Variation Genetic April 27, 2006 DNA Deoxyribonucleic acid Blueprint of life Double stranded molecule Ladder, double helix Each rung: Nucleotide Nucleotide Composed of one sugar (deoxyribose) one phospate, one nitrogenous base 4 types of nitrogenous bases: Adenine (A) Guanine (G) Thymine (T) Cytosine (C) Follows rules of complimentary base pairing Adenine only with thymine (AT) Cytosine only with guanine (CG) Base pairs held together by hydrogen bond Complimentary Base Pairing Bases on one strand are always complementary (matched) to bases on other strand A T C G G C T A Genetic information is encoded in the sequence of base pairs Genes
Certain segments of DNA function as genes Genes: direct the synthesis of a specific protein that can play either a structural or functional role in the cell Gene is expressed with protein it codes for is produced Protein produced by RNA: ribonucleic acid Backbone made of ribose instead of deoxyribose RNA DNA issues instructions to the cell via RNA RNA carries the information from DNA to the rest of the cell and directs synthesis of protein A small portion of DNA strand "unzips" RNA forms complimentary bases Bases in RNA different from DNA DNA: AdenineThymine, GuanineCytosine RNA: AdenineUracil, GuanineCytosine DNA: deoxyribose and phosphorus in backbone RNA: ribose and phosphorus makes up backbone AdenineThymine GuanineCytosine AdenineUracil GuanineCytosine Process of RNA getting the message from DNA is called transcription RNA then moves into cell and undergoes translation: cell translates RNA message into protein Replication DNA is passed from one generation to the next DNA must be copied
Enzyme breaks H bonds "unzipping" DNA strand Freefloating nucleotides attach to open DNA strands DNA polymerases link new nucleotides together and form new strand DNA in the Cells DNA packaged into chromosomes Combination of DNA and specialized proteins: histones Genes Genes arranged in fixed sequence along a specific chromosome Point on a chromosome where a particular gene is found: locus (pl: loci) Cell Division Each somatic cell in the body has 23 pairs of chromosomes Somatic cell: all cells except for eggs or sperm Somatic cells called diploid: they have 2 copies of each chromosome Cells with only one copy called haploid Cell Division Somatic cells divide growth, repair Each division yields an exact copy of the mother cell Divide via a process called mitosis Mitosis one phase in life
cycle of the cell Cell Cycle Cell cycle lasts 1624 hours Interphase: period of time between cell division Time of growth and function Lasts 2223 hours in actively dividing cells Some adult cells never divide Mitosis: time of active division Lasts 12 hours Mitosis Interphase: DNA duplicated Prophase: nuclear envelope breaks down, chromosomes condense, spindle fibers form Metaphase: Chromosomes line up along equatorial plate Anaphase: Chromosomes move towards opposite poles of cell Telophase: Nuclear envelope forms around chromosomes, Cytokinesis occurs Meiosis Mitosis forms 2 diploid cells Meiosis forms 4 haploid cells 2 cell divisions: Meiosis 1: Cell duplicates DNA and divides into 2 daughter cells Meiosis II: Sister chromatids separate and form haploid cells Recombination During process of reproduction, genes are shuffled around Recombination or crossing over: occurs
during meiosis I Pieces of chromatids are exchanged Daughter cells are genetically dissimilar Inheritance Genes passed from parent to offspring One haploid cell from father joins with one haploid cell from mother, creating a diploid cell Homologous pair of chromosomes Developing fetus has 2 copies of each Different gene forms of each gene: alleles Alleles produce different forms of the traits they determine Inheritance Genes direct synthesis of specific proteins Proteins influence the structure or action of the body's cells The genedetermined protein can influence whether a certain trait (characteristic) will develop Eye color, hair color, height, etc. One allele usually dominant over the other The dominant allele is expressed Alleles Individuals with 2 copies of the same allele of a gene are homozygous for that trait Individuals with different alleles for a gene are heterozygous for that trait Only one allele is expressed (shown) The allele that is expressed is called the dominant allele The allele whose effects are masked in a heterozygous condition is called a recessive allele
Recessive alleles are only expressed when the gene is homozygous for the recessive allele Each allele is given a letter depending on what it codes for If the allele is dominant, it gets a capital letter If it is recessive, it gets a lowercase letter Dark hairD Light haird If girl has dark hair, she either has one dominant allele and one recessive allele, or two dominant alleles: Dd or DD Genotype vs. Phenotype We observe the dominant trait whether person is homozygous dominant (TT) or heterozygous (Tt) for that trait May need to distinguish between persons genetic makeup and what is expressed Genotype: the precise alleles that are present If person is homozygous or heterozygous=genotype Phenotype: the observable physical traits of an individual Phenotype: Dark hair, no freckles Phenotype: Red hair, freckles Genotype: Dark hair dominant, no freckles recessive Hair: HH or Hh Freckles: ff Genotype: Red hair recessive, freckles dominant
Hair: hh Freckles: FF or Ff Punnett Square If you know the genotype of parents, you can figure out the possible genotypes and phenotypes of their children Phenotype: Brown hair Genotype: Bb Phenotype: Blonde hair Genotype: bb Codominance Codominance occurs when both alleles in a heterozygote are expressed Example: Blood type Blood type expressed by a gene denoted as "I" or "i" 3 alleles: i codes for type O (recessive), IA codes for type A (dominant), IB codes for type B (dominant) Genotype ii = type O blood Genotype IAi or IA IA = type A blood Genotype IBi or IBIB = type B blood Genotype IBIA = type AB blood Deletion A portion of the chromosome is lost All associated genes lost as well Called deletion Can happen during recombination, breakage, improper replication Loss of tip of chromosome 5 causes Criduchat
Down Syndrome Sometimes a haploid cell will end up with two copies of a particular chromosome If that cell is involved in fertilization, then there could be a problem Down Syndrome is when there are 3 copies of chromosome 21 Skeletal dysplasia or Dwarfism Most commonly caused by a mutation to or the expression of a gene on chromosome 4 Causes short stature in adults (under 4'10'' at full height) Can have problems with bones and joints Hermaphrodites Properly called "intersexual person" Person with both male and female reproductive organs Approx. 1% of live births exhibit some degree of sexual ambiguity Usually caused by incorrect hormone levels or inability to respond to hormones Albinism Governed by a recessive gene Person does not produce any melanin (stuff in skin, hair and eyes that gives tissues color) Angelman Syndrome Caused by deletion of segment of chromosome 15found in 1 in 25,000
Symptoms: Feeding problems, epilepsy, delay in sitting and speaking, affectionate nature and frequent laughter, light hair and blue eyes, wide stifflegged gait, tremors or jerky movements