Nucleic Acids: Structure and Function

Transcription

1 ucleic Acids: Structure and Function

2 Components of ucleotides The building blocks (monomers) of the nucleic acids are called nucleotides. ydrolysis of nucleotides gives phosphoric acid, a pentose sugar, and heterocyclic nitrogenous bases: P - P - - 3P4,Phosphoric Acid P4 3-,Phosphate

3 The pentose sugar found in DA is 2-deoxy-D-ribose; that found in RA is D-ribose. C 2 C 2 The bases found in DA are C 2 adenine cytosine guanine thymine The bases found in RA are 2 2 adenine cytosine guanine uracil 2

4 Structure of a ucleoside Structure of a ucleotide

5 Biosynthesis of ucleotides The net result of nucleotide biosynthesis can be represented as two dehydration (condensation) reactions: 2 2 P C P C 2 ucleotide

6 ucleotide umbering 2 P C ote that primed numbers are used for substituents on the sugar ring and unprimed numbers are used for substituents on the heterocyclic ring.

7 ucleotide omenclature ucleotides derived from ribose are called ribonucleotides; those from deoxyribose are called deoxyribonucleotides. ucleotide names are usually abbreviated: Deoxy- becomes: d- eterocyclic base names become: A, C, G, T, U 5 -Monophosphate becomes: MP P - C 2 - P - C 2 AMP dcmp

8 ucleic Acid Formation from ucleotides

9 ucleic Acid Formation from ucleotides The assembly of nucleotides into polynucleotides, or nucleic acids, can be thought of as a dehydration reaction between the 3 - of one nucleotide and the phosphate group of a second nucleotide to form a phosphodiester group. P C P C P C 2 - P C 2 By convention, nucleotide sequences are named in the 5 has one 5 -end and one 3 -end). 3 direction (a nucleic acid There may be a phosphate group attached at the 5 -end of the chain or at the 3 -end.

10 Polynucleotides

11 The Three-Dimensional Structure of DA The three-dimensional structure of DA was first deduced by James Watson and Francis Crick in 1953, on the basis of two key pieces of data: Chargoff rules: moles of A = moles of T and moles of C = moles of G for the DA of any species. The percentage of A+T or C+G varied from species to species. DA x-ray diffraction photographs obtained by Maurice Wilkins and Rosalind Franklin.

12 The Three-Dimensional Structure of DA

13 The three-dimensional structure of DA is called the double helix. In the double helix, two strands of DA coil about each other, running in opposite directions. The deoxy-sugars and phosphate groups are located on the outside of the helix and the heterocyclic bases are stacked on top of each other on the inside of the helix.

14 Base Pairing

15 Base Pairing The heterocyclic bases are stacked in pairs, each A across from a T (a), and each C across from a G (b). The pairs of bases are held together by hydrogen bonds: two for an AT base pair, and three for a CG base pair. Sugar Phosphate Backbone Sugar Phosphate Backbone Sugar Phosphate Backbone Sugar Phosphate Backbone

16 Illustrating Base Pairing with Electrostatic Diagrams

17 The Three-Dimensional Structure of DA The two deoxynucleotide strands in DA are complementary to each other: the sequence of bases on the 5 3 strand and the 3 5 strand are always matched: AT, TA, CG, or GC. ydrophobic interactions between the stacked base pairs also contribute significant stability to the double helix.

18 DA in the Cell DA strands have molecular masses estimated to be from a few billion to 100 billion. The human genome contains 23 pairs of chromosomes containing 3.2 billion base pairs. Each DA molecule is compacted by folding about a structure called a nucleosome core. Each nucleosome core consists of two pairs each of four different proteins called histones.

19 DA in the Cell Each nucleosome core consists of two pairs each of four different proteins called histones. Each DA molecule wraps around the nucleosome cores to form a chain of nucleosomes, each containing base pairs. The chain of nucleosomes is coiled to higher and higher levels to form a compact, highly supercoiled chromatin fiber.

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22 An Artists Conception of DA

23 Structure of RA Ribonucleic acids exist as single-stranded molecules. Several types of RA exist: Messenger RA - carries genetic information from DA to the site of protein synthesis (the ribosome) Transfer RA - helps in decoding genetic information by carrying specific amino acids to the ribosome Ribosomal RA - forms structural component of the ribosome Catalytic RAs - catalyze transformations of other RAs

24 Ribonucleic acids exist as single-stranded molecules but have extensive secondary and tertiary structure. A good example are the transfer RA s.

25 Artist s Rendition of Transfer RA s

26 Ribosomal RA Secondary Structure

27 Information Flow from DA to Protein The Central Dogma of Molecular Biology Replication is the copying of DA in the course of cell division. Transcription is the synthesis of RA from DA. Translation is the synthesis of polypeptides through the combined efforts of rra, mra, and tra. Approximately 2% of the nucleotides in DA result in the formation of polypeptides. These regions in the DA are called coding regions. The remaining 98% of the nucleotides do not appear to have any function and are called junk DA.

28 Basic Model of Replication

29 Semiconservative Replication