Chemistry 20 Chapter 17 Nucleotides and Nucleic acids
Introduction • Each cell of our bodies contains thousands of different proteins. • How do cells know which proteins to synthesize out of the extremely large number of possible amino acid sequences? • the transmission of hereditary information took place in the nucleus, more specifically in structures called chromosomes. • The hereditary information was thought to reside in genes within the chromosomes. • Chemical analysis of nuclei showed chromosomes are made up largely of proteins called histones and nucleic acids.
Nucleic acids Backbones of chromosomes Ribonucleic acids (RNA) Deoxyribonucleic acids (DNA) Nucleic acids RNA and DNA are polymers (monomers: nucleotides).
Nucleotide • A nucleotide is composed of: • Nitrogen-containing bases (amines) • Sugars (monosaccharides) • Phosphate Phosphate
Sugars (monosaccharide) • RNA contains: • Ribose sugar • DNA contains: • 2-Deoxy-D-ribose sugar(without O on carbon 2)
ß-N-glycosidic bond Nucleoside When a primidine or purine forms a glycosidic bond to C1’ of a sugar (ether ribose or deoxyribose). Base + Sugar Nucleoside
Nucleotide A nucleotide forms with the −OH on C5’ of a sugar bonds to phosphoric acid. Phosphate ester bond 5’ 5’ 1’ A nucleotide
Primary structure of DNA and RNA Carry all information for protein synthesis. Sequence of nucleotides. Each phosphate is linked to C3’ and C5’ of two sugars.
Primary structure of DNA and RNA A nucleoside = Base + Sugar A nucleotide = Base + Sugar + Phosphate A nucleic acid = A chain of nucleotides Like amino acids(C-terminal and N-terminal): Base sequence is read from the C5’ (free phosphate) end to the C3’ (free hydroxyl) end. -ACGU-
Secondary structure of DNA 5’ 3’ • Two strands of polynucleotide form a double helix structure like a spiral. • Hydrogen bonds link paired bases: • Adenine-Thymine (A–T) • Guanine-Cytosine (G-C) • Sugar-Phosphate backbone is hydrophilic and stays on the outside (bases are hydrophobic). 3D structure Sugar phosphate backbone 5’ 3’
Higher structure of DNA • DNA is coiled around proteins called histones. • Histones are rich in the basic amino acids • Acidic DNA basic histones attract each other and form units called nucleosomes. Core of eight histones
Higher structure of DNA Chromatin: Condensed nucleosomes
Higher structure of DNA Chromatin fibers are organized into loops, and the loops into the bands that provide the superstructure of chromosomes.
Difference between DNA & RNA • DNA has four bases: A, G, C, and T. • RNA has four bases: A, G, C, and U. 2. In DNA: Sugar is 2-deoxy-D-ribose. In RNA: Sugar is D-ribose. 3. DNA is almost always double-stranded (helical structure). RNA is single strand. 4. RNA is much smaller than DNA.
RNA molecules Transmits the genetic information needed to operate the cell. 1. Ribosomal RNA (rRNA) Most abundant RNA – Contains ribosomes: sites for protein synthesis. 2. Messenger RNA (mRNA) Carries genetic information from DNA (in nucleus) to ribosomes (in cytoplasm) for protein synthesis. They are produced in “Transcription”. 3. Transfer RNA (tRNA) Smallest RNA. Translates the genetic information in mRNA and brings specific Amino acids to the ribosome for protein synthesis.
Genes A section of a DNA molecule that contains a specific sequence of the four bases (A, G, T, and C) 1000 to 2000 nucleotides Base sequence of the gene carries the information to produce one protein molecule. Change of sequence New protein
Functions of DNA 1. It reproduces itself (Replication) 2. It supplied the information to make up RNA, proteins, and enzymes. (chapter 18)
Replication Separation of the two original strands and synthesis of two new daughter strands using the original strands as templates. By breaking H-bonds
Replication Replication is bidirectional: takes place at the same speed in both directions. Replication is semiconservative: each daughter molecule has one parental strand and one newly synthesized one. Origin of replication: specific point of DNA where replication begins. Replication fork: specific point of DNA where replication is proceeding.
Replication Leading strand: is synthesized continuously in the 3’ 5’ direction toward the replication fork. Lagging strand: is synthesized discontinuously in the 5’ 3’ direction away from the replication fork.
Replication Replisomes: assemblies of “enzyme factories”.
Helicases Unwinds the DNA double helix. • Replication of DNA starts with unwinding of the double helix. • Unwinding can occur at either end or in the middle. • Attach themselves to one DNA strand and cause separation of the double helix.
Primases Catalyze the synthesis of primers. Primers: are short nucleotides (4 to 15). • They are required to start the synthesis of both daughter strands. • Primases are placed at about every 50 nucleotides in the lagging strand synthesis.
DNA Polymerase Catalyze the formation of nucleotides. Joins the nucleotides to produce a new strands