CN1: Genetics II (Mendelian Disorder and Chromosomal Disorder) Flashcards
STRUCTURE AND COMPONENTS OF A GENE
● Humans have a mere 20,000 to 25,000 genes that code for proteins.
● Each gene is composed of one copy originating from the paternal side and the other from the maternal side.
○ Genes are composed of DNA, and the ultimate products of most genes are proteins.
○ A gene is a functional unit of DNA from which RNA is copied (transcribed).
○ Each gene is composed of a linear polymer of DNA.
DNA has several remarkable features that make it ideal for the transmission of genetic information:
○ It is relatively stable, at least in comparison to RNA or proteins.
○ The double-stranded nature of DNA and its feature of strict base-pair complementarity permit faithful
replication during cell division.
○ Complementarity also allows the transmission of genetic information from DNA → RNA → protein.
○ Messenger RNA is encoded by the so-called sense strand of the DNA double helix and is translated into proteins by ribosomes.
The presence of four different bases provides genetic
diversity:
○ In the protein-coding regions of genes, the DNA bases are arranged into Codons, a triplet of bases that specifies a particular amino acid.
○ It is possible to arrange the four bases into 64 different triplet codons (4^3).
○ Each codon specifies 1 of the 20 different amino acids, or a regulatory signal, such as stop translation because there are more codons than amino acids, the genetic code is degenerate: That is, most amino acids can be specified by several different combinations and in various lengths, it is
possible to generate the tremendous diversity of primary protein structure.
CHROMOSOMES AND GENES
● The human genome is divided into 23 different chromosomes, including 22 autosomes (numbered 1-22) and the X and Y sex chromosomes.
● Adult cells are diploid, meaning they contain two homologous sets of 22 autosomes and a pair of sex
chromosomes.
○ Females have two X chromosomes (XX), whereas males have one X and one Y chromosome (XY).
● The genome is estimated to contain about 30,000-40,000 that are divided among the 23 chromosomes.
● Historically, genes were identified because they conferred specific traits that are transmitted from one generation to the next.
● Human DNA is estimated to consist of about 3 billion base pairs (bp) of DNA for a haploid genome.
● DNA length is normally measured in units of 1000 bp (kilobases, kb) or 1,000,000 bp (megabases, Mb).
● Not all DNA encodes genes.
○ In fact, genes account for only about 10 to 15% of DNA.
○ Much of the remaining DNA consists of highly repetitive sequences the function of which is poorly understood.
○ These repetitive DNA regions, along with non-repetitive sequences that do not encode genes, may serve a structural role in the packaging of DNA
into chromatin (DNA bound to histone proteins) and chromosomes.
What is a functional unit that is regulated by transcription and encodes a product; either RNA or protein that exerts activity within the cell?
Gene
Every nucleated somatic cell in the human has a complete genome of about 6 x 10^9 base pairs of DNA,
with an uncoiled total length of
approximately 2 meters.
It is packaged by supercoiling into 46
chromosomes, consisting of 22 pairs of homologous chromosomes (identical in regard to morphology and constituent gene loci) and 1 pair of sex chromosomes (X and Y), one partner of each pair being derived from the mother and one from the father.
Genome
The 46 chromosomes in metaphase vary in length from
2 to 12 μm
The genes are arranged along the chromosomes in
linear order, with each gene having a precise position or locus
Genes that have their loci on the same chromosome are
said to be
syntenic
genes that are close together on the same chromosome and tend to travel together during meiosis (little crossing-over) are said to be linked.
Alternate forms of a gene that occupy the same locus are
called
Alleles
Any one chromosome bears only a single allele at a given locus, although in the population as a whole there may be multiple alleles, any one of which can occupy that specific locus.
Genetic information in DNA is transmitted to daughter
cells under two different circumstances:
○ Somatic cells divide by mitosis, allowing diploid (2n) genome to replicate itself completely in conjunction with cell division; and
○ Germ cells (sperm and ova) undergo meiosis, a process that enables the reduction of the diploid (2n) set of chromosomes to the haploid state (n)
when both members of a pair of alleles (alternative forms of a gene found at a given locus in the chromosome) are identical
Homozygous
○ genetic information defining the phenotype;
○ an individual’s full set of genes
○ describes the specific alleles at a particular locus.
Genotype
when alleles at a given locus are different.
Heterozygous
Overview: Transcription Factors
● The transcription of genes is controlled primarily by the transcription factors that bind to DNA sequences in the regulatory regions of genes.
● Gene expression requires a series of steps including mRNA processing, protein translation, and post-translational modifications, all of which are actively regulated.
● The regulatory regions most commonly involve sequence upstream (5’) of the translation start site, although there are also examples of control elements within introns or downstream of the coding regions of a gene.
○ The upstream regulatory regions are also referred to as the promoter.
○ The minimal promoter usually consists of a TATA box (which binds TATA-binding protein, TBP) and initiator sequences that enhance the formation of an active transcription complex.
○ Transcriptional termination signals reside downstream, or 3’, of a gene.
○ Specific groups of transcription factors that bind to these promoter and enhancer sequences provide a
combinatorial role for regulating transcription.
○ The transcription factors that bind to DNA actually represent only the first level of regulatory control.
○ Other proteins – coactivators and corepressors – interact with the DNA-binding transcription factors to generate large regulatory complexes.
● These complexes are subject to control by numerous cell-signaling pathways, including phosphorylation and acetylation.
○ Ultimately, the recruited transcription factors interact with and stabilize components of the basal transcription complex that assembles at the site of
the TATA box and initiator region.
○ This basal transcription complex that assembles at the site of the TATA box and initiator region, consists of > 30 different proteins.
○ Gene transcription occurs when RNA polymerase begins to synthesize RNA from the DNA template.
Transcription activation can be divided into 3 main
mechanisms;
■ Events that alter chromatin structure can enhance the access of transcription factors to DNA.
● e.g. histone acetylation opens chromatin structure and is correlated with transcriptional activation.
■ Post-translational modifications of transcription factors
● such as phosphorylation, can induce the assembly of active transcription
complexes.
■ Transcriptional activators can displace a repressor protein.
● This mechanism is particularly common during development when the pattern of transcription factor expression changes dynamically.
Overview: Epigenetic Events
● Includes X-inactivation and genomic imprinting, processes in which DNA methylation is associated with the silencing (i.e. suppression) of expression.
● Suppression of gene expression is as important as gene activation.
● Some mechanisms of repression are the corollary of activation.
○ For example, repression is often associated with histone and acetylation or protein dephosphorylation.
● For nuclear hormone receptors, transcriptional silencing involves the recruitment of repression complexes that contain histone deacetylase activity.
Prevents the expression of most genes on one of the two X-chromosomes in every cell of a female.
X-inactivation
Gene inactivation occurs on selected
chromosomal regions of autosomes, leading to preferential expression of an allele depending on its parental origin.
Genomic imprinting
Overview: Variations in Gene Expression
● Some genetic conditions segregate sharply; that is, the normal and abnormal phenotypes can be distinguished clearly.
● In ordinary experience, however, the clinical expression of a disorder may be extremely variable, the age of onset
may be late or variable, or the expression may be modified by other genes or by environmental factors.
● The problems are particularly characteristic of autosomal phenotypes and can lead to difficulties in diagnosis and confusion in pedigree interpretation.
Skipping of Generation (Penetrance and
Expressivity)
● When the frequency of expression of a trait is below 100 percent, that is, when some individuals who have the
appropriate genotype fail to express it, the trait is said to exhibit reduced penetrance.
○ A dominant gene is said to have full penetrance when the character it controls is always evident in an individual possessing the gene.
○ A gene controlling a recessive characteristic is fully penetrant if the characteristic is invariably manifest
when the gene is present in a double dose.
● If on the other hand, a trait takes different forms in different members of a kindred, it is said to have variable
expressivity.
○ Expression may range from mild to severe and members of the same kind may express the same gene in different ways as well severity.
○ A slight abnormality may not be obvious to the casual observer and may explain an apparent skipping of generation.
○ In other instances, the abnormality may be present in a mild form.
○ Some cases of gout or chronic familial hemolytic anemia are without symptoms and are discovered only when tests are made.
Pleiotropy: One Gene, Several Effects
● Multiple phenotypic effects produced by a single mutant gene or gene pair are called pleiotropic effects.
● Each gene has only one primary effect in that it directs the synthesis of a polypeptide chain.
● From this primary effect, however, many different consequences may arise.
● In any sequence of events, interference with one early step may have ramifying effects.
● Thus a single defect occurring early in development can lead to various abnormalities in fully differentiated
structures.
● In some cases a primary gene product might participate in a number of unrelated biosynthetic pathways, possibly at different times.
● In galactosemia, lack of enzyme galactose 1-phosphate uridyl transferase is the primary effect of homozygosity for the recessive gene concerned but there are multiple
secondary effects including mental retardation, cirrhosis of the liver, cataracts and galactosuria.
Genetic Heterogeneity: Several Genes, One Effect
● If mutations at different loci can independently produce the same trait, that trait is said to be genetically heterogenous.
● For example, congenital deafness can be caused by genes located at different loci.
● Thus a man with congenital recessively inherited deafness may marry a woman with congenital recessively inherited deafness, yet all offspring may be normal.
● A possible explanation is that the patient’s deafness is caused by different recessive genes and that each has normal alleles at the locus for which the other has only abnormal genes.
● Is also seen in osteogenesis imperfecta, an inherited disorder, or more specifically a group of disorders that have in common generalized connective tissue abnormality that leads to easy fracturing of bones with little trauma.
● Other defects seen in patients with osteogenesis imperfecta are blue sclerae, conductive hearing loss
relatively early in life, joint laxity and defective dentition.