1406- Majors Biology Exam 5 Review: Chapter 16 Chapter 16 Who were the four scie

Question

1406- Majors Biology Exam 5 Review: Chapter 16 Chapter 16 Who were the four scientists fundamental in discovering the molecular structure of DNA? Describe the structure of a DNA molecule, including the composition of the backbone and specific base pairings Describe the process of DNA replication (for both leading and lagging strands). What enzymes are involved? What is the origin of replication? What differences exist between DNA replication in prokaryotes and eukaryotes? What is the semiconservative model of DNA replication? How is the DNA proofread and repaired? What enzymes are involved? Why is the replication of chromosome ends a problem in eukaryotic cells but not prokaryotic cells? What is a telomere and why is it important? What is telomerase and what kinds of cells express telomerase? How is DNA packaged? What is chromatin? What is euchromatin and heterochromatin?

Explanation / Answer

Four scientists are Rosalinda Frnklin, Maurice wilkins: X ray difraction studies of DNA and they thought that DNA is a double helix

Francis crick, James watson: deduced the double helix structure of DNA

DNA is made up of 4 nirogenoue base pairs Adenine (A), Thiamine (T), Guanine (G), Cytocine (C) are called nitrogenoue bases. When attached to deoxyrobose or ribose sugars called nucleoside eg: adenine+ribose = nucleoside, when phosphate backbone is attached it is called nucleotide. eg: adenine+ribose+phosphate = nucleotide. In molecular biology or genetics generally ATGC called as nucleotides but in reality Eg A meaning complete package of adenine+ribose(in RNA) or deoxyribose (in DNA)+phosphate = nucleotide. A pairs with T, G pairs with C by two and three hydrogen bonds respectively. Paralelly they are bounded by hydrogen bonds and perpendicularly or Side wise each nucleotide is attached to each other by phosphodiester linkage. Two strands of the DNA are anti parallel meaning one strands runs from 5 prime to 3prime and other strands runs from 3 prime to 5prime direction. In secondary structure DNA assumes the shape of double helix with major and minor groove patterns. One 360 degree turn of the DNA double helix is 34 nm and holds 10 base pairs. The distance between each base pair is 3.4nm. Diameter of the DNA is 2nm.

deoxyribose (in DNA) = + Phosphate =

Adenine Ribose + Adenine (nucleoside) + Phosphate = Nucleotide

In DNA replication process itself i will mention the enzymes

Helicases untwist or open the double helix separating the two parental strands and making them available as template strands. After the parental strands separate, single-strand binding proteins bind to the unpaired DNA strands, keeping them from re-pairing. The untwisting of the double helix causes tighter twisting and strain ahead of the replication fork. Topoisomerase helps relieve this strain by breaking,

swiveling, and rejoining DNA strands and these strands are available to serve as templates for the synthesis of new complementary DNA strands. However, the enzymes that synthesize DNA cannot initiate the synthesis of a polynucleotide; they can only add DNA nucleotides to the end of an already existing chain that is base-paired with the template strand. The initial nucleotide chain that is produced during DNA synthesis is actually a short stretch of RNA, not DNA. This RNA chain is called a primer and is synthesized by the enzyme primase. Primase starts a complementary RNA chain from a single RNA nucleotide, adding more RNA nucleotides one at a time, using the parental DNA strand as a template. The completed primer, generally 5–10 nucleotides long, is thus base-paired to the template strand. The new DNA strand will start from the 3?end of the RNA primer. Enzymes called DNA polymerases catalyze the synthesis of new DNA by adding nucleotides to a preexisting chain. Each nucleotide to be added to a growing DNA strand consists of a sugar attached to a base and to three phosphate groups. DNA

polymerases can add nucleotides only to the free 3? end of a primer or growing DNA strand, never to the 5? end, a new DNA strand can elongate only in the 5?à 3? direction. DNA polymerase can synthesize a complementary strand continuously by elongating the new DNA in the mandatory 5?à 3? direction. DNA polymerase remains in the replication fork on that template strand and continuously adds nucleotides to the new complementary strand as the fork progresses. The DNA strand made by this mechanism is called the leading strand. To elongate the other new strand of DNA in the mandatory

5?à 3? direction, DNA pol III must work along the other template strand in the direction away from the replication fork. The DNA strand elongating in this direction is called the lagging strand. In contrast to the leading strand, which elongates continuously, the lagging strand is synthesized discontinuously, as a series of segments. These segments of the lagging strand are called Okazaki fragments,

Origin of replication is the point from which two DNA strands separate and replication enzymes/replication fork starts synthesizing/replicating new DNA

Prokaryotic DNA is arranged in a circular shape as a single chromosome, and has only one replication origin or origin of replication or replication starts from one point only. eukaryotic DNA arranged in chromosomes and it is is linear. When replicated, there are as many as one thousand origins of replications. In otherwords its start replicating at many points compared to prokaryotes.

Semiconservative mode of replication is two strands of DNA will seperate and a new complementary strand is synthesized, new DNA molecule one strand from old template one is newly synthesized, hence named as semiconservative replication or synthesis of DNA.

In Prokaryotes (e.coli) all three DNA polymerases (I, II and III) have the ability to proofread, using 3’ ? 5’ exonuclease activity. When an incorrect base pair is recognized, DNA polymerase reverses its direction by one base pair of DNA and excises the mismatched base. Following base excision, the polymerase can re-insert the correct base and replication can continue. In eukaryotes only the polymerases that deal with the elongation (delta and epsilon) have proofreading ability (3’ ? 5’ exonuclease activity)

As eukaryotic cells chromosomes/DNA is lenear unlike circular in prokaryotes in each cell division telomere shortening of DNA happens. Telomeres prevent the loss of genes as chromosome ends wear down, the tips of eukaryotic chromosomes have specialized DNA “caps” called telomeres with a repeating TTAGGG for thousands of base pairs, which act as molecular clocks to define how many times a cell has to devide. Telomerase is expressed in cancer cells.

DNA is packaged with histone basic proteins, like string on beads this is the primary structure, secondary structure 8 histone proteins come together and form a bigger bead and in tertiary structure these beads arrange like a cheomosome.

Chromatin is a complex of macromolecules found in cells, consisting of DNA, protein, and RNA.[1] The primary functions of chromatin are 1) to package DNA into a more compact, denser shape, 2) to reinforce the DNA macromolecule to allow mitosis, 3) to prevent DNA damage, and 4) to control gene expression and DNA replication.

Based on the staining property of chromosomes its classified as euchromatin stains less intensely and packaged loosely where active genes and transcription taking place where as heterochromatin stains intensely and is tightly packed and less with genes and active transcription

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