DNA Structure and Function Chapter 6

1.1 Impacts/Issues Here Kitty, Kitty, Kitty, Kitty, Kitty Making clones (exact genetic copies) of adult animals is now a common practice in research and animal husbandry

6.2 Chromosomes A eukaryotic chromosome is a molecule of DNA together with associated proteins Chromosome Structure made of DNA and associated proteins Carries part or all of a cell’s genetic information

Chromosome Structure Sister chromatid Centromere One of two attached members of a duplicated eukaryotic chromosome Centromere Constricted region in a eukaryotic chromosome where sister chromatids are attached

Chromosome Structure Proteins organize DNA structurally Histone Allow chromosomes to pack tightly Histone Type of protein that structurally organizes eukaryotic chromosomes Nucleosome A length of DNA wound around a spool of histone proteins

Chromosome Structure

Chromosome Number A eukaryotic cell’s DNA is divided into a characteristic number of chromosomes Chromosome number Sum of all chromosomes in a cell of a given type A human body cell has 23 pairs of chromosomes Diploid Cells having two of each type of chromosome characteristic of the species (2n)

Examples of Chromosome Number

Types of Chromosomes There are two types of eukaryotic chromosomes: autosomes and sex chromosomes Autosomes Paired chromosomes with the same length, shape, centromere location, and genes Any chromosome other than a sex chromosome Sex chromosomes Members of a pair of chromosomes that differ between males and females

Sex Chromosomes: Sex Determination in Humans

diploid reproductive cell in female diploid reproductive cell in male X X X Y eggs sperm X Y X Y Figure 6.3: Animated! Pattern of sex determination in humans. The grid shows how sex chromosome combinations result in female (pink) or male (blue) offspring. Figure It Out: About what proportion of human newborns would you expect to be male? Answer: About 50 percent. XX XY union of sperm and egg at fertilization Stepped Art Fig. 6-3a, p. 104

Karyotype Karyotyping reveals characteristics of an individual’s chromosomes Karyotype Image of an individual’s complement of chromosomes arranged by size, length, shape, and centromere location

Constructing a Karyotype

6.3 Fame and Glory Erwin Chargaff Rosalind Franklin Discovered the relationships between DNA bases Rosalind Franklin Discovered the basic structure of DNA by x-ray crystallography James Watson and Francis Crick Built the first accurate model of a DNA molecule

Key Players Rosalind Franklin, Maurice Wilkins, James Watson, and Francis Crick

The Double Helix A DNA molecule consists of two strands of nucleotide monomers running in opposite directions and coiled into a double helix DNA nucleotide A five-carbon sugar (deoxyribose) Three phosphate groups One nitrogen-containing base (adenine, thymine, guanine, or cytosine)

The Double Helix Two double-helix strands are held together by hydrogen bonds between nucleotide bases Chargaff’s rules Bases of the two DNA strands in a double helix pair in a consistent way: A = T and C = G Proportions of A and G vary among species

The Four DNA Nucleotides

DNA Structure

Patterns of Base Pairing The order of bases (DNA sequence) varies among species and among individuals Each species has characteristic DNA sequences DNA sequence The order of nucleotide bases in a strand of DNA

6.4 DNA Replication and Repair A cell replicates its DNA before it divides Each strand of the double helix serves as a template for synthesis of a new, complementary strand of DNA DNA replication results in two double-stranded DNA molecules identical to the parent

DNA Replication and Repair During DNA replication, the double-helix unwinds DNA polymerase uses each strand as a template to assemble new, complementary strands of DNA from free nucleotides DNA ligase seals any gaps to form a continuous strand

DNA Replication and Repair Duplication of a cell’s DNA before cell division DNA polymerase DNA replication enzyme; assembles a new strand of DNA based on sequence of a DNA template DNA ligase Enzyme that seals breaks in double-stranded DNA

DNA Replication

1) The two strands of a DNA molecule are complementary: their nucleotides match up according to base-pairing rules (G to C, T to A). 2) As replication starts, the two strands of DNA unwind at many sites along the length of the molecule. 3) Each parent strand serves as a template for assembly of a new DNA strand from nucleotides, according to base-pairing rules. Figure 6.8: Animated! DNA replication. Each strand of a DNA double helix is copied; two double-stranded DNA molecules result. 4) DNA ligase seals any gaps that remain between bases of the “new” DNA, so a continuous strand forms. The base sequence of each half-old, half-new DNA molecule is identical to that of the parent. Stepped Art Fig. 6-8, p. 108

Animation: DNA replication

DNA Replication: The Double Helix

Checking for Mistakes DNA repair mechanisms fix damaged DNA Proofreading by DNA polymerase corrects most base-pairing errors DNA repair mechanisms Any of several processes by which enzymes repair DNA damage

Mutations Uncorrected errors in DNA replication may become mutations A permanent change in DNA sequence

6.5 Cloning Adult Animals Reproductive cloning technologies produce an exact genetic copy of an individual (clone) Reproductive cloning Technology that produces genetically identical individuals

Somatic Cell Nuclear Transfer Somatic cell nuclear transfer (SCNT) Method of reproductive cloning in which nuclear DNA from an adult somatic cell is transferred into an unfertilized, enucleated egg Therapeutic cloning Using SCNT to produce human embryos for research

Somatic Cell Nuclear Transfer

Clones Clone produced by somatic cell nuclear transfer

Digging into Data: The Hershey Chase Experiments

Figure 6.12: Animated! The Hershey–Chase experiments. Alfred Hershey and Martha Chase tested whether the genetic material injected by bacteriophage into bacteria is DNA, protein, or both. The experiments were based on the knowledge that proteins contain more sulfur (S) than phosphorus (P), and DNA contains more phosphorus than sulfur. Fig. 6-12a, p. 113

35S remains outside cells Virus proteins labeled with 35S DNA being injected into bacterium Virus DNA labeled with 32P 32P remains inside cells Figure 6.12: Animated! The Hershey–Chase experiments. Alfred Hershey and Martha Chase tested whether the genetic material injected by bacteriophage into bacteria is DNA, protein, or both. The experiments were based on the knowledge that proteins contain more sulfur (S) than phosphorus (P), and DNA contains more phosphorus than sulfur. Labeled DNA being injected into bacterium Fig. 6-12b-c, p. 113