1 / 72

CHAPTER 5

CHAPTER 5. Genetics: A Review. Mendel’s Investigations. Gregor Johann Mendel (1822-1884) First person to formulate the principles of heredity: genes guide organization and sequence. Conducted breeding experiments in monastery garden from 1856 to 1864

bracha
Download Presentation

CHAPTER 5

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CHAPTER 5 Genetics: A Review

  2. Mendel’s Investigations • Gregor Johann Mendel (1822-1884) • First person to formulate the principles of heredity: genes guide organization and sequence. • Conducted breeding experiments in monastery garden from 1856 to 1864 • Discoveries were published in 1866 but remained unappreciated and forgotten until 1900

  3. Mendel’s Investigations • Mendel chose to work with pure strains of garden peas • Ex: dwarf and tall varieties • The plants were self-fertilizing but subject to experimental cross-fertilization • He studied single characteristics that displayed sharply contrasting traits

  4. Chromosomal Basis of Inheritance • Meiosis: Reduction Division of Gametes • Sex cells (gametes) transmit genetic information from parents to offspring in sexually reproducing organisms • Chromosomes occur in pairs: homologs • One member or the pair is donated by the mother, the other by the father • Homologs • Contain similar genes encoding the same set of characteristics • Usually have the same size and shape

  5. Chromosomal Basis of Inheritance • Meiosis • Special type of nuclear division • Associated with gamete production • Genetic material: doubles once (2n), divides twice • Produces 4 daughter cells • Each with only 1 member of each homologous chromosome pair or 1 set of chromosomes (haploid)

  6. Chromosomal Basis of Inheritance • Fertilization • Reestablishes the diploid chromosome number (n) • Union of egg and sperm produces a zygote (single cell) • Contains chromosomes of egg and sperm or 2 sets of chromosomes (diploid)

  7. Chromosomal Basis of Inheritance • Meiosis I Prophase I • The two members of each pair of homologs make side-by-side contact (synapsis) to form a bivalent • Bivalent composed of 2 pairs of chromatids (each pair termed a dyad) or 4 future chromosomes and is called a tetrad • Homologs cross-over and exchange genetic information

  8. Chromosomal Basis of Inheritance Metaphase I • Homologs line up side by side on the metaphase plate Anaphase I • Homologs are separated and moved to opposite poles of cell

  9. Chromosomal Basis of Inheritance Telophase I • Nuclear membrane forms around separated homologs • Each of these cells (now haploid) enter Meiosis II • No interphase between Meiosis I and Meiosis II.

  10. Chromosomal Basis of Inheritance • Meiosis II Prophase II • No crossing-over occurs Metaphase II • All chromosomes line up single file at metaphasic plate

  11. Chromosomal Basis of Inheritance Anaphase II • Centromeres of dyads are replicated and single-stranded chromosomes move toward each pole Telophase II • Nuclear membrane forms around separated chromosomes • Each daughter cell contains one complete haploid set of chromosomes

  12. Chromosomal Basis of Inheritance • Sex Determination in Humans • 46 chromosomes (23 pair) in somatic cells • Pairs 1-22: Autosomes (do not determine sex) • Pair 23: Sex Chromosomes X, Y (determine sex) • Males: XY Females: XX • Heteromorphic: do not contain the same linear sequence of genes • In birds, moths, butterflies and some fish female XY. Bugs: female XX male XO • Some animals use environmental temps to determine sex of offspring • Some fish are hermaphroditic; sensory stimulates male/female.

  13. Mendelian Laws of Inheritance • Mendel’s First Law • In the formation of gametes, paired factors that may specify alternative phenotypes (visible traits) separate so that each gamete receives only one member of the pair. • Homozygote: TT or tt • Heterozygote: Tt

  14. Mendelian Laws of Inheritance • Mendel cross-pollinated pure-line tall plants with pollen from pure-line dwarf plants • Monohybrid cross • A cross involving variation at a single locus • Results • Progeny of F1 generation: All Tall Plants • Reciprocal cross gave same results

  15. Mendelian Laws of Inheritance • Next Mendel self-fertilized the tall F1 plants • The progeny (F2 generation) produced • Tall and dwarf plants in a ration of 3:1 • Called the tall factor dominant and the short factor recessive • Used capital letter to denote dominate trait and corresponding lowercase letter to denote the recessive trait • Same results for 6 other contrasting traits in the pea plants

  16. Mendelian Laws of Inheritance • Finally, Mendel allowed plants in the F2 generation to self-fertilize: • All dwarf plants produced only dwarf plants • 1/3 of tall plants produced tall plants • 2/3 of tall plants produced tall plants and dwarf plants • Mendel’s “factors” are called genes today. • Alternate forms of a gene for the same trait are termed alleles

  17. Mendelian Laws of Inheritance • Testcross • If an allele is dominant • Heterozygous individuals have the same phenotype as homozygous dominant individuals • Impossible to determine the genotype by observing the phenotype • A testcross is performed to determine the genotype • Crossing an individual of unknown genotype with a homozygous recessive individual

  18. Mendelian Laws of Inheritance • Intermediate Inheritance • Neither allele is completely dominant over the other • The heterozygous phenotype is distinct from those of the parents, often intermediate between them

  19. Mendelian Laws of Inheritance • Mendel’s Second Law • Genes located on different pairs of homologous chromosomes assort independently during meiosis • Pertains to studies of 2 pairs of hereditary factors at the same time • Mendel performed experiments using pea strains differing by 2 or more phenotypic characters controlled by genes located on different (nonhomologous) chromosomes

  20. Mendelian Laws of Inheritance • Multiple Alleles • Individuals can have no more than 2 alleles at a given locus • Many more dissimilar alleles can exist in a population • Ex: • Human ABO blood groups • Multiple alleles arise through mutations at the same gene locus at different times

  21. Mendelian Laws of Inheritance • Gene Interaction • Many different genes can affect a single phenotype: polygenic inheritance • Pleiotropy: Genes having more than a single effect on phenotypes • Epistasis: Allele at one locus can mask or prevent the expression of an allele at another locus acting on the same trait • Quantitative inheritance: Characters show continuous variation between 2 extremes

  22. Mendelian Laws of Inheritance • Sex-Linked Inheritance • Traits specified by genes located on sex chromosomes • Designated as X – Linked or Y – Linked • X – Linked Traits • Most sex-linked traits are X-linked • Genes located on the X sex chromosome. • Ex: Red – Green Color Blindness Hemophilia

  23. Mendelian Laws of Inheritance • Autosomal Linkage • Genes on the same chromosome • Said to be linked • Tend to be inherited together • However, linkage groups may be broken up during meiosis (crossing over) •  distance between the 2 loci,  probability that alleles will be inherited together •  distance between the 2 loci,  probability that alleles will be inherited together.

  24. Mendelian Laws of Inheritance • Chromosomal Aberrations • Structural and numerical deviations from the norm that affect many genes at once • 5 out of every 1000 humans are born with serious genetic defects attributable to chromosomal anomalies • Changes in chromosome number • Euploidy • Aneuploidy

  25. Mendelian Laws of Inheritance • Euploidy • Addition or deletion of whole sets of chromosomes • Polyploidy • Most common type of euploidy • The carrying of 3 or more sets of chromosomes by an organism • More common in plants than animals

  26. Mendelian Laws of Inheritance • Aneuploidy • Usually caused by failure of chromosomes to separate during meiosis (nondisjunction) • Monosomy • 1 less chromosome relative to the diploid parental number • Trisomy • 1 extra chromosome relative to the diploid parental number

  27. Mendelian Laws of Inheritance • Changes in Chromosome Structure • Inversion • Portion of a chromosome reversed • Deletion • Entire blocks of genes lost • Translocation • Nonhomologous chromosomes exchange sections • Duplication • Extra section of chromosomes attached • Rare • Important for evolution: Supply additional genetic information that may enable new functions

  28. Gene Theory • Gene Concept • W. Johannsen coined the term “gene” in 1909 to name the hereditary factors referred to by Mendel. • GENE: Nucleotide sequence that encodes a functional polypeptide or RNA sequence

  29. Storage and Transfer of Genetic Information • Cells contain 2 kinds of nucleic acids • DNA: deoxyribonucleic acid • RNA: ribonucleic acid • DNA and RNA are polymers of nucleotides • A nucleotide composed of • Pentose sugar • Nitrogenous base • Phosphate group

  30. Storage and Transfer of Genetic Information • Nitrogenous Bases • Purines • Adenine and Guanine • Larger than pyrimidines • 2 fused rings • Pyrimidines • RNA: Cytosine and Uracil • DNA: Cytosine and Thymine • Smaller than purines • 6-membered ring

  31. Storage and Transfer of Genetic Information • In DNA • PURINES base pair with the PYRIMIDINES • GCAT • 3 hydrogen bonds between C and G • 2 hydrogen bonds between A and T RNA GCAU Uracil instead of Thymine

  32. Storage and Transfer of Genetic Information • DNA is a double stranded molecule. • Strands are joined by hydrogen bonding between the bases. • Strands are complementary • The base sequence in one strand determines the base sequence in the other.

  33. Storage and Transfer of Genetic Information • DNA synthesis occurs in the 5' to 3' direction in both strands. • The DNA strands are antiparallel • 5' end of one is associated with the 3' end of the other. • The DNA ladder is twisted into a double helix • Ten base pairs occur per turn.

  34. Storage and Transfer of Genetic Information • DNA Replication (3 steps) Step 1 • Helicase enzyme • Separates the DNA strands • Each parent strand serves as a template for synthesis of a complementary strand

  35. Storage and Transfer of Genetic Information • Step 2 • DNA Polymerase enzyme • Catalyzes assembly of new strand • Synthesis occurs in 5' to 3' direction only • Since parent DNA strands are antiparallel • One strand is synthesized continuously • Leading strand • One strand is synthesized in fragments, each in the 5' to 3' direction • Lagging strand • Step 3 DNA Ligase enzyme Joins fragments of the lagging strand

More Related