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BIOL 1010U (101)
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Biology 1 - Genetics: Chromosomal Basis of Inheritance.docx

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School
Department
Biology
Course
BIOL 1010U
Professor
Sylvie Bardin
Semester
Fall

Description
Chromosomal Basis of Inheritance Genes can be tagged with a fluorescent marker and visualized on chromosomes Morgan’s experiment • Evidence that genes are on chromosomes • Fruit flies were studied because • A generation can be bred every two weeks • Produce hundreds of offspring • Only 4 pairs of chromosomes; 3 pairs of autosomes and 1 pair of sex chromosomes • In Morgan’s experiment, reciprocal crosses gave different outcomes • Only the males were affected by the trait • Mated male flies with white eyes (mutant) with female flies with red eyes (wild type) • The F 1eneration all had red eyes • The F 2eneration showed the 3:1 red:white eye ratio, but only males had white eyes → The gene for eye color must be located on the X chromosome with no corresponding gene on the Y chromosome → This finding supported the chromosome theory of inheritance Sex-Linked inheritance • X and Y chromosomes: – Differ in size, shape and gene content – Contains genes for many characters unrelated to sex – Synapse during meiosis (behave as homologous chromosomes) and segregate in different gametes Sex-linked Gene – A gene located on either sex chromosome - In humans, sex-linked usually refers to a gene on the larger X chromosome (also called X-linked inheritance) - Males only inherit one X chromosome from their mother - Both dominant and recessive alleles located on X chromosome will be expressed in males! - Sex-linked recessive disorders are more common in males than in females EXAMPLES - Color blindness - Duchenne muscular dystrophy - Hemophilia X-Linked Vs. Autosomal Inheritance • X-linked inheritance – Males tend to be more affected than women. – Mother-to-sons transmission can occur. – Father-to-sons transmission cannot occur. – Father-to-daughters transmission will occur. • Autosomal inheritance (from genes located on autosomal chromosomes) – Both male and female are affected by the trait. – Parents and/or grand-parents must carry or be affected by the trait Sex-linked genes exhibit unique patterns of inheritance • SRY gene (sex-determining region of Y) activation in mammal embryo (2 month old), lead to development of male characteristics • In absence of SRY, embryo develops into a female X Inactivation in Female Mammals • In mammalian females, one of the two X chromosomes in each cell is randomly inactivated • The inactive X condenses into a Barr body • If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character. – Some cells will express the trait while some other will not • Inactivation of X involves DNA modification (methylation of bases) Linked genes tend to be inherited together because they are located near each other on the same chromosome • Morgan crossed Drosophila that differed in traits of body color and wing size (these 2 are linked) • He noted that these genes do not assort independently, and reasoned that they must be on the same chromosome • Linked genes - genes on the same chromosome that tend to be inherited together Dependent assortment • When genes are on the same chromosome • Genes are linked and tend to be inherited together Recombination of Linked Genes: Crossing Over • Incomplete linkages are possible; evident from the ratio of recombinant phenotypes • Crossing over during meiosis I explains these phenotypes • The closer the genes on the chromosome the less likely they are to be separated by a crossover event in meiosis I Linkage map (Sturtevant) • Genetic map: Location of genes on a chromosome • Linkage map: Genetic map based on recombination frequencies • One map unit (centimorgan) = 1% recombination frequency. They indicate relative distance and order, not precise locations of genes Linkage map • The further apart the genes, the higher the recombination frequency. • Genes that are physically linked (on the same chromosome), but genetically unlinked (recombination frequency near 50%) behave as if they are found on different chromosomes. • Frequency of cross-over is not uniform along the chromosome. Cytogenetic map: indicate the positions of genes with respect to chromosomal features (e.g. stained bands) Physical maps: determined by nucleotide sequences Genetic recombination and linkage (summary) • Genes on different chromosomes: – Recombination occurs due to independent assortment of chromosomes; Follow Mendel’s law. – 50% of the offspring will have a recombinant genotype due to random orientation of the homologous chromosomes at metaphase I. • Genes on the same chromosome: – Recombination will occur if crossing over between linked genes occurs during prophase I. – The recombination frequency is proportional to the distance between the two genes. – If the genes are close to one another →Do not follow Mendel’s law of independent assortment – If the genes are far away and recombination frequency is ≥ 50% due to high frequency of crossing over, the genes will behave as if they are not linked! →Follow Mendel’s law of independent assortment Alterations of chromosome number or structure cause some genetic disorders • Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders • Abnormal Chromosome Number is caused by nondisjunction: – Pairs of homologous chromosomes do not separate normally during meiosis I or II. – One gamete receives two of the same type of chromosome, and another gamete receives no copy Aneuploidy: – results from the fertilization of gametes in which nondisjunction occurred – Offspring have an abnormal number of a particular chromosome • A monosomic zygote has only one copy of a particular chromosome (2n -1 chromosomes) • A trisomic zygote has three copies of a particular chromosome (2n +1 chromosomes) – Mitosis will transmit the anomaly to all embryonic cells. – Nondisjunction can also occur during mitosis; harmful effect if it happens early in embryonic development. Human disorders caused by aneuploidy • Down syndrome (Trisomy 21): – Facial features; short stature; heart defects; susceptibility to respiratory infections; mental retardation; sterile – Incidence increases with the age of the mother – Trisomy or monosomy of sex chromosomes – Not as detrimental as aneuploidy in autosomes  Klinefelter syndrome: extra chromosome in a male, producing XXY individuals; usually sterile; develop some female body characteristics.  Turner syndrome: Monosomy X, produces X0 females, who are sterile; it is the only known viable monosomy in humans Polyploidy • Condition in which an organism has more than two complete sets of chromosomes – Triploidy (3n) is three sets of chromosomes – Tetraploidy (4n) is four sets of chromosomes – Polyploidy is common in plants, but not animals (in fishes and amphibians) • Polyploids are more normal in appearance than aneuploids Alterations of Chromosome Structure • Due to meiosis error and/or radiation damages Disorders Caused by Structurally Altered Chromosomes • The syndrome cri du chat (“cry of the cat”), results from a specific deletion in chromosome 5 – A child born with this syndrome is mentally retarded and has a catlike cry; individuals usually die in infancy or early childhood – Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes Some inheritance patterns are exceptions to the standard chromosome theory Genomic Imprinting: • For a few mammalian traits (mainly on autosomes), the phenotype depends on which parent passes along the alleles for those traits • Involves the silencing of certain alleles during gamete production. Allele are “stamped” with an imprint (CH 3n cytosine); This causes inactivation/activation of the allele. → In the zygote, only the male or female allele is expressed → All the cells will behave the same way. • In a given species, imprinted genes are always imprinted the same way. • Aberrant imprinting leads to abnormal development and certain cancers. Inheritance of Organelle Genes • Mitochondria, chloroplasts, and other plant plastids carry small circular DNA molecules • Extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg • Some defects in mitochondrial genes prevent cells from making enough ATP and result in diseases that affect the muscular and nervous systems – Mitochondrial myopathy – Leber’s hereditary optic neuropathy Molecular Basis of Inheritance DNA is the genetic material • Used in information storage and transfer. • Nucleic acid - Polymer of nucleotide monomers consisting of • A pentose sugar: deoxyribose • Phosphate group (on carbon 5’) • Nitrogenous bases (on carbon 1’) • Purine (ring:5 + 6): Adenine and Guanine • Pyrimidine (ring: 6); Cytosine and Thymine • Made of nucleodide monomers attached via covalent phosphodiester bond between the 3’- OH and the 5’-OH of the deoxyribose. • Sugar-phosphate make up the backbone of the structure. • The polymer is directional; 5’-phosphate and 3’-OH Structure of DNA • Erwin Chargaff (1950): – DNA composition varies from one species to the next. – Amount of A = amount of T – Amount of C = amount of G • Rosalind Franklin: – produced pictures of the DNA molecule using X-ray crystallography – concluded that that there were two antiparallel sugar-phosphate backbones, with the nitrogenous bases paired in the molecule’s interior.  Watson and Crick: using Rosalind X-ray pictures of DNA and what was known about DNA chemistry determined the structure of DNA as we know it today.  Two polynucleotide chains coiled around a common axis forming a “right-handed” helix.  Two strands run antiparallel.  Strands are held together via hydrogen bonds between bases on opposite chains (weak bonds)  Stabilized by base stacking (hydrophobic)  Helix diameter: Purine pairs with pyrymidine Complementary Base pairing • Adenine always pairs with Thymine – Forms 2 hydrogen bonds – Guanine always pairs with Cytosine – Form 3 hydrogen bonds → Consistent with Chargaff’s rule (A=T and G=C) →Selective base pairing provides basis for DNA replication Human has 33% of A; how many T; C and G? Differences between DNA and RNA DNA RNA Used in storage and transmission of genetic Involved in protein synthesis information Deoxyribose Sugar Ribose Sugar Thymine Base Uracil Base Two anti-parallel Strands Single stranded (can also be double stranded) • Nucleoside: pentose + base • Nucleotide: pentose + base + phosphate (up to 3) • Pentose: ribose (RNA)/deoxyribose (DNA) • Bases: – Purine: adenine / guanine – Pyrimidine: thymine (DNA)/ uracyl (RNA)/ cytosine – Form by condensation of 3’-OH of one nucleotide and 5’-phosphate of the next nucleotide; 3’-5’-phosphodiester bond. • Molecule directional: 5’ → 3’ Many proteins work together in DNA replication and repair • Mechanism of DNA replication: – DNA is made of two complementary strands. Replication is semi-conservative: each parental strand serves as a template for synthesis of the new strand DNA replication • DNA unwinds and separates at the origin of replication; Formation of replication bubbles. – One in prokaryotes – Hundreds (thousand) in eukaryotes • Replication proceeds at the replication folks in both directions from each origin. DNA replication requires: DNA + dTTP, dATP, dGTP, dCTP → 2 x DNA + PPi – DNA template – dNTPs (deoxynucleoside triphosphate) – RNA primer (Primase + NTPs) A variety of enzymes and proteins DNA polymerase III • Catalyze the addition of nucleotides complementary to the template DNA • Cannot initiate replication • Require a RNA primer (primase enzyme) to provide 3’OH. • Formation of phosphodiester between 3’ OH of the growing chain and the 5’ phosphate of the incoming nucleotide triphosphate releasing pyrophospate that breaks down into 2 Pi (energy). • Chain grows in the 5’ to 3’ direction • DNA gyrase is a topoisomerase: relieve over twisting of the DNA helix ahead of the replication fork by breaking and reforming phosphodiester bonds. • http://www.youtube.com/watch?v=k4fbPUGKurI • Helicase: Converts double-stranded DNA into single-stranded DNA by breaking the hydrogen bonds between bases at the replication fork. • SSB: single-strand binding proteins. Bind to the single-stranded created by helicase to prevent the re-formation of double-stranded DNA. Primase • Synthesize RNA primer complementary to the DNA template • Provide the 3’-OH needed for DNA polymerase III to start replicating DNA. DNA replication – summary (1) • Helicase and topoisomerase: Unwind the DNA double helix into single-stranded DNA. • SSB protein: protect and prevent rewinding of single-stranded DNA. • Primase: Synthesize a short RNA primer (5-10 nucleotides) following DNA template (base- pairing rule) to provide the 3’-OH group for DNA polymerase III to function. • DNA polymerase III: covalently add nucleotides to the 3’-OH end of the RNA primer or pre- existing DNA strand, following the DNA template (Base pairing rule).
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