Bio notes 3 - instructor: Tyler Hodges PDF

Preview

Summary

instructor: Tyler Hodges...

Description

CHAPTER 13: MEIOSIS AND SEXUAL LIFE CYCLES Heredity—the transmission of traits from one generation to the next Variation—the differences in appearance that offspring show from parents and siblings Genetics—the scientific study of heredity and variation Genes—units of heredity; made of segments of DNA (DNA is packaged into chromosomes) Somatic cells—body cells (46 chromosomes; 23 pairs) Gametes—reproductive cells (sperm and egg) Locus—gene’s specific position along a chromosome Asexual reproduction—single individual passes all of its genes to offspring Clone—group of genetically identical individuals from the same parent Sexual reproduction—two parents make offspring with unique combination of genes from both parents Karyotype—ordered display of the chromosome pairs from a cell Homologous chromosomes (homologs)—two chromosomes in each pair 

One from mom and one from dad



The pair are same length, same shape, and carry genes controlling the same inherited characters

SEX CHROMOSOMES Determine the sex of the individual (X and Y) Females—XX males—XY Autosomes—the remaining 22 pairs of chromosomes (23 in all; one from each parent gives 46) 

Each pair of homologous chromosomes includes one chromosome from each parent

Diploid cell—(2n) has two sets of chromosomes (2n=46) Haploid cell—(n) single set of chromosomes (gametes are haploid) (n=23) 

egg gives X, sperm gives X or Y

BEHAVIOR OF CHROMOSOME SETS Fertilization—the union of gametes (sperm and egg) 

zygote—fertilized egg o has one set of chromosomes from each parent o produces somatic cells by mitosis

gametes are the only types of human cells produced by meiosis fertilization and meiosis alternate in sexual life cycles to maintain chromosome number THREE TYPES OF SEXUAL LIFE CYCLES Haploid or diploid cells can divide by mitosis depending on the type of life cycle (only diploid cells can undergo meiosis) The halving and doubling of chromosomes contribute to genetic variation in offspring MEIOSIS Meiosis is preceded by the replication of chromosomes  

results in four daughter cells (haploid) each one has only half as many chromosomes as the parent cell

chromosomes duplicate during interphase I (never interphase II) sister chromatid cohesion—sister chromatids are closely associated in length MEIOSIS I Prophase I—each chromosome pairs with its homolog and crossing over occurs 

chiasmata—sites of crossover; hold homologs together as the spindle forms for the first meiotic division

Metaphase I—pairs of homologs line up at metaphase plate (middle), with one chromosome facing each pole 

microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad



microtubules from other pole are attached to the kinetochore of the other chromosome

Anaphase I—pairs of homologous chromosomes separate  

one chromosome of each pair moves toward opposite poles, guided by the spindle apparatus sister chromatids remain attached at the centromere

Telophase I and cytokinesis—cleavage furrow or cell plate forms, and cell splits, forming two haploid daughter cells 

no chromosome replication occurs between meiosis I and meiosis II

MEIOSIS II Prophase II—spindle apparatus forms, chromosomes move toward metaphase plate Metaphase II—sister chromatids are arranged at metaphase plate; kinetochores of sister chromatids attach to microtubules extending from opposite poles 

because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical

Anaphase II—sister chromatids separate, move as two individual chromosomes toward opposite poles Telophase II and cytokinesis—chromosomes arrive at opposite poles, nuclei form, chromosomes begin decondensing, cells separate  

at the end, there are four daughter cells, each with a haploid set of unreplicated chromosomes each daughter cell is genetically distinct from the others and parent cell

CROSSING OVER AND SYNAPSIS DURING PROPHASE I Cohesins—proteins holding together sister chromatids after interphase Crossing over occurs between nonsister chromatids Synaptonemal complex—zipper-like structure that holds the homologs together tightly DNA breaks are repaired, joining DNA from one nonsister chromatid to the corresponding segment of another THREE EVENTS UNIQUE TO MEIOSIS 

Synapsis and crossing over in prophase I

 

Homologous pairs at the metaphase plate Separation of homologs during anaphase I

Sister chromatid cohesion allows them to stay together through meiosis I  

In mitosis—cleaved at the end of metaphase In meiosis—cleaved in anaphase I (separation of homologs) and in anaphase II (separation of sister chromatids)

Meiosis 1 reduces the number of chromosomes per cell GENETIC VARIATION Mechanisms that contribute to genetic variation: 

 

Independent assortment of chromosomes o 2n—the number of combinations possible when chromosomes assort independently into gametes (n is haploid number)  For humans, n=23, 223 possible combinations of chromosomes Crossing over o Produces recombinant chromosomes—combine DNA inherited from each parent Random fertilization

CHAPTER 14: MENDEL AND THE GENE IDEA Mendel’s experimental, quantitative approach      

Character—heritable feature that varies among individuals (such as flower color) Trait—each variant for a character (purple or white flower color) Advantage of using peas: short generation time, large numbers of offspring, mating could be controlled Mendel tracked only those characters that occurred in two distinct alternative forms Used true-breeding—plants that produce offspring of the same variety when they selfpollinate Hybridization—mating two contrasting, true-breeding varieties o P generation—the true-breeding parents o F1 generation—the hybrid offspring of the P generation o F2 generation—F1 self-pollinate or cross pollinate with other F1 hybrids

The Law of Segregation 

Ration of about 3:1, purple to white flowers in the F2 generation o Purple color was dominant, white color was recessive o The factor for white flowers reappeared in the F2 generation o What Mendel called a “heritable factor” is now called a gene

Mendel’s Model 

Four related concepts make up the 3:1 inheritance pattern

o o

o

o

First: Alternative versions of genes (alleles) account for variations in inherited characters (gene exists in one for purple and one for white) Second: for each character, an organism inherits two alleles, one from each parent  the two alleles at a locus may be identical (P generation) or different (F1 generation) third: if the two alleles at a locus differ, then one (dominant) determines the organism’s appearance, and the other (recessive) has no noticeable effect on appearance fourth: (law of segregation) the two alleles for one gene for a heritable character separate during gamete formation and end up in different gametes  an egg or a sperm gets only one of the two alleles that are present in the organism (the distribution of homologous chromosomes to different gametes)

Punnett square Possible combinations of sperm and egg Capital letter (dominant), and lowercase letter (recessive) Genetics       

homozygous—two identical alleles for a character (AA or aa) heterozygous—two different alleles for a character (Aa) o not true breeding because of the different effects of dominant and recessive alleles, traits do not always reveal its genetic composition phenotype—physical appearance genotype—genetic makeup (PP and Pp plants have the same phenotype but different genotypes) testcross—breeding the mystery individual with a homozygous recessive individual

LAW OF INDEPENDENT ASSORTMENT 

law of segregation follows a single character (monohybrids— heterozygous for one character)







 

o monohybrid cross—crossing between these heterozygotes second law of inheritance—two characters at the same time o dihybrid—crossing two true-breeding parents differing in two characters; shows in F1 generation o dihybrid cross—between F1 dihybrids (can determine whether two characters are transmitted to offspring as a package of independently Law of Independent Assortment—each pair of alleles segregates independently of each other pair of alleles during gamete formation o Applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome  Genes located near each other on the same chromosome tend to be inherited together Multiplication rule—the probability that two or more independent events will occur together is the product off their individual probabilities o Segregation in a heterozygous plant—each gamete has a ½ change of carrying the dominant allele and ½ change of carrying the recessive allele (½ * ½ = ¼) Addition rule—used to determine probability of heterozygote o ¼ + ¼ = ½ probability of heterozygote Aa Use the multiplication and addition rules to predict the outcome of crosses involving multiple characters o Each character is considered separately, and the individual probabilities are multiplied (Forked Line Method)

EXTENDING MENDELIAN GENETICS 

Inheritance of characters by a single gene may deviate from simple mendelian patterns in the following situations: o When alleles are not completely dominant or recessive o When a gene has more than two alleles o When a gene produces multiple phenotype

DEGREES OF DOMINANCE   



Complete dominance—phenotypes of the heterozygote and dominant homozygote are identical (both purple) Incomplete dominance—phenotype of F1 hybrids is a mix of phenotypes of the two parents Codominance—two dominant alleles affect the phenotype in separate, distinguishable ways (both purple and white present) A dominant allele does not subdue a recessive allele



Dominant alleles are not necessarily more common than recessive alleles

OTHER STUFF  

 

 

Most genes exist in populations in more than two allelic forms Pleiotropy—genes with multiple phenotypic effects o These alleles are responsible for the multiple symptoms of certain diseases, such as cystic fibrosis paired with sickle-cell Epistasis—a gene at one locus alters the phenotypic expression of a gene at a second locus o Ex: coat color in Labrador retrievers is determined by two genes Quantitative characters—vary in the population along a continuum o Usually indicates polygenic inheritance—an additive effect of two or more genes on a single phenotype (skin color in humans) Multifactorial—traits that depend on multiple genes combined with environmental influences Caveat—humans are not good subjects for genetic research (generation time is too long, parents produce few offspring, breeding experiments are unacceptable

PEDIGREE ANALYSIS   

Pedigree—a family tree that describes the interrelationships of parents and children across generations Inheritance patterns can be traced and used to make predictions about future offspring Can use the multiplication and addition rules to predict the probability of specific phenotypes

AUTOSOMAL RECESSIVE TRAITS     

Only show up in individuals homozygous for the allele Carriers—heterozygous individuals who carry the recessive allele but are phenotypically normal; most individuals with recessive disorders are born to carrier parents Consanguineous mating— (mating between close relatives) increase the chance of mating between two characters of the same rare allele Cystic fibrosis—most common lethal genetic disease; results in defective or absent chloride transport channels in plasma membranes leading to a buildup of chloride ions outside the cell Sickle-cell—caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells

o o o

In homozygous individuals, all hemoglobin is abnormal (sickle-cell Heterozygotes are usually healthy but may suffer some symptoms Heterozygotes are less susceptible to the malaria parasite (advantage)

AUTOSOMAL DOMINANT TRAITS   

Dominant alleles that cause a lethal disease are rare and arise by mutation Achondroplasia—form of dwarfism caused by a rare dominant allele Huntington’s disease—degenerative disease of the nervous system o Has no phenotypic effects until 35-40 years of age o Once the deterioration of the nervous system begins the condition is irreversible and fatal

MULTIFACTORIAL DISORDERS   

Heart disease, diabetes, alcoholism, mental illnesses, and cancer have both genetic and environmental components No matter the genotype, our lifestyle has a tremendous effect on phenotype Genetic counselors—provide info to prospective parents concerned about specific diseases in a family’s history o Help couples determine the odds that their children will have genetic disorders o The genotype of one child is unaffected by the genotypes of older siblings

CHAPTER 15: THE CHROMOSOMAL BASIS OF INHERITANCE   

Mendel’s “hereditary factors” (genes) were purely abstract when first proposed Sutton and Boveri and others independently noted the parallels and the chromosome theory of inheritance began to form Thomas Hunt Morgan notes wild type, or normal (natural), phenotypes that were common in the fly populations o Traits alternative to the wild type are called mutant phenotypes

CORRELATING BEHAVIOR OF A GENE’S ALLELES WITH BEHAVIOR OF A CHROMOSOME PAIR 

Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)



o F1 generation all had red eyes o F2 generation had a 3: 1 red to white eye ratio, but only males had white eyes The white-eyed mutant allele must be located on the X chromosome o Supports the chromosome theory of inheritance o Discovered that traits can correlate with sex

THE CHROMOSOMAL BASIS OF SEX    

Two varieties of sex chromosomes: larger X chromosome and smaller Y chromosome Short segments at the ends of the Y chromosomes are homologous with the X, allowing the two to behave like homologues during meiosis in males Sex-linked gene—gene that is located on either sex chromosome X chromosomes have genes for many characters unrelated to sex, whereas most Y-linked genes are related to sex determination

INHERITANCE OF X-LINKED GENES    



Follow specific patterns of inheritance For a recessive X-linked trait to be expressed, a female needs two copies of the allele (homozygous), a male needs only one copy of the allele (hemizygous) X-linked recessive disorders are much more common in males than in females Some recessive disorders on the X chromosome o Color blindness (mostly X-linked) o Duchenne muscular dystrophy hemophilia In females, one of the X chromosomes in each cell is randomly inactivated during embryonic development o The inactive X condenses in a Barr body o If female is heterozygous for a gene on the X chromosome, she will be a mosaic for that character

LINKED GENES  

Tend to be inherited together because they are located near each other on the same chromosome Morgan did fruit fly experiments to see how linkage affects inheritance of two characters o Crossed flies that differed in traits of body color and wing size o Found that body color and wing size are usually inherited together in specific combinations (parental phenotypes) o These genes do not assort independently, and reasoned that they were on the same chromosome

GENETIC RECOMBINATION AND LINKAGE    

Nonparental phenotypes were also produced This result involves exploring genetic recombination, the production of offspring with combinations of traits differing from either parent Genes can be linked, but the linkage was incomplete, because some recombinant phenotypes were observed Crossing over of homologous chromosomes—process that occasionally breaks the physical connection between genes on the same chromosome

VARIATION FOR NATURAL SELECTION   

Recombinant chromosomes bring alleles together in new combinations in gametes Random fertilization increases even further the number of variant combinations that can be produced This abundance of genetic variation is the raw material upon which natural selection works

ABNORMAL CHROMOSOME NUMBER 





 

Nondisjunction—pairs of homologous chromosomes do not separate normally during meiosis o One gamete receives two of the same type of chromosome, and another gamete receives no copy Aneuploidy—the fertilization of gametes in which nondisjunction occurred o Have an abnormal number of a particular chromosome o XXX females are healthy, with no unusual features o Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals Monosomic zygote—has only one copy of a particular chromosome o Monosomy X (turner syndrome), produces X0 females, who are sterile; it is the only known viable monosomy in humans Trisomic zygote—three copies of a particular chromosome (down syndrome: trisomy 21) Polyploidy—organism has more than two complete sets of chromosomes (common in plants, normal in appearance) o Triploidy—(3n) three sets of chromosomes o Tetraploidy—((4n) four sets

ALTERATIONS OF CHROMOSOME STRUCTURE 

Breakage of a chromosome can lead to four types of changes in chromosome structure o Deletion—removes a chromosomal segment o Duplication— repeats a segment o Inversion—reverses orientation of a segment within a chromosome o Translocation—moves a segment from one chromosome to another

DOWN SYNDROME (TRISOMY 21) 

Aneuploid condition that results from three copies of chromosome 21



Frequency of down syndrome increases with the age of the mother, a correlation that has not been explained

INHERITANCE OF ORGANELLE GENES  

Extranuclear genes (cytoplasmic genes) are in organelles in the cytoplasm Inherited maternally because the zygote’s cytoplasm comes from the egg

CHAPTER 16: THE MOLECULAR BASIS OF INHERITANCE James Watson and Francis Crick introduced double-helix for DNA Two components of chromosomes: DNA and protein Frederick Griffith—worked with two strains of a bacterium; pathogenic “S” and harmless “R” to discover the genetic role of DNA     

Mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain, some living cells became pathogenic Called it transformation: a change in genotype and phenotype due to assimilation of foreign DNA Determined that only DNA worked in transforming harmless bacteria into pathogenic bacteria Number of A=T; number of G=C Rosalind Franklin develop first pic of DNA using X-ray crystallography

DNA STRUCTURE  

Sugar-phosphate backbones run in opposite 5’ to 3’ directions from each other (antiparallel) The nitrogenous bases in DNA form hydrogen bonds in a complementary fashion: A with T, G with C

DNA SYNTHESIS    

The two strands are complementary and antiparallel, each strand is a template for building a new strand in replication Begins at origins of replication—where the two DNA strands are separated, opening up a replication “bubble” Bacteria typically have only a single origin of replication per circular chromosome DNA polymerase—enzymes that catalyze synthesis with assistance from other proteins that serve to configure parental DNA strands so that they can serve as templates for synthesis of new “daughter” DNA strands o C...