Genes, Chromosomes and DNA

1. Genes, Chromosomes and DNA (Chapter 2)

2. The structure of DNA • Composed of 4 nucleotide bases, 5 carbon sugar and phosphate. • Base pair = rungs of a ladder. • Edges = sugar-phosphate backbone. • Double Helix • Anti-Parallel

3. Figure 2.21 The structure of DNA

4. DNA Replication • Adenine (A) always base pairs with thymine (T) • Guanine (G) always base pairs with Cytosine (C) • ALL Down to HYDROGEN Bonding • Requires steps: • H bonds break as enzymes unwind molecule • New nucleotides (always in nucleus) fit into place beside old strand in a process called Complementary Base Pairing.

5. Figure 2.22a DNA Replication Remember – the two strands run in opposite directions Synthesis of a new (daughter) strand occurs in the opposite direction of the old (parental) strand. Complementary base-pairing occurs A with T and G with C G and C have three hydrogen bonds A and T have two hydrogen bonds

6. Figure 2.22b DNA Replication

7. Figure 2.22c DNA Replication New nucleotides joined together by enzyme DNA Polymerase

8. DNA Replication • Each new double helix is composed of an old (parental) strand and a new (daughter) strand. • As each strand acts as a template, process is called Semi-conservative Replication. • Replication errors can occur. Cell has repair enzymes that usually fix problem. An error that persists is a mutation. • This is permanent, and alters the phenotype.

9. Gregor Mendel observed phenotypes and formed hypotheses • How do offspring come to resemble their parents physically? • Genetics begins with the unifying assumption that biological inheritance is carried by structures called Genes. • The same basic patterns of inheritance apply to most organisms. • The inheritance of some human traits can be explained from work on plants • Sex-linked traits in humans is more complicated

10. Gregor Mendel • Was the first person to analyze patterns of inheritance • Deduced the fundamental principles of genetics

11. Phenotype Terms: • An organism’s physical traits • Genotype • An organism’s genetic makeup

12. Allele • Allele: Alternate form of a gene at same position on pair of chromosomes that affect the same trait. • Dominant Allele: Capital Letter--O • Recessive Allele: lowercase letter--o • Homozygous Dominant--OO • Homozygous Recessive--oo • Heterozygous--Oo

13. Mendel’s Peas • These plant are easily manipulated • These plants can self-fertilize Stamen Carpel Garden pea

14. Dominant Recessive Recessive Dominant Pod shape Constricted Inflated Flower color Purple White Pod Color Yellow Green Flower position Axial Terminal Seed color Yellow Green Stem length Dwarf Tall Seed shape Round Wrinkled

15. Monohybrid Crosses P Generation (true-breeding parents) Purple flowers Whiteflowers All plants have purple flowers F1 Generation Fertilization among F1 plants (F1  F1) F2 Generation 1/4 of plants have white flowers F2 = 3:1 ratio 3/4 of plants have purpleflowers

16. Using a Punnett square to explain the results of a monohybrid cross PP PP P plants All P Gametes p All F1 plants: (hybrids) All Pp P Gametes 1/2 1/2 p P P Sperm Eggs F2 plants: PP p p Phenotypic ratio 3 purple : 1 white Pp Pp pp Genotypic ratio 1 PP : 2 Pp : 1 pp Figure 9.8b

17. from the monohybrid crosses, Mendel derived 4 hypotheses….combined, we now refer to these as… = Mendel’s Principle of Segregation • There are alternative forms of genes, now called alleles • For each characteristic, each organism has two genes • Gametes carry only one allele for each inherited characteristic • Alleles can be dominant or recessive

18. What happens when you follow the inheritance of more than a single trait at one time? How do two different traits get passed to offspring? Mendel’s Principle of Independent Assortment A Dihybrid Cross

19. Dihybrid Cross RRYY rryy ry Gametes RY RrYy RY RY Sperm Eggs RrYy RrYy rY rY RRYY Ry Ry RrYY RrYY ry ry RRYy rrYY RRYy RrYy RrYy RrYy RrYy Yellow round 9/16 Green round RRyy rrYy rrYy 3/16 Yellow wrinkled Rryy Rryy 3/16 rryy Green wrinkled 1/16 9:3:3:1

20. Mendel’s principle of independent assortment • Each pair of alleles segregates independently of the other pairs during gamete formation a P B a b P aa Bb PP Genotype:

21. A testcross is a mating between: Using a Testcross to Determine an Unknown Genotype Testcross: • An individual of unknown genotype and • A homozygous recessive individual Genotypes P_ pp Two possibilities for the purple flower: PP Pp Gametes P p P P p pp Pp Pp Offspring All purple 1 purple : 1 white

22. In incomplete dominance F1 hybrids have an appearance in between the phenotypes of the two parents IncompleteDominance in Plants and People Red RR White rr r Gametes R Pink Rr 1/2 1/2 r R Gametes 1/2 Sperm R R 1/2 Eggs Red RR 1/2 r r 1/2 Pink rR Pink Rr White rr Figure 9.18

23. Types of cells • Not all cells of an organisms have the • same number of chromosomes. • Two types of cells: • Somatic Cells • Gametes

24. Somatic Cells • Non-sex Cells. • These cells do not carry genetic information for sexual reproduction. • Contain a full compliment of chromosomes • Characteristic to their species. • Referred to as the diploid number of chromosomes. • Diploid • Means double number. • Designated 2n • All somatic cells in an organism have the 2n or diploid number of chromosomes.

25. Gametes • Haploid • Means single number. • Designated n • All gametes formed by an organism have the n or haploid number of • chromosomes. • Sex Cells • Cell which carry genetic information for sexual reproduction. • Contain one half the compliment of chromosomes characteristic to their species. • Referred to as the haploid number of chromosomes.

27. Human Life Cycle • Adults produce gametes--egg and sperm. • Gametes fuse to produce zygote. • Zygote grows and develops to produce baby. • Meiosis--process of division that produces gametes. • Mitosis--process of replication and division required for growth. • Adults, zygote and baby--2n. 2n=diploid • Gametes--n. n=haploid

28. Mitosis • Process of division that produces two daughter cells with identical chromosomal content of parent cell. • Mitosis is one stage of the cell cycle. • Cell cycle--cycle of stages a cell goes through in order to grow and divide. • Stages: I=Interphase, Growth 1=G1, DNA synthesis=S, Growth 2=G2, Mitosis=M

29. The Human cell cycle • Interphase--G1, S, G2 • Mitosis--M • G1--growth • S--DNA Synthesis, replication • G2--growth • M: • mitosis-- nuclear division • cytokinesis--cell division

30. Stages of Division- Mitosis • Prophase--nuclear envelope breakdown, chromosome condensation, spindle formation. • Metaphase--chromosomes are lined up precisely on the metaphase plate, or middle of the cell. • Anaphase--spindle pulls sister chromatids apart. • Telophase--chromatids begin to decondense and become chromatin. Spindle disappears. • Cytokinesis--divide cell and organelles. Actin ring, or cleavage furrow splits cell.

33. Gamete Production -Meiosis • In order to reproduce we must produce gametes. • Gametes are sperm and egg. • Why is that siblings are not identical? • Meiosis blends DNA from parental contributions to produce a mixed up “half” or haploid, set of DNA. • Crossing over is critical for producing haploid DNA with genetic diversity.

34. The Process of Meiosis Interphase • Haploid gametes are produced in diploid organisms • Two consecutive divisions occur, meiosis I and meiosis II, preceded by interphase Centrosomes (with centriole pairs) Nuclear envelope Chromatin Chromosomes duplicate

35. Prophase -I Replicated pairs of chromosomes line up side by side. These pairs are called Homologous--both have same gene order (gene for eye color, hair color, etc). Sister chromatid from one pair interact with a Sister chromatid from another pair. One sister is from father, one sister from mother, but they have same gene order.

36. Prophase -I • This interaction is called Synapsis. • Synapsis results in the formation of a Tetrad (4 sisters together). • Crossing over swaps sections of homologous genes.

37. Figure 2.9 (1) Meiosis - I Prophase I Metaphase I Anaphase I Telophase I

38. Meiosis - II Figure 2.9 (2) Prophase II Metaphase II Anaphase II Telophase II

39. Meiosis I Meiosis I: Homologous chromosomes separate Telophase I and Cytokinesis Prophase I Metaphase I Anaphase I Sites of crossing over Microtubules attached to Chromosomes Sister chromatids remain attached Cleavage furrow Spindle Sister chromatids Tetrad Centromere Tetrads line up Homologous chromosomes pair and exchange segments Pairs of homologous chromosomes split up Two haploid cells form: chromosomes are still double

40. Meiosis II Meiosis II: Sister chromatids separate Telophase II and Cytokinesis Prophase II Anaphase II Metaphase II Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes

41. Sexual life cycles • Haploid Gametes join to form a zygote • Somaticcells divide by Mitosis to produce adult organism • Meiosis produces gametes in sex cells

42. Genes on sex chromosomes determine Sex and sex-linked traits • Micrograph of the chromosomes of an organism paired and numbered. • Used to check for chromosomal abnormalities in individuals.

43. Sex Determination • All embryos start on a neutral or "indifferent" path. The 4 week old embryo is indifferent • By 7 weeks, the SRY (sex-related) gene encoded by the short arm of the Y chromosome begins to roar! • Testis determining factor converts progesterone to testosterone

44. Sex Determination • Indifferent embryos have two sets of ducts: • Müllerian ducts--will be come the future oviducts--thus female. • Wolfian ducts--will become the future vas deferens--thus male • dependent on testosterone for its continued development • The testes also produce an anti-Müllerian hormone that promotes regression of the Müllerian ducts • without SRY, the indifferent embryo will naturally develop into a female

45. Sex Determination Two copies of DAX (double X) inactivate SRY, thus this individual would be genetically male, but look female.

46. Sex Determination

47. Figure 2.16 (3) Sex Determination

48. Sex Chromosomal Disorders • Turner Syndrome – XO only one sex chromosome • Short, thick neck and stature • Do not undergo puberty, or menstruate, • no breast development • Kleinfelter Syndrome – XXY • Testis and prostate underdeveloped • No facial hair • Brest development • Long arms and legs: big hands and feet • Can be mentally retarded

50. An XY Individual with Androgen Insensitivity Syndrome Androgen Insensitivity Syndrome is a sex reversal condition where XY individuals look female. These individuals have the Y chromosome and functional SRY. These individuals have testis which generate AMH and testosterone. However, the genetic mutation results in a lack of the testosterone receptor. Estrogens are made in the adrenal gland which drive phenotypic development. As adults, these individuals have testes in the abdomen and lack a uterus and oviducts.