Human Genetics (Lec 6) PDF

Human Genetics FOLDER No.6 Department of Medical Laboratory Techniques 3rd Stage Al-Salam University College Baghdad, Iraq Asst.Inst. Omer. S. Ghalib Human Genetics...

Human Genetics FOLDER No.6 Department of Medical Laboratory Techniques 3rd Stage Al-Salam University College Baghdad, Iraq Asst.Inst. Omer. S. Ghalib Human Genetics Asst.Inst. Omer S. Ghalib Lec: 6 3rd stage Chromatin Chromatin is a complex of DNA and protein found in eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important roles in reinforcing the DNA during cell division, preventing DNA damage, and regulating gene expression and DNA replication. During mitosis and meiosis, chromatin facilitates proper segregation of the chromosomes in anaphase; the characteristic shapes of chromosomes visible during this stage are the result of DNA being coiled into highly condensed chromatin. The total DNA in the cell is about 5 to 6 feet long which has to fit inside the nucleus of a cell in an orderly fashion. DNA molecules first wrap around the histone proteins forming beads on string structure called nucleosomes. Nucleosomes further coil and condense/gather to form fibrous material which is called chromatin. Chromatin fibers can unwind for DNA replication and transcription. When cells replicate, duplicated chromatins condense further to become a lot like chromosomes, visible under microscope which are separated into daughter cells during cell division. The overall structure of the chromatin network further depends on the stage of the cell cycle. During interphase, the chromatin is structurally loose to allow access to RNA and DNA polymerases that transcribe and replicate the DNA. The local structure of chromatin during interphase depends on the specific genes present in the DNA. Regions of DNA containing genes which are actively transcribed ("turned on") are less tightly compacted and closely associated with RNA polymerases in a structure known as euchromatin, while regions containing inactive genes ("turned off") are generally more condensed and associated with structural proteins in 2 Human Genetics Asst.Inst. Omer S. Ghalib Lec: 6 3rd stage heterochromatin. Epigenetic modification of the structural proteins in chromatin via methylation and acetylation also alters local chromatin structure and therefore gene expression. There is limited understanding of chromatin structure and it is active area of research in molecular biology. - Chromatin is the complex combination of DNA and proteins that makes up chromosomes. - It is found inside the nuclei of eukaryotic cells. - The function of chromatin is: -to package DNA into a smaller volume to get in the cell to strengthen the DNA to allow mitosis and meiosis. 3 Human Genetics Asst.Inst. Omer S. Ghalib Lec: 6 3rd stage Types of Chromatin 1- Euchromatin 2- Heterochromatin 1-Euchromatin Euchromatin (also called "open chromatin") is a lightly packed form of chromatin (DNA, RNA, and protein) that is enriched in genes, and is often (but not always) under active transcription. Euchromatin stands in contrast to heterochromatin, which is tightly packed and less accessible for transcription. 92% of the human genome is euchromatic. In eukaryotes, euchromatin comprises the most active portion of the genome within the cell nucleus. In prokaryotes, euchromatin is the only form of chromatin present; this indicates that the heterochromatin structure evolved later along with the nucleus, possibly as a mechanism to handle increasing genome size. Functions of euchromatin Euchromatin is the part of the chromatin involved in the active transcription of DNA into mRNA. As euchromatin is more open in order to allow the recruitment of RNA polymerase complexes and gene regulatory proteins, so transcription can be initiated. There is a direct link between how actively productive a cell is and the amount of euchromatin in its nucleus. 4 Human Genetics Asst.Inst. Omer S. Ghalib Lec: 6 3rd stage 2-Heterochromatin Heterochromatin is a tightly packed form of DNA or condensed DNA, which comes in multiple varieties. These varieties lie on a continuum between the two extremes of constitutive heterochromatin and facultative heterochromatin. Both play a role in the expression of genes. Because it is tightly packed, it was thought to be inaccessible to polymerases and therefore not transcribed.much of this DNA is in 5 Human Genetics Asst.Inst. Omer S. Ghalib Lec: 6 3rd stage fact transcribed, but it is continuously turned over via RNA-induced transcriptional silencing (RITS). Heterochromatins are two types: - A- Constitutive heterochromatin - Constitutive heterochromatin domains are regions of DNA. - Found throughout the chromosomes of eukaryotes. - Heterochromatin is found at the pericentromeric regions of chromosomes but is also found at the telomeres and throughout the chromosomes. - Has a structural function.  Made up of satellite DNA. 6 Human Genetics Asst.Inst. Omer S. Ghalib Lec: 6 3rd stage - Constitutive heterochromatin can affect the genes near itself (e.g. position-effect variegation). It is usually repetitive and forms structural functions such as centromeres or telomeres, in addition to acting as an attractor for other gene- expression or repression signals. B- Facultative heterochromatin - In contrast facultative heterochromatin consists of euchromatin that takes on the staining and compactness characteristics of heterochromatin during same phase of development. - The inactive x-chromosomes is made up of facultative heterochromatin. - It may convert to euchromatin depending upon requirement. - Facultative heterochromatin is the result of genes that are silenced through a mechanism such as histone deacetylation or Piwi-interacting RNA (piRNA) through RNAi. It is not repetitive and shares the compact structure of constitutive heterochromatin. However, under specific developmental or environmental signalling cues, it can lose its condensed structure and become transcriptionally active. 7 Human Genetics Asst.Inst. Omer S. Ghalib Lec: 6 3rd stage Functions of Heterochromatin Heterochromatin has been associated with several functions, from gene regulation to the protection of chromosome integrity; some of these roles can be attributed to the dense packing of DNA, which makes it less accessible to protein factors that usually bind DNA or its associated factors. For example, naked double-stranded DNA ends would usually be interpreted by the cell as damaged or viral DNA, triggering cell cycle arrest, DNA repair or destruction of the fragment, such as by endonucleases in bacteria. Some regions of chromatin are very densely packed with fibers that display a condition comparable to that of the chromosome at mitosis. Heterochromatin is generally clonally inherited; when a cell divides, the two daughter cells typically contain heterochromatin within the same regions of DNA, resulting in epigenetic inheritance. Variations cause heterochromatin to encroach on adjacent genes or recede from genes at the extremes of domains. Transcribable material may be repressed by being positioned (in cis) at these boundary domains. This gives rise to expression levels that vary from cell to cell, which may be demonstrated by position- effect variegation. Insulator sequences may act as a barrier in rare cases where constitutive heterochromatin and highly active genes are juxtaposed (e.g. the 5'HS4 insulator upstream of the chicken β-globin locus, and loci in two Saccharomyces spp Nucleosome / Nucleosomes A nucleosome is a section of DNA that is wrapped around a core of proteins. Inside the nucleus, DNA forms a complex with proteins called chromatin, which allows the DNA to be condensed into a smaller volume. When the chromatin is extended and viewed under a microscope, the structure resembles beads on a string. Each of these tiny beads is a called a nucleosome and has a diameter of approximately 11 nm. The nucleosome is the fundamental subunit of chromatin. Each nucleosome is composed of a little less than two turns of DNA wrapped around a set of eight proteins called histones, which are known as a histone 8 Human Genetics Asst.Inst. Omer S. Ghalib Lec: 6 3rd stage octamer. Each histone octamer is composed of two copies each of the histone proteins H2A, H2B, H3, and H4. The chain of nucleosomes is then compacted further and forms a highly organized complex of DNA and protein called a chromosome. The DNA Packaging - DNA packaging. Each chromosome consists of one continuous thread-like molecule of DNA coiled tightly around proteins, and contains a portion of the 6,400,000,000 basepairs (DNA building blocks) that make up your DNA. The way DNA is packaged into chromatin is a factor in how protein production is controlled. - Watson and Crick gave the DNA structure. According to their model DNA is a double-helical structure with two polynucleotide strands that run antiparallel. Due to phosphate groups in the DNA backbone, this double helix is negatively charged. The cell produces histone proteins that bind to the DNA to counteract the negative charge. These histone proteins have a role in the packaging of DNA. 9 Human Genetics Asst.Inst. Omer S. Ghalib Lec: 6 3rd stage The Nucleosome and DNA Packaging - Although the DNA helical diameter is only 2 nm, the entire DNA strand in a single cell will stretch roughly 2 meters when completely unwound. The entire DNA strand must fit within the nucleus of a cell, so it must be very tightly packaged to fit. This is accomplished by wrapping the DNA around structural histone proteins, which act as scaffolding for the DNA to be coiled around. The entire structure is called a nucleosome, each of which includes an octamer of histone proteins and 146 to 147 base pairs of DNA. The millions of nucleosomes tightly coil the continuous DNA strand into chromatin which is further condensed into the chromosome classically visualized during cell division. - The tight structure of chromatin brings about the problem of accessibility to the DNA by enzymes involved in DNA replication and transcription. Chromatin exists in one of two states: heterochromatin, which is condensed and allows little access by transcription enzymes, and euchromatin, which is loose to allow for interaction with transcription enzymes. The transition between these two states is determined by interactions between the DNA and histone proteins via post- translational modifications to the histone proteins like methylation and acetylation. Methylation generally increases interactions between the DNA and histone, thus suppressing gene expression, whereas acetylation will loosen interactions resulting in greater access by transcription enzymes resulting in increased gene expression. The post-translational modifications to histone proteins underlie the mechanisms of epigenetics, which are defined as alterations to gene expression without changes to the DNA sequence. - The ability for DNA packaging to be modified at various stages of the cell cycle is important in both DNA replication and cell division as well as transcription. 10 Human Genetics Asst.Inst. Omer S. Ghalib Lec: 6 3rd stage Replication occurs at many origins of replication throughout the DNA strand to accelerate the replication of the entire genome, with each origin separated by approximately 100,000 base pairs. The DNA does not interact with histones during this process to allow for the propagation of the polymerase enzymes. However, when the process is complete, the DNA must reintegrate with the histones to reform nucleosomes and eventually the supercoiled chromosome structure during mitosis. Following cell division, the DNA must again separate from the histone proteins to undergo transcription. This capability for the DNA- histone interactions to be modulated is crucial for the proper growth and function of a cell with malfunctions contributing to disease like hypermethylation in cancer. 11