Heterochromatin is a tightly packed DNA region where genes in such regions are usually not transcribed. Numerous transposable elements (TEs) and repetitive DNA are found in heterochromatic regions. As they can transpose along the genome and disrupt gene functions, it is essential to repress such TEs and DNA repeats (Lippman et al., 2004). Heterochromatin is able to maintain internucleosomal interactions as well as chromatin fiber interactions between cis-elements. It can be passed on to subsequent generations and can control gene expressions by inhibiting transcription epigenetically, a process known as silencing.
INTRODUCTION Two states of chromatin are known as euchromatin and heterochromatin. Tightly packed form of DNA is called heterochromatin which is enriched at the centromeres and telomeres of chromosomes. The dense packing of DNA makes it less accessible to several protein factors and hence is often described as the gene-poor part of the genome. Heterochromatin is primarily composed of highly repetitive sequences such as satellite sequences, ribosomal DNA (rDNA) and transposable elements (1). It localizes at the nuclear periphery and nucleolus.
Retrieved 11 December, 2013 from http://www.oculist.net/downaton502/prof/ebook/duanes/pages/v8/ch025/005f.html Karp, G. (2013). Cell and molecular biology: Concepts and experiments (7th ed.). USA: John Wiley & Sons. Lefranc, M. P., & Huret, J. L. (2002). Atlas of genetics and cytogenetics in oncology and haematology.
While the members of the FA complementation group do not share sequence similarity, they are related by their assembly into a common nuclear protein complex. Beside these sixteen FA proteins, there are several other proteins associating with the FA core complex, known as the FA Associated Proteins (FAAPs): -100, -24, -20, -16/MHF1, and -10/MHF2. FA plays an important role in the genomic stability through DNA repair of interstrand crosslinks (ICLs). When mutations occur in these genes, however, abnormal cell division, which eventually causes cancer and congenital defects occurs in most patients (Nalepa, et al., 2013; Tomida, et al., 2013). Fanconi anemia is caused by mutations in one of the Fanconi anemia genes leading to lack of interstrand crosslink repair.
In highlighting similarities between p53 protein-DNA interactions to other protein-DNA complexes, Cho et al. (1994), point out that p53 uses a loop packing at the NH2-terminal part of the alpha helix to make extra connections to the bases in the major groove of DNA. The C-terminal (carboxyl terminal) —61 important amino aci... ... middle of paper ... ...anaka, S., 2009. Suppression of induced pluripotent stem cell generation by the p53-p21 pathway. Nature 460, 1132-1135.
1.6.1 Genetic engineering Genetic engineering depends on the location and analysis of genes on chromosomes and ultimately DNA sequencing. The early cartography of the genes used the principles of Mendelian genetics . It is assumed that alleles that are transmitted together side by side are located on the same chromosome : it is said that are connected or linkage . These genes form a bridging group - linkage group : are the same for gametes and are usually transmitted together , so they do not have independent distribution. Crossing-over occurring during meiosis may cause these alleles can be exchanged between the chromosomes of a homologous pair .
Chromosome cohesion in mitosis and meiosis. Journal of Cell Science,120, 367-369. Karp, G. (1999). Cell and molecular biology concepts and experiments (2nd ed.). America, A: John Wiley & Sons, Inc. McIntosh, J. R., & McDonald, K. L. (1989).
References 1. “Biochemistry of the cell cycle: A review”, Baserga. Cell Proliferat.1(2): 167-191. 2. “The effect of protein synthesis inhibition on the entry of messenger RNA into the cytoplasm of sea urchin embryos”, Hogan and Gross.
Those errors that remain buried within a cell’s genome and are sometimes passed on to the next generation, go under the name of DNA mutation. Because DNA codes for proteins and RNA molecules, almost every change in the DNA sequence, left unrepaired, can cause physiological malfunctions, known as genetic diseases. According to The Global Genes Project, there are more than 7,000 rare genetic diseases alone. Despite those alarming figures, genetic diseases are not uncommon since DNA is rather a fragile molecule prone to damage. Many types of mutations have been analyzed and classified, the simplest of which is point mutation.
DNA methylation is a postreplicative modification that occurs when a methyl (-CH3) group is added at position 5 of the cytosine pyrimidine ring and “establishes a silent chromatin state by collaborating with proteins that modify nucleosomes.” (Rudolf Jaenisch, 2003). It is found in cytosines of the dinucleotide sequence CpG, which when methylated within a gene in mammals can turn the gene off. This silencing of genes process can be found in cases of cancer in humans. (Wikipedia, DNA methylation in Cancer, 2012). The process of forming 5-methylcytosine (5mC), which makes up 1% of all DNA bases (Mamta Tahiliani, 2009), is catalysed by DNA methytransferase proteins (DNMTs).