science genetics

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Information about science genetics

Published on October 3, 2007

Author: Sudiksha


Slide1:  $ $ $ $ $ $ $ Karl Popper Logic der Forschung, 1914:  Karl Popper Logic der Forschung, 1914 Science never pursues the illusory aim of making its answers final, or even probable. Its advance is, rather, towards an infinite yet attainable aim: that of ever discovering new, deeper, more general problems, and of subjecting our ever tentative answers to ever renewed and ever more rigorous tests. Some Genetic Vocabulary:  Some Genetic Vocabulary Nucleotides: the base (G,A,T,C) building blocks of deoxyribonucleic acid (DNA) . Sequence: the order in which the bases (G, A, T, C) are found in DNA or other nucleic acids. DNA consists of a double helix (actually two DNA strands) with specific physical properties. DNA is a very stable chemical polymer molecule Genetic code: The sequence of bases; the “meanings” of the order of the sequence. From Mendel to the Human Genome:  From Mendel to the Human Genome Staining and visualization of chromosomes (cytogenetics; 1970’s) Slide5:  Cytogenetics is the study of aberrant chromosomes. Women who are planning to have children in their 30’s and 40’s, or later, are counseled about the risks of Down’s syndrome What Exactly is a chromosome?:  What Exactly is a chromosome? A chromosome is a structure in a cell’s nucleus that carries human genes. A chromosome consists of a continuous molecule of DNA and has proteins wrapped around it. DNA is composed of four different nucleotide letters, designated A, T, C, and G in a repeating polymer molecule that is also built with sugar and phosphate. Human Chromosomes:  Human Chromosomes Humans possess 23 “pairs” of chromosomes (46 chromosomes) 22 chromosome “pairs” (44 chromosomes) are autosomes (non-sex chromosomes) 1 chromosome “pair” (2 chromosomes) are sex chromosomes; XX in females and XY in males Mendelian traits are conditions caused by a single gene:  Mendelian traits are conditions caused by a single gene The modes of inheritance reveal whether the trait is dominant or recessive, and whether the gene that controls the trait is carried on an autosome (one of the 22 paired non-sex chromsomes) or a sex chromosome (X or Y). Since HD is an autosomal dominant disorder, we can infer that the HD gene is a dominant gene that is present on an autosome. What is a Gene?:  What is a Gene? A gene is the smallest unit of heredity. Beyond this, the precise definition of a gene today is still debated. Many scientists think of genes as the linear segment of DNA which is the genetic element that maintains/creates the gene product. Many genes code for a protein product made by transcription of the gene into messenger RNA, then translation of the messenger RNA in the protein. Many genes code for only one specific protein product Some genes code for RNA, not protein. What is a Gene?:  What is a Gene? Most human genes have sequences that are expressed as “protein” (called exons) and sequences intervening that are not expressed as protein (called introns). All sequences in the gene are first transcribed into a big RNA molecule called precursor RNA. Then cell machinery “splices” out the intron, and joins the exons in order into the mature messenger RNA that carries the genetic information for the protein. The cell structure called the ribosome can then translate the messenger RNA sequences into the protein product. The genetic code for translation uses three base letter to code for each of twenty different protein amino acids All genes are regulated correctly (developmental, temporal, exons/introns, processing) DNA is the Genetic Material:  DNA is the Genetic Material The order of the DNA letters make up three letter “words” (the genetic code) each of which denotes one of twenty different amino acids that in turn are joined together to form polymers (proteins). “one gene” = “one protein” most frequently, genes are discontinuous flows of genetic information. Some Genetic Vocabulary:  Some Genetic Vocabulary Allele: An alternate form of a gene. Dominant: A gene allele that is expressed in even a single copy; if one allele of a gene masks the effect of the other allele it is said to be dominant. Recessive: An allele whose expression is masked by another allele. Slide13:  Dominant Recessive + = Dominant How is an allele different from a mutant gene?:  How is an allele different from a mutant gene? An allele is any form of a gene A mutation is defined as a change in DNA A mutant is described as an allele that differs from the wild-type (“normal”) allele, altering the phenotype. Remember that the term mutant was developed to describe non-human experimental model organisms. It is not—and probably should not—be applied to human variations. Slide15:  Figure 3. Examples of Point Mutations. Panel A shows the normal sequence of DNA from one exon and the protein product it encodes. Panel B shows a silent mutation Panel C a conservative missense mutation (serine and threonine have very similar structures) Panel D a nonconservative missense mutation (serine and proline have very different structures) Panel E a nonsense mutation Panel F a frame-shift mutation. In Panel F, the insertion of a single G throws off the reading frame, so that all amino acids downstream are changed radically. Slide16:  Mutation is the process that drives the formation of variant alleles. Mutation is important for the generation of genetic diversity, which in turn is important for the “health” of the human population as well as all other species. Genotype: The allele combinations in an individual that cause a particular trait. Phenotype: The expression of a gene in traits. Slide17:  Mammalian species (including humans) are diploid organisms, that is, each cell contains two complete sets of chromosomes and therefore two alleles of every gene. Homozygous: When the two alleles of a gene are identical. Heterozygous: When the two alleles of a gene are different. Slide18:  There may be as few as several hundred to as many as several hundred million different alleles of every human gene if one considers the entire human population. You cannot reliably predict interaction between two different alleles of a gene (e.g. whether one allele or the other will be dominant, recessive, or “co-dominant”) unless you know beforehand the inheritance pattern of the alleles. This is determined by observation of phenotype/genotype correlation between generations. Genetics in Medicine:  Genetics in Medicine Inherited genetic diseases: Variant forms of genes passed from one generation to the next. Some diseases are caused by single genes, like HD. Many conditions (traits) are cause by the action(s) of more than one gene. Furthermore, the action(s) of ALL genes are influenced to some degree by the environment. Slide20:  CF gene HD gene HD CF Environmental influence on trait Genes or Environment? Genetics in Medicine:  Genetics in Medicine Genetic diseases are not always inherited. Somatic genetic disease is caused by the appearance of a variant form of a gene in one part of the human body. Cancer is perhaps the most significant example of this type of genetics. While the “risk” of cancer frequently has an inherited component, cancer arises from somatic mutation. Genetics in Medicine :  Genetics in Medicine Chromosomal aberrations represent the third type of genetic condition (inherited; somatic; chromosomal). These are changes in the structure or number of human chromosomes. Many important human conditions are due to chromosomal aberrations, some of which are inherited, some of which arise anew, as de novo cases. New cases are frequently associated with the age of the mother or father. Back to Mendel::  Back to Mendel: Autosomal dominant disorders never skip generations. In contrast, autosomal recessive traits can skip generations and can affect both sexes. An example of an autosomal recessive disorder is cystic fibrosis. Sex-linked Mendelian traits affect males and females differently. Slide24:  Every human being, in their genome, carries at least 6 lethal autosomal recessive disorders. Since they are recessive traits they do you no harm. However, these rare autosomal recessive disorders do sometimes appear in families where blood relatives have children together. Mendel’s first law: Gene segregation:  Mendel’s first law: Gene segregation During meiosis (sex cell formation) homologous* pairs of chromosomes separate from one another and are packaged into separate gametes (ova or sperm). Mendel’s first law, which can predict the probability that a child will inherit a Mendelian trait, applies anew to each child (Punnett squares). Mendel’s second law: The law of independent assortment:  Mendel’s second law: The law of independent assortment When examining more than one trait, where each trait is caused by a different gene that has two different alleles, Mendel concluded that a gene for one trait does not influence the transmission of a gene for another trait. E.g. height and haircolor. Morgan:  Morgan Mendel’s second law only applies to genes that reside on DIFFERENT chromosomes. Genes on the same chromosome are physically linked. Genes reside at different chromosomal addresses. Linkage: The relationship between DNA sequences (genes) located on the same chromosome. LINKAGE:  LINKAGE Genes on the same chromosome are linked. However, occasionally expected linkage does not occur. Genes that are close together on the same chromosome are more closely linked than genes that are farther away from each other. Linkage can be used to generate a Genetic Map (chromosomal gene addresses). Meiotic Recombination; also called “crossing over”.:  Meiotic Recombination; also called “crossing over”. Meiotic recombination is the “force” that occasionally destroys linkage. Recombination is an event that occurs during meiosis. When meiotic chromosomes are aligned together (“paired”) they can occasionally exchange or trade “homologous” parts of their chromosomes with each other. x Human Diversity:  Human Diversity Crossing Over and the Independent Assortment of chromosomes through meiosis are the two major mechanisms that generate diversity. All human individuals are genetically unique. With 223 chromosomal arrangements and and many meiotic crossing over possibilities there are literally an infinite number of separate genotypes. Human Genes:  Human Genes The 46 human chromosomes contain about 30,000 human genes. Each gene is believed to be present in two copies. An exception to universal diploidy of genes occurs in males since males have only one X chromosome (and one Y chromosome) some genes are represented only once. Junk DNA:  Junk DNA By our current definitions, genes represent only about 1 to 2 % of our genome. The other ~98% of the genome has been commonly called junk DNA. Most of the repetitive DNA used for loci identification resides in this domain of junk DNA. Many scientists believe this DNA is not actually junk, and has been described as junk DNA primarily from ignorance of roles this DNA may play. Possible roles of junk DNA: regulation and timing of of gene expression, telomeres (cell aging), X-chromosome inactivation, regulation of chromosome behavior, control of cell division, new genes (unidentified, and “evolving” genes) How does one go about finding a human gene that is the cause of an inherited human genetic disease? :  How does one go about finding a human gene that is the cause of an inherited human genetic disease? The case of Huntington’s Disease. What is the HD gene?:  What is the HD gene? The HD gene is represented by a segment of DNA (approximately 300,000 letters) found on human chromosome 4. Approximately 3% of the letters of the HD gene are expressed as the HD protein, which has been named huntingtin. Although HD is relatively rare (both inherited and de novo case), every human possesses two alleles of the HD gene. What’s wrong with the HD gene?:  What’s wrong with the HD gene? The variant HD alleles which cause HD have a molecular abnormality consisting of an expanding triplet repeat (CAG). The expanding triplet repeat causes extra glutamine amino acids to be placed within the front end of the huntingtin protein, making huntington bigger. The genetic test that measures the presence of the HD disease alleles.:  The genetic test that measures the presence of the HD disease alleles. The original HD test was based on genetic markers that were close to the HD gene. These tests were about 95% reliable (sometimes crossing over would “break” the linkage). The current test is based on the direct assessment of the exact size of the “expanding triplet repeat” region of the HD gene. This is a much more reliable test, more than 99.5% accurate. The genetic test that measures the presence of the HD disease alleles.:  The genetic test that measures the presence of the HD disease alleles. Alleles of HD which have between 10 and 35 (CAG) repeats never cause HD. Alleles of HD which have more than 40 (CAG) repeats always cause HD. Alleles of HD which have between 36 and 39 (CAG) repeats almost always causes HD. HD is nearly completely penetrant:  HD is nearly completely penetrant Penetrance is a measure of the percent of individuals affected. Alleles of HD which have more than 40 (CAG) repeats always cause HD. Alleles of HD which have between 36 and 39 (CAG) repeats almost always causes HD. Expressivity of HD:  Expressivity of HD Expressivity is a measure of the intensity of symptoms. All people who clinically present with HD have an approximately similar course of symptoms followed by inevitable death. (Big CAG repeats do lead to early-onset)

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