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General Biology Notes 2

Terms in this set (391)

- Sickle-cell disease is caused by the substitution of a single amino acid in the hemoglobin protein of red blood cells; in homozygous individuals, all hemoglobin is of the sickle-cell (abnormal) variety
- When the oxygen content of an affected individual's blood is low (at high altitudes or under physical stress, for instance), the sickle-cell hemoglobin molecules aggregate into long rods that deform the red cells into a sickle shape
- Sickled cells may clump and clog small blood vessels, often leading to other symptoms throughout the body, including physical weakness, pain, organ damage, and even paralysis
- Regular blood transfusions can ward off brain damage in children with sickle-cell disease, but there is no cure
- Although two sickle-cell alleles are necessary for an individual to manifast full-blown sickle-cell disease, the presence of one sickle-cell allele can affect the phenotype
- At the organismal level, the normal allele is incompletely dominant to the sickle-cell allele
- Heterozygotes (carriers), said to have sickle-cell trait, are usually healthy, but they may suffer some sickle-cell symptoms during prolonged periods of reduced blood oxygen
- At the molecular level, the two alleles are codominant; both normal and abnormal (sickle-cell) hemoglobins are made in heterozygotes
- About one out of ten African-Americans have sickle-cell trait, an unusually high frequency of heterozygotes for an allele with severe detrimental effects in homozygotes
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- Having a single copy of the sickle-cell allele reduces the frequency and severity of malaria attacks, especially among young children
- The malaria parasite spends parasite spends part of its life cycle in red blood cells, and the prescence of even heterozygous amounts of sickle-cell hemoglobin results in lower parasite densities and hence reduced malaria symptoms
- In tropical Africa, where infection with the malaria parasite is common, the sickle-cell alllel confers an advantage to heterozygotes even though its harmful in the homozygous state
- This shows that the behavior of homologous chromosomes during meiosis can account for the segregation of the alleles at each genetic locus to different gametes
- The figure also shows that the behavior of nonhomologous chromosomes can account for the independent assortment of the alleles for two or more genes located on different chromosomes
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- Most of one X chromosome in each cell in female mammals becomes inactivated during early embryonic development
- As a result, the cells of females and males have the same effective dose (one copy) of most X-linked genes
- Females consist of a mosaic of two types of cells: those with the active X derived from the father and those with the active X derived from the mother
- After an X chromosome is inactivated in a particular cell, all mitotic descendants of that cell have the same inactive X
- If a female is heterozygous for a sex-linked trait, about half her cells will express one allele, while the others will express the alternate allele
- In humans, mosaicism can be observed in a recessive X-linked mutaton that prevents the development of sweat glands (a woman who is heterozygous for this trait has patches of normal skin and patches of skin lacking sweat glands)
- Inactivation of an X chromosome involves modification of the DNA and the histone proteins bound to it, including attachment of methyl groups to one of the nitrogenous bases of DNA nucleotides
- A particular region of each X chromosome contains several genes involved in the X inactivation process
- The two regions, one in each X chromosome, associate briefly with each other in each cell at an early stage of embryonic development
- Then one of the genes, called XIST (X-inactive specific transcript) becomes active only on the chromosome that will become the Barr body
- Multiple copies of the RNA product of this gene apparently attach to the X chromosome on which they are made, eventually almost covering it
- Interaction of this RNA with the chromosome seems to initiate X inactivation, and the RNA products of other genes nearby on the X chromosome help to regulate the process
- Morgan proposed that some process must occasionally break the physical connection between specific alleles of genes on the same chromosome
- Crossing over accounts for the recombination of linked genes
- In crossing over, which occurs while replicated homologous chromosomes are paired during prophase of meiosis I, a set of proteins orchestrates an exchange of corresponding segments of one maternal and on paternal chromatid
- In effect, end portions of two nonsister chromatids trade places each time a crossover occurs---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
- Nondisjunction of sex chromosomes produces a variety of aneuploid conditions
- Most of these conditions appear to upset the genetic balance less than aneuploid conditions involving autosomes (this may be because the Y chromosome carries relatively few genes and because extra copies of the X chromsome become inactivated as Barr bodies in somatic cells)
- An extra X chromsome in a male, producing XXY, occurs approximately once in every 500 to 1,000 live male births
- People with this disorder, called Klinefelter syndrome, have male sex organs, but the testes are abnormally small and the man is sterile
- Even though the extra X is inactivated some breast enlargement and other female body characteristics are common (affected individuals may have subnormal intelligence)
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- About 1 of every 1,000 males is born with an extra Y chromosome (XYY)
- These males undergo normal sexual development and do not exhibit any well-defined syndrome, but they tend to be somewhat taller than average
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- Females with trisomy X (XXX), which occurs once in approximately 1,000 live female births, are healthy and have no unusual physical features other than being slightly taller than average
- Triple-X females are at risk for learning disabilities but are fertile
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- Monosomy X, called Turner syndrome, occurs about once in every 2,500 female births and is the only known viable monosomy in humans
- Individuals are phenotypically female, but they are sterile because their sex organs do not mature
- When provided with estrogen replacement therapy, girls with Turner syndrome do develop secondary sex characteristics and most have normal intelligence
- Geneticists have identified two to three dozen traits in mammals that depend on which parent passed along the alleles for those traits
- Such variation in phenotype depending on whether an allele is inherited from the male or female parent is called genomic imprinting
- Most imprinted genes are on autosomes
- Genomic imprinting occurs during gamete formation and results in the silencing of a particular allele of certain genes
- Because these genes are imprinted differently in sperm and eggs, a zygote expresses only one allele of an imprinted gene, that inherited from either the female or the male parent
- The imprints are then transmitted to all body cells during development
- In each generation, the old imprints are "erased" in gamete-producing cells, and the chromosomes of the developing gametes are newly imprinted according to the sex of the individual forming the gametes
- In a given species, the imprinted genes are always imprinted the same way
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- The mouse gene for insulin-like growth factor 2 (Igf2), one of the first imprinted genes to be identified
- Although this growth factor is required for normal parental growth, only the paternal allele is expressed
- Evidence that the Igf2 gene is imprinted came initially from crosses between normal-sized (wild-type) mice and dwarf (mutant) mice homozygous for a recessive mutation in the Igf2 gene
- The phenotypes of heterozygous offspring (with one normal allele and one mutant) differed, depending on whether the mutant allele came from the mother or the father
------------------------------------------------------------- A genomic imprint, in many cases, seems to consist of methyl groups that are added to cytosine nucleotides of one of the alleles
- Such methylation may silence the allele, an effect consistent with evidence that heavily methylated genes are usually inactive
- However, for a few genes, menthylation has been shown to activate expression of the allele
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- The inconsistency as to whether methylation activates or silences alleles was resolved in part when researches found that DNA methylation operates indirectly by recruiting enzymes that modify DNA-associated proteins (histones), leading to condensation of the local DNA
- Depending on the original function of the condensed DNA in regulating allele expression the result is either silencing or activation of a given allele
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- Genomic imprinting is thought to affect only a small fraction of the genes in mammalian genomes, but most of the known imprinted genes are critical for embryonic development
- Not all of a eukaryotic cell's genes are located on nuclear chromosomes, or even in the nuceleus; some genes are located in organelles in the cytoplasm (extranuclear or cytoplasmic genes)
- Mitochondria, as well as chloroplasts and other plastids in plants, contain small circular DNA molecules that carry a number of genes
- These organelles reproduce themselves and transmit their genes to daughter organelles
- Organelle genes are not distributed to offspring according to the same rules that direct the distribution of nucelar chromosomes during meiosis, so they do not display Mendelian inheritance
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- In most plants, a sygote receives all its plastids from the cytoplasm of the egg and none from the sperm, which contributes little more than a haploid set of chromosomes
- An egg may contain plastids with different alleles for a pigmentation gene
- As the zygote develops, plastids containing wild-type or mutant pigmentation genes are distributed randomly to daughter cells (pattern of leaf coloration exhibited by a plant depends on the ratio of wild-type to mutant pastids in its various tissues)
------------------------------------------------------------- Maternal inheritance is also the rule for mitochondrial genes in most animals and plants, because almost all the mitochondria passed on to a zygote come from the cytoplasm of the egg the products of most mitochondrial genes help make up the protein complexes of the electron transport chain and ATP synthase
- Defects in one or more of these proteins, therefore, reduce the amount of ATP the cell can make and have been shown to cause a number of rare human disorders
- The parts of the body most susceptible to energy deprivation are the nervous system and the muscles, most mitochondrial diseases primarily affect these systems
------------------------------------------------------------- Mitochondrial myopathy causes weakness, intolerance of exercise, and muscle deterioration
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- Leber's hereditary optic neuropathy, which can produce sudden blindness in people as young as their 20's or 30's
- Four mutations have been found that cause this disorder--they affect oxidative phosphorylation during cellular respiration, a crucial function for the cell
- Alfred Hershey and Martha Chase performed experiments showing that DNA is the genetic material of a phage known as T2
- Phage T2 is one of many phages that infect E. coli
- T2 is composed of almost entirely DNA and protein
- Hershey and Chase found that the phage DNA entered the host cells but the phage protein did not
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- Consists of three components: a nitrogenous base, a pentose sugar called deoxyribose, and a phosphate group
- The bases are adenine (A), thymine (T), guanine (G), or cytosine (C)
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- James Watson and Francis Crick were the first to establish the correct structure
- Presence of two strands accounts for the double helix
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