DNA Structure and Replication

DNA Structure and Replication The Objective : To give information about : 1- Human genome and its components . 2- What chromosome means and how DNA condensed into chromosome to fit in the cell nucleus . 3- Forms of chromosomes which necessary in karyotypes 4- The typical structure and characters of DNA and their function . 5- The molecular structure of gene and its function . 6- Replication of DNA and enzymes involved in replication .

Human Genome : Genome : the complete set of chromosomes and genes in an organism. Human genome contained in every cell and consist of : 23 chromosome pairs and a small mitochondrial DNA . 2 meters of DNA 3 billion DNA subunits ( the bases A, T, C, G ) Approximately 30000 genes code for proteins that perform most life function . Genes only make up ( 5 % ) of human genomic DNA and the rest is junk DNA . Genes consist of exon ( protein coding portions ) and intron (non coding regions ) . ( 99.7 % ) of human genomes are identical . ( 0.3 % ) of human genomes are different .

Chromosome : The structure by which hereditary information is transmitted from one generation to the next . Located in the nucleus, it is consists of tightly coiled thread of DNA with associated proteins and RNA . The genes are arranged in linear order along the DNA . Chromosome consist of two chromatids that held together by centromere.

Chromatin : Is the nucleoprotein material of Eukaryotic chromosome Chromatin : Is the nucleoprotein material of Eukaryotic chromosome . Chromatin is divided into Euchromatin and Heterochromatin . Euchromatin : is chromatin that stains lightly , it is uncoiled during interphase but becomes condensed during mitosis . Most of genome consists of euchromatin . Genetically active ( that is , it contains genes that are being expressed . Heterochromatin : is chromatin that remains condensed during interphase , stained darkly and is genetically inactive . It is found at spesific sites in chromosomes, often in centromeric or telomeric regions of chromosomes .

Chemical Composition of Eukaryotic Chromosomes : 1- DNA 2- Proteins ( Histones and non Histone Proteins ) 3- RNA in small amounts Histones : basic ( positively charged ) proteins , thus facilitating their binding to the negatively charged DNA . Histones H2a , H2b , H3 , H4 are constant from cell to cell with in an organism . They function in the coiling of DNA in chromosomes and in the regulation of gene activity .

Non Histone Chromosomal Proteins : Consist of large number of very heterogeneous acidic proteins , the composition of it varies widely among different cell types of the same organism . Have roles in the regulation of expression of specific genes or sets of genes .It fall into several functional categories : scaffold proteins and chromatin – bound enzymes and transcription factor .

Forms of Chromosomes : Chromosomes can be classified according to the Position of Centromere into : ( Figure 3 ) 1- Metacentric : The Centromere in the Middle of Chromosome . 2- Submetacentric : The centromere between middle and end of chromosome 3- Acrocentric : The Centromere very near the end of Chromosome . 4- Telocentric : The Centromere at the end of Chromosome .

ANATOMY OF MITOTIC CHROMOSOMES FROM HIGHER EUKARYOTES

Karyotype The Chromosome constitution of a cell or an individual . Chromosomes arranged in pairs according to their Length and Position of Centromere in mitotic metaphase .

DNA Organization in Eukaryotic Chromosomes : DNA of human cell is 2 meters in length while the nucleus is a 5-10 micrometers, therefore DNA wraps at several levels until it is compacted into chromosome to fit in the cells nucleus . First level : Nucleosomes ( Beads on a String ) DNA wraps around one histone octamer after another to produce Nucleosomes ( chromatin arranged like beads on a string under an electrone microscope ) this structure produces 10 nm chromatin fiber . Nucleosome contains core consisting of a 146 Nucleotide-pair long segment of DNA wrapped around an histone Octamer (two molecules each of H2a , H2b , H3 , H4 ) . H1 linker histone which holds the nucleosome structure together. The DNA that is between each nucleosomes is called the linker DNA .

A possible nucleosome structure

10 nm chromatin is produced in the first level of packaging. Nucleosomes connected together by linker DNA and H1 histone to produce the “beads-on-a-string” extended form of chromatin 10 nm chromatin is produced in the first level of packaging. Linker DNA H1 Histone octomer

Second level : Solenoid or 30 nm chromatin fiber : 10 nm chromatin fiber folds into 30 nm chromatin fiber or solenoid ( each turn of solenoid includes six nucleosomes ) with aid of H1 . H1 : attached to nucleosome and involved in further compaction of DNA ( conversion of 10 nm chromatin to 30 nm chromatin ) .

Packaging of nucleosomes into the 30-nm chromatin fiber .

Third level : Radial loop domains : Model for the organization of 30 nm chromatin fiber into looped domains , which seems to compact the DNA to 300 nm chromatin . Model for the organization of 30-nm chromatin fiber into looped domains that are anchored to a nonhistone protein chromosome scaffold

Forth level : Metaphase chromosome : Chromatin is looped and attached to a non histone protein scaffold and further compaction of all radial loops to form metaphase chromosome .

DNA Structure : DNA : Deoxyribonucleic acid : is composed of two chains of repeating nucleotides . Each nucleotide consists of three components : 1- Phosphate group 2- Five carbon sugar : Deoxyribosr sugar 3- Nitrogenous bases : Adenine (A) and Guanine (G) are purine which have a two ring structure Cytosine (C) and Thymine (T) are pyrimidine which have a single ring structure .

Characters of DNA : 1- Double Helix : Watson and Crick ( 1953 ) proposed that DNA exist as a Double Helix in which the 2 Polynucleotide chains (strands) are coiled a bout one another in a Spiral . (Figure 11 ) . Each strand consist of a sequence of Nucleotides linked together by Phosphodiester bonds , joining adjacent Deoxyribose moieties . The 2 strands are held together in their helical configuration by Hydrogen bonding between bases in opposing strands .

2- Complementary Base Pairing : The Rule of Complementarity is : Adenin always pairs with Thymine by 2 Hydrogen bonds , and Guanine always pairs with Cytosine by 3 Hydrogen bonds . Once the sequence of bases in one strand of a DNA is known , the sequence of bases in the other strand is also known because of the specific base- pairing .. It is this property complementarity of the 2 strands that makes DNA uniquely suited to store and transmit genetic information .

3-Antiparallel : Antiparallelism in a DNA molecule arises from the orientation of the deoxyribose sugars . One strand of the double helix runs in 5 to 3 direction, and the other strand runs in 3 to 5 direction .

4- DNA resemples a spiral staircase or double helix in which the rails or backbone of alternating sugars and phosphates is the same from molecule to molecule but the steps are pairs of four types of DNA bases whose sequence varies . 5- DNA carry negative charges because of their phosphate groups . 6- DNA is the same in all nucleated cells of persons body .

Characteristics of the Genetic Material : The molecule to serve as the genetic material it must possess four major characteristics : 1- Replication 2- Storage of information 3- Expression of information 4- Variation by mutation

Gene : is acertain segment of DNA that contains the necessary code to make RNA or polypeptide (proteins). Gene accomplish their function through 1-replication that result in more units like themselves . 2-transcription and translation whereby proteins that function as determiners in the metabolism of the cell are synthesized

Gene

DNA – Replication : Replication of DNA is Semiconservative . During Replication the 2 Complementary strand unwind and each single strand serve as template directing the synthesis of a new Complementary strand . ( Figure 16 ) . The new result of Replication is thus 2 progeny DNA molecules identical to the parental Double Helix . Asite where DNA is locally opened called replication fork .

Enzymes involved in DNA Replication and Their Function : 1- Helicase : unwinds parental double helix . (Figure 17 ) . 2- Binding Proteins : stabilize separate strands . 3- Primase : adds short RNA Primer to template strand .

4- DNA Polymerase : a- Adding bases to the new DNA chain , Because DNA Polymerase added Bases in 5  3 direction only , and DNA is antiparallel, the new Strand is synthesized continuously in 5  3 direction and called Leading strand . While the other daughter strand is synthesized discontinuously by short segments of DNA called Okazaki Fragments (synthesized 5  2 ) that are joined together by ligase and called Lagging strand . (Figure 18). b- Proofreading activity checks and replaces incorrect bases . c- Removing RNA primer 5- Ligase : joins okazaki fragments , and seals other nicks in sugar phosphate backbone .

for your attention Thanks