroxyl group at the 3Ј end and a phosphate group at the 5Ј end. Note also that two phosphoester bonds link adjacent nucleotides; this two-bond linkage commonly is referred to as a phosphodiester bond. (b) In the "stick" diagram (top), the sugars are indicated as vertical lines and the phosphodiester bonds as slanting lines; the bases are denoted by their single-letter abbreviations. In the simplest representation (bottom), only the bases are indicated. By convention, a polynucleotide sequence is always written in the 5Јn3Ј direction (left to right) unless otherwise indicated. 287:755; part (b) from R. E. Dickerson, 1983, Sci. Am. 249:94.]

Biology summary (practice 7-12

Practice 7-Structure and replication of DNA F. Griffith (1928) (S. pneumoniae) " genetic transformation, transforming principle " : living s(smooth)-pneumococcus bacteria causes pneumonia and death in mice, r(rough)-bacteria don't, heat killed s-bacteria do not kill the mouse, when mixed with living r bacteria the mouse dies. Griffith concluded that the live R strain bacteria absorbed genetic material from the dead S strain bacteria. Transformation: one strain of a bacterium absorbs genetic material from another strain of bacteria and " turns into " the type of bacterium whose genetic material it absorbed. O. Avery, C. MacLeod and M. McCarty (1944) experimental demonstration that DNA is the genetic material: molecules of s-strain cells (RNA, protein, DNA, lipids, carbonhydrate) were tested for transformation of R strain cells to S strain cells. DNA transformed the cells → DNA carries heritable information. A. D. Hershey and M. Chase (1952) Bacteriophage (virus) in which the protein capsule is labeled with radioactive sulfur and the DNA core is labeled with radioactive phosphorus infected cells. Results: Viral proteins labeled with radioactive isotope of sulfur, remained outside of host. In contrast, radioactively labeled (32P) DNA entered the bacterial cell. Those bacteria containing viral DNA, when cultured, gave rise to more phages. Conclusion: The active component of the bacteriophage that transmits the infective characteristic is the DNA. → clear correlation between DNA and genetic information DNA primary structure: Purine-(Adenosine, Guanine) + pyrimidine-(Uracil, Cytosine, Thymine)bases attached to C1 of deoxyribose, 2 molecules of deoxyribose connected by phosphoric ester connected with C3 on one and C5(prim. alcohol group) on the other molecule. DNA secondary structure J. Watson and F. Crick, 1953 Nobel prize for physiology and medicine – 1962: Antiparallel DNA strands, Complementary base pairing, Hydrogen bonds Nucleosome is a structural unit of chromosomes, complex of DNA and histones.. The complex of DNA and proteins that forms a chromosome is called chromatin (the smallest unit of the chromatine = nucleosome). Levels of chromatin packing: beads on a string (chromatin fiber of packed nucleosomes).As the chromosome prepare or enter mitosis, the chromatin thread coils up further, adopting more and more compact structure, until the highly condensed mitotic chromosome has been formed.Each DNA molecule has been packed into a mitotic chromosome that is 50,000x shorter than its extended length. Centromere-a region of DNA near the middle of a chromosome where two identical sister chromatids come in contact. It is the point of mitotic spindle attachment. Telomere-region of repetitive DNA nucleotides (G especially) at the end of a chromosome. Shortening of chromosome ends occurs during chromosome replication due to inability of linear chromosomal ends synthesis (enzyme telomerase).