DNA Organization PDF

DNA organization Learning Objectives: DNA organization : To Identify what are histones, chromatin, chromosomes karyotyping Cell cycle 1-The different phases of cell cycle 2-The proteins and enzymes involved in regulation of cell cycle How the cell cycle is regulated To include an...

DNA organization Learning Objectives: DNA organization : To Identify what are histones, chromatin, chromosomes karyotyping Cell cycle 1-The different phases of cell cycle 2-The proteins and enzymes involved in regulation of cell cycle How the cell cycle is regulated To include an outline of the use of checkpoints to control the cycle DNA organization 1. Nucleosomes :made of nucleoproteins (DNA & basic proteins) 2. Chromatin : DNA since it is 1 meter long in one human cell ( cell is a very small space) must be condensed in to a compact structure chromatin (condensed DNA) is consist of — 1. long double strand DNA — 2. Histones which are basic proteins, positively charged with high content of lysine & arginine — 3. other basic proteins — 4. small quantity of RNA 24-Jul-24 3 Histones form complexes called nucleosomes.(DNA is packaged) Each nuclesome is composed of DNA wound around eight histone proteins. Ø Histones are termed H1, H2A, H2B, H3, and H4 Two each of the histones H2A, H2B, H3, and H4 come together to form a histone octamer, which binds about 146 base pairs. The addition of one H1 protein wraps another 20 base pairs. 24-Jul-24 4  H1 Histone is the least Attached and the least tightly bound to the chromatin. Easley removed by salt solution ØThe chromatin becomes soluble (The linker histone) 24-Jul-24 5 24-Jul-24 6 24-Jul-24 8  3. Genome ( total chromosomal content of DNA) 23 pair of chromosomes in somatic cell 4. Gene: is part of chromosome which occupy a specific position ( locus) 24-Jul-24 9 Ø Chromosomes come in matched sets known as homologous pairs /diploid (2n) The 46 chromosomes of a human cell are organized into 23 pairs, and the two members of each pair are said to be homologues of one another (with the slight exception of the X and Y chromosomes Human sperm and eggs, which have only one homologous chromosome from each pair, are said to be haploid (1n). When a sperm and egg fuse, their genetic material combines to form one complete, diploid set of chromosomes The two chromosomes in a homologous pair are: 1. very similar to one another 2. have the same size and shape. 3. They carry the same type of genetic information: that is, they have the same genes in the same locations. 4. The sex chromosomes, X and Y, determine a person's biological sex: XX specifies female and XY specifies male. These chromosomes are not true homologues and are an exception to the rule of the same genes in the same places 5.The 44 non-sex chromosomes in humans are called autosomes. 6. As a cell prepares to divide, it must make a copy of each of its chromosomes. The two copies of a chromosome are called sister chromatids. 7. As long as the sister chromatids are connected at the centromere, they are still considered to be one chromosome. However, as soon as they are pulled apart during cell division, each is considered a separate chromosome.  Karyotypes The chromosomes from a dividing cell can be photographed and organised into the pairs – one from the mother and one from the father Each pair has a characteristic size, shape and banding pattern (except for the sex chromosomes where the X and Y chromosomes are different) Karyotype from a Autosomes female 22 pairs Make a karyotype Try it Heterosomes yourself 1 pair Chromatids Each molecule of DNA in the nucleus replicates to produce 2 identical strands The strands are called chromatids They remain joined together at the centromere Homologous chromosomes The 22 pairs of autosomes are known as homologous chromosomes – they have the same genes in the same places Ø centromere is an adenine-thymine (A–T)-rich region containing repeated DNA sequences The ends of each chromosome contain structures called telomeres. Telomeres consist of short TG-rich repeats. Human telomeres have a variable number of repeats of the sequence 5ʹ-TTAGGG-3ʹ, which can extend for several kilobases. Telomerase, is the enzyme responsible for telomere synthesis and thus for maintaining the length of the telomere.  telomerase employs its enzyme component reverse transcriptase to extend the telomeres, using its RNA as a template Telomere shortening has been associated with both malignant transformation and aging this enzyme has become an attractive target for cancer chemotherapy and drug development Chromatin containing active genes , transcriptionally has been shown to differ in several ways from that of inactive region Ø transcriptionally inactive chromatin is densely packed during interphase as observed by electron microscopic studies and is referred to as heterochromatin; Transcriptionally active chromatin stains less densely and is referred to as euchromatin. Types of heterochromatin: 1. Constitutive heterochromatin is always condensed and thus essentially inactive. It is found in the regions near the chromosomal centromere and at chromosomal ends (telomeres). 2. Facultative heterochromatin is at times condensed, but at other times it is actively transcribed and, thus, uncondensed and appears as euchromatin. one X chromosome is almost completely inactive transcriptionally and is heterochromatic. DNA methylation is the attachment by specific enzymes of methyl groups (-CH 3 ) to DNA bases after DNA synthesis. Inactive DNA is generally highly methylated compared to DNA that is actively transcribed. Demethylating inactive genes turns them on. Histone acetylation: (addition of an acetyl group ) play a direct role in the regulation of gene transcription. Acetylated histones grip DNA less tightly, providing easier access for transcription proteins in this region Noncoding intervening sequences (introns). The protein coding regions of DNA, the transcripts of which ultimately appear in the cytoplasm as single mRNA molecules, are usually interrupted in the eukaryotic genome by large intervening sequences of nonprotein-coding DNA ØIn most cases, the introns are much longer than the coding regions (exons). The DNA in a eukaryotic genome can be divided into different “sequence classes.” 1. These are unique-sequence DNA, or non repetitive DNA / single copy genes that code for proteins 2. Repetitive-sequence DNA / sequences that vary in copy number from 2 to as many as 107 copies per cell. Ø In Human DNA, at Least 30% of the Genome Consists of Repetitive Sequences  Repetitive-Sequence DNA Both moderately repetitive and highly repetitive DNA sequences are sequences that appear many times within a genome. These sequences can be arranged within the genome in one of two ways: – distributed at irregular intervals—known as 1. dispersed repeated DNA or interspersed repeated DNA – 2. or clustered together so that the sequence repeats many times in a row—known as tandemly repeated DNA. Interspersed genome-wide repeats consist of families of repeated sequences interspersed throughout the genome. They can be either short or long and many have the added distinction of being either an actual mobile elements (transposons or retrotransposons) Transposons are mobile DNA sequences which migrate to different regions of the genome via transposition.  Two types of dispersed repeated sequences are known: – 1. Long interspersed elements (LINEs), in which the sequences in the families are about 1,000– 7,000bp long; and – 2. Short interspersed elements (SINEs), in which the sequences in the families are 100–400 bp long. Satellite DNA Human satellite DNA is comprised of very large arrays of tandemly repeated DNA with the repeat unit being a simple or moderately complex sequence (100kb to several Mb not ususally transcribed ONE PERCENT OF CELLULAR DNA IS IN MITOCHONDRIA The majority of the polypeptides in mitochondria (about 54 out of 67) are encoded by nuclear genes, while the rest are coded by genes found in mitochondrial (mt) DNA. Human mitochondria contains 2 to 10 copies of a small circular ~16 kbp dsDNA molecule Ø mtDNA codes for mt-specific rRNA and tRNA Ø and for 13 proteins that play key roles in the respiratory  because all mitochondria are contributed by the ovum during zygote formation—it is transmitted by maternal nonmendelian inheritance. Thus, in diseases resulting from mutations of mtDNA, an affected mother would in theory pass the disease to all of her children but only her daughters would transmit the trait.