Chapters 10 & 11

1. Professor E. U. Gene Chapters 10 & 11 Mendel, DNA & Genes

2. Gregor Mendel • Austrian monk, gardener, scientist • First acknowledged to study heredity – the passing on of characteristics from parents to offspring • Traits – characteristics that are inherited • Father of genetics – the branch of biology that studies heredity

3. The Peas • Mendel chose garden peas • They reproduce sexually, so they have both male and female sex cells, called gametes • Pollen and egg unite in a process called fertilization • Fertilization results in a fertilized cell, called a zygote, that develops into a seed

4. The Peas Mate • Pollination is the transfer of pollen grains from a male reproductive organ to a female productive organ • Both male and female organs are close together in the same pea flower • As a result, peas normally self-pollinate • This is what Mendel wanted in most cases • Mendel also removed the male organ and dusted pollen on flower of another plant • Called cross-pollination

5. Mendel’s Peas • Mendel’s peas had been self-pollinating for a long time • This meant that the tall ones had been tall for a long time and the short ones had been short for generations • Called purebreds

6. Mendel’s Monohybrid Crosses • Mendel performed cross-pollination with a tall pea plant (6 foot purebred) and a short pea plant (2 foot purebred) – these are the parental generation (P1) • Hybrid – offspring of parents that have different forms of a trait • All of the offspring grew as tall as the tall parent – first filial generation (F1) • The “short trait” seemed to disappear

7. Monohybrid Crosses • Mendel let his F1 plants self pollinate • Second filial generation (F2) • He counted over 1000 plants • About 75% were tall • About 25% were short • Ratio of 3:1

8. Monohybrid Crosses • Mendel did these same types of crosses for 7 traits • Seed shape: round vs. wrinkled • Seed color: yellow vs. green • Flower color: purple vs. white • Flower position: axial vs. terminal • Pod color: green vs. yellow • Pod shape: inflated vs. constricted • Plant height: tall vs. short • back

9. The Rule of Unit Factors • Mendel concluded that each organism has two factors that control each of its traits • We now know these factors are genes and that they are located on chromosomes • Alternate forms of genes are called alleles • Each of Mendel’s traits had two forms of genes – two alleles • One comes from mother, one from father

10. The Rule of Dominance • Some alleles are dominant over recessive alleles • Dominant alleles cover recessive alleles • In genetics, capital letters are used to express dominant alleles and lower case letters are used to express recessive alleles • Ex: “T” for tall allele, “t” for short allele • So what will a “TT” plant look like? • A Tt plant? • A tt plant? • Earlobes, Forelock, Dimples, Straight thumb, Bent Pinky, Mid-digit hair

11. The Law of Segregation • Mendel’s first law of heredity • Every individual has two alleles of each gene and when gametes are produced, each gamete receives one of these alleles • So how did this work in Mendel’s F2 generation ?

12. Phenotypes • Two organisms can look alike on the outside, but have different allele combinations • Phenotype is the way an organism looks and behaves • ex: yellow seeds can be TT or Tt

13. Genotypes • The allele combination an organism contains is known as its genotype • You can’t always see this because of dominiance • TT and Tt are different genotypes • An organism is homozygous for a trait if the two alleles for the trait are the same • Ex: TT or tt • An organism is heterozygous for a trait if the two alleles for the trait are different • Ex: Tt

14. Mendel’s Dihybrid Crosses • Mendel crossed peas that differed from each other in two traits • He crossed plants that were homozygous for round yellow seeds (RRYY) with plants that were homozogous for green wrinkled seeds (rryy) • F1 generation • All plants produced round, yellow seeds • What was dominant?

15. The Second Generation (F2) • Let F1 plants self-pollinate (were heterozygous for two traits) • He got all four combinations in a certain ratio • Round, yellow – 9 • Round, green – 3 • Wrinkled, yellow – 3 • Wrinkled, green - 1

16. The Law of Independent Assortment • Mendel’s second law of heredity • Genes for different traits are inherited independently of each other • Inheritance of one trait has no influence on another trait • Instead of a ratio of 9:3:3:1, what would the dihybrid cross have looked like?

17. Punnett Squares • 1905, Reginald Punnet, English biologist created a shorthand way of finding EXPECTED proportions of possible genotypes in the offspring of a cross • Monohybrid crosses • See overhead • Dihybrid crosses • See overhead

18. Probability • Reality is rarely like a Punnett square • When you toss a coin, what’s the likelihood it will be heads? • You toss 20 heads in a row, what’s the likelihood the next will be heads? • TOSS COINS • ASSIGNMENT • P. 262, Problem-Solving Lab • P. 276, Connection to Math

19. Section 1 Review • What structural features of pea plants made them suitable for Mendel’s genetic studies? • What are genotypes of a homozygous and a heterozygous tall pea plant? • One parent is homozygous tall and the other is heterozygous. How many offspring will be heterozygous? • How many different gametes can an RRYy parent form? What are they? • What is the law of segregation? • What is the law of independent assortment? • What is the rule of dominance? • In garden peas, the allele for yellow peas is dominant to the allele for green peas. Suppose you have a plant that produces yellow peas, but you don’t know whether it is homozygous dominant or heterozygous. What experiment could you do to find out?

21. Genetic Variation • Crossing over during meiosis provides variability • How many different kinds of gametes can a pea plant produce? • Each cell has 7 pairs of chromosomes • Each can line up at the equator in two different ways and separate by segregation • 2n = 27 = 128 possible combinations without crossing over • When you include the egg, 128 x 128 = 16,384 different combinations of offspring

22. Genetic Variation • In humans, how many possible combinations are there in a single sperm or egg? • 223 = 8,388,608 combinations • How many possible combinations with fertilization • 8,388,608 x 8,388,608 = 7.04 x 1013 (over 70 trillion)

23. Genetic Recombination • With crossing over, additional variation is added providing an almost endless amount of variation possible • This reassortment of chromosomes and the genetic information they carry, either by crossing over or by independent segregation of homologous chromosomes is called genetic recombination • Variation is the raw material that forms the basis for evolution to act on

24. Nondisjunction • Go back to anaphase I • The failure of homologous chromosomes to separate properly during meiosis is called nondisjunction • In one type of nondisjunction, two kinds of gametes result • One with an extra chromosome (trisomy) • Trisomy 21 – Down syndrome • One missing a chromosome (monosomy, usually don’t survive) • One exception – Turner syndrome, female is XO

25. Nondisjunction • The other type of nondisjunction involves a total lack of separation of homologous chromosomes • Gamete inherits a complete diploid set • Zygote has polyploidy • Not uncommon in plants – often larger and healthier • chrysanthemum (tetraploid, 4n) • Wheat (hexaploid, 6n) • Apples (3n) • There are even chemicals that help plant breeders do this artificially

26. Gene Linkage • Genes that are close together on a chromosome are often inherited together • Crossing over rarely works for just one gene • These genes are said to be linked • So chromosomes, not genes follow Mendel’s independent assortment

27. Chromosome Mapping • Crossing over occurs • Geneticists use the frequency of crossing over to map the relative position of genes on a chromosome • Genes that are further apart are more likely to have crossing over occur

28. Chromosome Mapping • Suppose there are 4 genes on a chromosome – A, B, C, D • Frequencies of recombination as follows: • Between A & B: 50% (50 map units) • Between A & D: 10% (10 map units) • Between B & C: 5% (5 map units) • Between C & D: 35% (35 map units) • These give a relative distance between genes • A -10 units- D -35units- C -5 units- B (whole thing is 50 units)

30. Section 2 Review • How are the cells at the end of meiosis different from the cells at the beginning of meiosis? • What is the significance of meiosis to sexual reproduction? • Why are there so many varied phenotypes within a species such as humans? • How does meiosis support Mendel’s law of independent assortment? • What is crossing over? • How can you use crossing over to map a chromosome? • What are linked genes?

31. What is DNA? • DNA contains the complete instructions for manufacturing all the proteins for an organism • DNA achieves its control by determining the structure of proteins • The moral - DNA codes for PROTEINS • Let’s start with an animated tour

32. Alfred Hershey & Martha Chase • For a long time, many scientists believed protein was the genetic material • Protein is more complex than DNA • 1952, Hershey & Chase experimented with radioactively labeled viruses (bacteriophages) made only of protein & DNA • DNA was labeled with one isotope • Protein with a different isotope

33. Hershey & Chase • Bactriophages inject genetic material into bacteria, causing the bacteria to create more bacteriophages • Hershey & Chase “followed” the radioactively labeled viruses • Discovered that DNA was the genetic material being injected into the bacteria

34. Structure of Nucleotides • DNA is a polymer of nucleotides • Nucleotides have 3 parts • A simple sugar (deoxyribose in DNA) • A phosphate group • A nitrogenous base • 4 possible nitrogenous bases in DNA • Adenine (A) • Thymine (T) • Guanine (G) • Cytosine (C)

35. Watson & Crick • In 1953, published a paper proposing that DNA is made of two chains of nucleotides held together by nitrogenous bases • Hydrogen bonds hold strands together (weak) • A only bonds with T, C only bonds with G • DNA is a double helix • Also correctly described now DNA replicates • Won the Nobel Prize

36. DNA- Common Thread • DNA is the same among ALL organisms with DNA • The differences come with the different sequences of the four bases • Like different words meaning different things, even though the letters are the same • The more alike the sequences, the more related he organisms are

37. Replication of DNA • Without replication, new cells (after mitosis) would only have half the DNA of their parents • All organisms undergo DNA replicaton

38. How DNA Replicates • Due to complementary base pairing, “knowing” the sequence of one strand helps you predict what the other will look like • Replication begins when an enzyme breaks the hydrogen bonds between bases • Called “unzipping” the DNA • Cells have stockpiles of nucleotides and these bond with the newly exposed bases • Done by a set of enzymes called DNA polymerases

39. How DNA Replicates • Each DNA strand has 2 ends, labeled the 5’ and and the 3’ end. • Replication proceeds in the 5’ 3’ direction • Each new strand formed is a complement of one of the parents strands • Half old, Half new - called semiconservative replication • Genetic continuity is maintained

40. Section 1 Review • Describe the structure of a nucleotide. • How do the nucleotides in DNA bond with each other within a strand? How do they bond with each other across strands? • Explain why the structure of a DNA molecule is often described as a zipper. • How does DNA hold information? • The sequence of nitrogenous bases on one strand of a DNA molecule is GGCAGTTCATGC. What would be the sequence of bases on the complementary strand? • In general, describe the steps that occur during DNA replication.

41. Genes & Proteins • The information in DNA is put to work through the production of proteins • Some proteins become important structures • Other proteins (enzymes) control chemical reactions • Remember that proteins are polymers of amino acids • The sequence of nucleotides determines the string of amino acids and the protein

42. RNA • RNA differs from DNA in 3 ways: • RNA is single stranded (DNA is double) • The sugar in RNA is ribose (DNA is deoxyribose) • Instead of thymine, RNA contains the base uracil • 3 types of RNA • Messenger RNA (mRNA) • rRNA • tRNA

43. Transcription • In the nucleus (if cell is eukaryotic), enzymes make an RNA copy of a portion of a DNA strand • This process is called transcription • Begins as enzymes unzip the molecule of DNA in region of gene being transcribed • RNA polymerases pair free nucleotides with their complementary strands • The mRNA strand breaks away

44. RNA Processing • Genes usually contain long non-coding sequences of bases, called introns • Regions that contain coding sequences are called exons • Enzymes in the nucleus cut out the intron segments from the mRNA strand and then paste it back together

45. Genetic Code • A code is needed to convert the language of mRNA (4 letters) into the language of proteins (20 amino acids) • How big do the words have to be?????? • A codon is a group of 3 nitrogenous bases in mRNA that codes for an amino acid • 1st one found was UUU - coding for phenalynine • 64 possible codons, so some amino acids correspond to multiple codons

46. Genetic Code • Some codons don’t code for an amino acid - they code for “instructions” • UAG - codon for “stop” • AUG is the “start” codon as well as coding for methionine • The code is universal • Used in ALL organisms the same

47. Translation • Translation is the process of converting the information in a sequence of nitrogenous bases in mRNA into a sequence of amino acids in a protein • Takes place at the ribosomes in the cytoplasm

48. Transfer RNA • Transfer RNA (tRNA) molecules bring the amino acids (which are in the cytoplasm) to the ribosome • Each tRNA molecule attaches to only one type of amino acid • There is a sequence of 3 nucleotides on the opposite side of the tRNA molecule from the amino-acid attachment site • It is complementary to a specific codon, thus it is called an anti-codon

49. At the Ribosome • tRNA attaches to mRNA at the ribosome • mRNA strand “slides along” and another tRNA attaches at second bonding site • two amino acids bond together with a peptide bond • mRNA “slides along” again, allowing another tRNA to come on in • stops at certain codon and is released