What is DNA, and How is it Used in

Today’s Society? • Deoxyribonucleic Acid (DNA)

– DNA is found in all living things (all life related?)– The hereditary material; found in the nucleus of

eukaryotes (copied before each cell division; passes codes for physical traits to offspring)

– Today, segments of DNA (genes) can be manipulated, and can be removed from/inserted into organisms (biotechnology, transgenic organisms)

– Your DNA code is unique (excl. identical twins) criminal and paternity applications

– Genetic diseases linked to various genes genetic screenings and counseling

What is the Structure of DNA, and How is it Copied Before Cell Division?

• Structure of DNA– A polymer, composed of nucleotides (which consist of

a sugar, a phosphate group, and a nitrogenous base)

• Sugar is deoxyribose• Nitrogenous bases: guanine, cytosine, adenine, and thymine

– Double-stranded molecule, wound in helix (Watson, Crick, and Wilkins Nobel Prize)

• Two strands joined by hydrogen bonds (two bonds between T/A; three bonds between C/G); unzip at high temperature or via enzyme action

• DNA Replication (occurs during S-phase)– Code of new strand based on original template– Enzymes involved: DNA Polymerase, Helicase

Figures 5.13 and 5.16

Figure 5.17

What is the Role of RNA in Gene Expression?

• Gene Expression:Gene (DNA) message (m-RNA) polypeptide (protein)

TRANSCRIPTION TRANSLATION

• Transcription (gene m-RNA)– Occurs in nucleus– Nucleotide sequence of m-RNA based on code of DNA

(gene)• RNA polymerase enzyme involved in process

– In eukaryotes, m-RNA often edited into exons and introns; exons processed into mature m-RNA that enters cytoplasm and is used for protein synthesis

Fig. 5.18

How are Proteins Synthesized Based on Genetic Instructions?

• Translation (Protein synthesis: m-RNA polypeptides)– Occurs at ribosomes (in rough ER or cytoplasm)– t-RNA, bound to amino acids, associates with ribosome– Order of amino acids determined by GENETIC CODE:

m-RNA codons (base triplets) bind to anticodons of t-RNAs; amino acids join (peptide bonds) to form

polypeptides – Polyribosomes found in cells that exhibit high levels of

protein synthesis (when many copies of same poly- peptide are routinely synthesized)

Figures 5.21 and 5.22

Table 5.3

What are Chromosomes, Genes, Genomes, and Karyotypes?

• Chromosomes and Genes– Chromosome: genetic material (DNA) packaged with

histone proteins in discrete units (nucleosomes)• Centromere joins two chromatids; can be central or off-center• Categorized by size, centromere location, and band patterns

– Gene: nucleotide sequence that codes for a functional polypeptide (affect traits), as opposed to “junk

DNA”• Genome

– The entire genetic sequence of an organism (all chromo- somes, all nucleotides); chromosome numbers

vary by species• Karyotypes

– Photograph of entire set of an organism’s chromosomes– Used for diagnosis of chromosomal abnormalities

What are the Differences Between Sexual and Asexual Reproduction?

• Asexual Reproduction: offspring are clones of a single parent; variation among individuals limited to mutations

– Bacteria divide by binary fission – Spores in many eukaryotes (environmentally resistant

reproductive cells that can develop alone)• Sexual Reproduction: two parents donate genes to

offspring via gametes (sex cells)– Gametes are haploid, must fuse diploid zygote– Several sources of variation in addition to mutation

great physical diversity among individuals’ traits • Many organisms with both asexual and sexual

cycles (alternate, depending on various factors)– Example: parthenogenesis in some fishes and reptiles

RED CLONE

YELLOW CLONE ORANGE CLONE

How are Gametes Produced? What are Some Sources of Variation Among Individuals?

• Gametogenesis: formation of gametes from somatic cells (via MEIOSIS, cell differentiation)

– Spermatogenesis: formation of sperm cells; occurs in testes• Haploid spermatids differentiate into sperm cells (with cap and

flagellum)– Oogenesis: formation of egg cells (oocytes); occurs in ovaries

• One of four grand-daughter cells absorbs cytoplasms of others (egg cells are very large cells, with RNA-rich and protein-

rich cytoplasms); polar bodies remain after formation of oocyte

• Important sources of variation for sexually reproducing organisms (other than mutation):

1. Independent assortment of chromosomes in meiosis2. Crossing over (genetic recombination) among homologous

chromosomes; more likely to occur away from centromere 3. Gene duplications, inversions, translocations, and deletions4. Non-disjunction of chromosomes / duplication of chromosomes5. Whole-genome duplications

Figure 5.2

Fig. 5.10

What are the Two Laws of Mendelian (Classical) Genetics? What are Alleles?

• Developed by Gregor Mendel (1822-1884): studied heredity in pea plants (mainly texture and color of seeds); based solely on observations (no knowledge of

DNA or meiosis) – see cartoon– Law of Segregation: there are two sets of genes for a particular trait

(one from each parent), but only one gets into gamete during gametogenesis

– Law of Independent Assortment: during gametogenesis, a gene that enters a gamete does so independently of those for other traits (ex. if red hair expressed, blue eyes not necessarily expressed)

• Alleles: different forms of same gene (found at same locus)– Dominant allele: the form expressed in offspring (if present)– Recessive allele: masked by dominant allele (not expressed if

dominant allele present), but can still be passed on to next generation (by a carrier)

What are Genotypes and Phenotypes?How do we Solve Genetics Problems?

• Phenotype: description of form of physical trait an individual exhibits (ex. trait of hair color, “red hair” is a

phenotype)• Genotype: description of individual’s condition at the genetic

level; three possible genotypes:– Homozygous dominant (AA): both genes for trait instruct to produce

dominant phenotype– Homozygous recessive (aa): both genes for trait instruct to produce

recessive phenotype– Heterozygous (Aa): genetic instructions conflict; for Mendelian traits,

dominant phenotype results (recessive masked)

• Solving Genetics Problems: Mendel described traits in P (parental) generation and F1, F2 (filial) generations

– Monohybrid Cross: single trait; parents’ genotypes crossed using Punnett Square

– Dihybrid Cross: two traits; find results for each single trait with Punnett Square, then multiply probabilities (ex. ¼ X ¼ = 1/16)

Figure 5.6

What are Some Modern Additions to Mendelian Genetics?

• Polygenic traits: traits caused by multiple genes– Variation in population often follows bell curve when frequency is

plotted against measurement of phenotype (ex. height)

• Multiple alleles: ex. blood types– Only two alleles in any cell, but more than two in population

• Linked genes: loci typically in close proximity• Incomplete Dominance (and Co-dominance)

– Phenotype for heterozygous genotype is a mixture (blend) of those caused by homozygous genotypes

– Problem-solving is the same as Mendelian traits; need to evaluate genotypes differently; any mention of three pheno-types in problem?

• Examples: color of petals in roses; Sickle-cell anemia

Figure 5.5

How is Sex Determined? What are Some Examples of Sex-linked Traits?

• Sex Determination: 23rd pair of human chromo-somes are the sex chromosomes, others are autosomes - see cartoon

– Females: XX; Males: XY

• Sex-linked traits: X-sex chromosome has many genes other than those for sex determination, but Y-sex chromosome does not no male carriers for sex-linked traits

– Examples: color blindness, hemophilia – Same methods to solve problems, but must account for

sex of parents and offspring [ex. XHXh x XHY XHXH, XHY, XhXH (= XHXh), XhY]

Figures 5.3 and 5.4