DNA and RNA structure The painting “Dawn of the Double Helix” composes the DNA duplex as human figures. The theme in this painting is “Life forms: The basic structures that make our existence possible”.

Two types of nucleic acids – DNA and RNA Genome - the genetic information of an organism. The genomes of all cells are composed of DNA. Nucleic acids are biopolymers consisting of nucleotides Nucleotides have three components: (1) A weakly basic nitrogen base (2) A five-carbon sugar (3) Phosphate

Ribose and Deoxyribose Рибоза Дезоксірибоза Ribose is the constituent of RNA Deoxyribose is the constituent of DNA

Nitrogen Bases of Nucleic Acids Adenine Guanine Cytosine Thymine Uracil The nitrogen bases are derivatives of either pyrimidine or purine.

Nucleosides Nucleosides are composed of ribose or deoxyribose and a heterocyclic base.

Structure of mononucleotide Adenosine mononucleotide

Formation of DNA chain (5’-3’ direction)

Nucleotides joined by 3’-5’ phosphodi-ester linkages One end of the polynucleotide chain is said to be 5’ (no residues are attached to 5’-carbon) and other is said to be 3’. Direction from the top to bottom is called 5’→3’, from bottom to top – 3’→5.

Primary structure of nucleic acids

Two Antiparallel Strands Form a Double Helix DNA is Double-Stranded Two Antiparallel Strands Form a Double Helix Two strands run in opposite directions Bases in opposite strands pair by complementary hydrogen bonding Adenine (A) - Thymine (T) Guanine (G) - Cytosine (C) Crick Francis Watson James The double helix of DNA was discovered in 1953 by Crick F. and Watson J. Nobel prize in 1962.

Chemical structure of double-stranded DNA Chemical structure of double-stranded DNA. The two strands run in opposite directions. Adenine in one strand pairs with thymine in the opposite strand, and guanine pairs with cytosine

Comple-mentary base pairing and stacking in DNA

Two-stranded struc-ture of DNA

DNA in cells is a constituent of chromatin Chromatin – DNA plus different proteins Histones – main proteins of chromatin

Chromatin structure DNA is packaged by coiling into a solenoid (helix) structure

Types of RNA (1) Transfer RNA (tRNA) Carries amino acids to translation machinery Very stable molecules (2) Ribosomal RNA (rRNA) Makes up much of the ribosome Very stable, majority of cellular RNA (3) Messenger RNA (mRNA) Encodes message from DNA to ribosomes Rapidly degraded by nucleases

DNA Replication

The flow of genetic information in a typical cell The main postulate of molecular biology DNA  RNA protein

Replication – synthesis of DNA on the DNA template Semiconservative mechanism of DNA replication The two strands separate, and each strand is copied to generate a comple-mentary strand. Each parental strand remains associated with its newly synthesized complement, so each DNA duplex contains one parental strand and one new strand.

A model for DNA replication

Components which are necessary for replication Enzymes (most important – DNA-dependent DNA polymerase) Protein factors Parental DNA ATP, GTP, ТТP, CТP Ions Mg і Zn

DNA replication In eukaryotes the replication begins in many points simultaneously V-shape – replication forks - the point of the beginning of replication Helicase – enzyme untwisting the double strand 5’ 3’ Helicase Primase 5’ Okazaki fragments 3’ Primer Leading strand Lagging strand 5’ 3’

Replisome - protein machinery for replication Replisome contains: primosome, DNA polymerase III, proteins Helicase is a constituent of primosome

Bidirectional DNA replication in E. coli New strands of DNA are synthesized at the two replication forks where replisomes are located

DNA polymerase DNA polymerase III – the main enzyme of replication responsible for the chain elongation Appropriate nucleotides are inserted in the correct positions according to the complementary principle DNA polymerases only synthesize new strand in the 5’-3’ direction.

Direction of synthesis 5’-3’, antiparalelly to matrix chain

DNA polymerase synthesizes two strands simultaneously Because DNA polymerases only polymerize nucleotides 5 ’3’, both strands must be synthesized in the 5’3’ direction. Thus, the copy of the parental 3’5’ strand is synthesized continuously; this newly made strand is designated the leading strand. As the helix unwinds, the other parental strand (the 5’3’, strand) is copied in a discontinuous fashion through synthesis of a series of fragments - Okazaki fragments; the strand constructed from the Okazaki fragments is called the lagging strands

Синтез відстаючого ланцюга відбувається дискретно Lagging strand is copied in a discontinuous fashion (Okazaki fragments) The formation of a phosphodi-ester linkage between of adjacent Okazaki fragments is catalized by ligase

Okazaki Model Reiji Okazaki provided experimental evidence for discontinuous DNA synthesis

RNA Primer Begins Each Okazaki Fragment Primosome is a complex containing primase enzyme which synthesizes short pieces of RNA at the replication fork - primer DNA pol III uses the RNA primer to start the lagging-strand DNA synthesis Replisome - includes primosome, DNA pol III Each Okazaki fragment has the primer

DNA polymerase I activities Okazaki Fragments Are Joined by Action of DNA Polymerase I and DNA Ligase DNA polymerase I activities DNA pol I removes the RNA primer at the beginning of each Okazaki fragment Synthesizes DNA in place of RNA DNA ligase Catalyzes the formation of a phosphodiester linkage between of adjacent Okazaki fragments

Repair of Damaged DNA DNA is the only cellular macromolecule that can be repaired DNA damage includes: -base modifications -nucleotide deletions or insertions -cross-linking of DNA strands -breakage of phosphodiester backbone

Reparation – enzymatic deletion and synthesis of the damaged DNA fragments Recombination - exchange or transfer of pieces of DNA from one chromosome to another or within a chromosome Transposition – dislocation of gene or group of genes from one place to another