Chapter6

BIOLOGY: Today and TomorrowBIOLOGY: Today and Tomorrow, 4e, 4estarr starr evers evers starrstarr

Chapter 6 DNA Structure and Function

6.1 A Hero Dog’s Golden Clones

Trakr, the search dog that located the final survivor of the 9/11 attacks, later died of toxic exposure

Trakr’s DNA lives on in his genetic copies (clones)

To make a clone from an adult cell, researchers must reprogram its DNA to function like the DNA of an egg

Cloning mammals is unpredictable; few implanted embryos result in a live birth, and many have serious health problems

James Symington and Trakr at Ground Zero, September 2001

6.2 Chromosomes

Each DNA molecule consists of two strands twisted into a double helix

In cells, DNA molecules and their associated proteins are organized into chromosomes

DNA wraps around “spools” of proteins called histones that allow chromosomes to pack tightly

Chromosome Structure

When the cell prepares to divide, it duplicates all of its chromosomes

Sister chromatid One of two attached members of a duplicated eukaryotic

chromosome

Centromere Constricted region in a eukaryotic chromosome where

sister chromatids are attached

A Duplicated Chromosome

centromere

one chromatid its sister chromatid

a chromosome (unduplicated)

a chromosome (duplicated)

Chromosome Structure

A) Proteins structurally organize DNA into chromosomes.

Chromosome and Uncondensed DNA

B) A human chromosome.

C) DNA, the substance, extracted from human cells.

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 46 chromosomes

Diploid Cells having two of each type of chromosome

characteristic of the species (2n)

Two Types of Eukaryotic 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 In humans, XY and XX

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

Human Female Karyotype

A) Karyotype of a female human, with identical sex chromosomes (XX).

Bird Female Karyotype

B) Karyotype of a female chicken, with nonidentical sex chromosomes (ZW).

ANIMATED FIGURE: Chromosome structural organization

ANIMATION: Karyotype preparation

6.3 Fame, Glory, and DNA Structure

A DNA molecule consists of two strands of nucleotide monomers running in opposite directions and coiled into a double helix

DNA nucleotide One five-carbon sugar (deoxyribose) Three phosphate groups One nitrogen-containing base (adenine, thymine, guanine,

or cytosine)

adenine (A)deoxyadenosine

triphosphate

three phosphate

groups

baseFour nucleotides in DNA

thymine (T)deoxythymidine

triphosphate

Four nucleotides in DNA

cytosine (C)deoxycytidine triphosphate


guanine (G)deoxyguanosine

triphosphate


Discovering DNA Structure

Erwin Chargaff Discovered the relationships between DNA bases

Rosalind Franklin Discovered structure of DNA by x-ray crystallography

Maurice Wilkins Experimental evidence of DNA structure

James Watson and Francis Crick Built the first accurate model of a DNA molecule

Chargaff’s Rules

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

Rosalind Franklin’s x-ray diffraction image of DNA

C) Maurice Wilkins.

Watson, Crick, and Wilkins

B) Watson (left) and Crick (right) with their model of DNA.

The Double Helix

Watson and Crick proposed that DNA consists of two strands of nucleotides, running in opposite directions, coiled into a double helix

Hydrogen bonds between bases hold the two strands together

Only two kinds of base pairings form: A to T, and G to C (Chargaff’s rule)

DNA Structure

Covalent bonds between sugar and phosphate form the backbone of each chain

sugar–phosphate backbonehydrogen bonds link internally positioned nucleotide bases

A) Structure of DNA, as illustrated by a composite of three different models. The two sugar–phosphate backbones coil in a helix around internally positioned bases.

DNA’s Base-Pair Sequence

The order of nucleotide bases in a DNA strand (DNA sequence) is genetic information

The order of bases varies among species and among individuals

The two strands of a DNA molecule are complementary

Complementary Base Pairs

ANIMATED FIGURE: DNA close up

ANIMATED FIGURE: Subunits of DNA

6.4 DNA Replication and Repair

DNA replication Process by which a cell copies its DNA before it divides Each strand of the double helix serves as a template for

synthesis of a new, complementary strand of DNA Results in two double-stranded DNA molecules identical to

the parent

Semiconservative replication One strand of each molecule is parental (old), and the

other is new

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

Each type of DNA polymerase requires a primer in order to initiate DNA synthesis

DNA ligase seals any gaps to form a continuous strand

DNA Replication and Repair

DNA polymerase DNA replication enzyme; assembles a new strand of DNA

based on sequence of a DNA template

Primer Short, single strand of DNA that base-pairs with a targeted

DNA sequence

DNA ligase Enzyme that seals breaks in double-stranded DNA

DNA Replication

enzymes

primer

DNA polymerase

DNA ligase

How Mutations Arise

Proofreading by DNA polymerase corrects most base-pairing errors during DNA replication

Uncorrected errors in DNA replication may become mutations

Mutation A permanent change in DNA sequence

How Mutations Arise

Ionizing radiation (x-rays, most UV light, and gamma rays) can knock electrons out of atoms, breaking DNA

Non ionizing radiation (UV light 320–380 nm) forms nucleotide dimers that kink DNA and increase mutation rate

Chemicals in tobacco smoke transfer methyl groups (CH3) to nucleotide bases in DNA, causing mispairs during replication

Breakdown products of many environmental pollutants bind irreversibly to DNA, causing replication errors

Effects of Ionizing Radiation

A) Major breaks (red arrows) in chromosomes of a human white blood cell after exposure to ionizing radiation. Pieces of broken chromosomes often become lost during DNA replication.

thymine dimer

B) Thymine dimer. This type of DNA damage is caused by exposure to UV light between 230 and 380 nanometers in wavelength.

Effects of Thymine Dimers

ANIMATED FIGURE: DNA replication details

ANIMATED FIGURE: Duplication

ANIMATED FIGURE: Translocation

6.5 Cloning Adult Animals

Reproductive cloning Technology that produces an exact genetic copy of an

individual (clone)

Somatic cell nuclear transfer (SCNT) Nuclear DNA from an adult somatic cell is transferred into

an unfertilized, enucleated egg Common in livestock breeding

A) A cow egg is held in place by suction through a hollow glass tube called amicropipette. DNA is identified by a purple stain.

Somatic Cell Nuclear Transfer

B) Another micropipette punctures the egg and sucks out the DNA. All that remains inside the egg’s plasma membrane is cytoplasm.


C) A new micropipette prepares to enter the egg at the puncture site. The pipettecontains a cell grown from the skin of a donor animal.


D) The micropipette enters the egg and delivers the skin cell to a region between the cytoplasm and the plasma membrane.


E) After the pipette is withdrawn, the donor’s skin cell is visible next to the cytoplasm of the egg. The transfer is now complete.


F) An electric current causes the foreign cell to fuse with and empty its nucleus into the cytoplasm of the egg. The egg begins to divide, and an embryo forms.


Clone produced by SCNT

A) Liz the championship Holstein cow (right) with her clone.

ANIMATED FIGURE: How Dolly was created

6.6 A Hero Dog’s Golden Clones (revisited)

B) Trakr’s clones with James Symington. Today, the clones are search and rescue dogs for Team Trakr Foundation, an international humanitarian organization that Symington established.

Digging into Data:The Hershey Chase Experiments

Virus coat proteins labeled with 35S

35S remains outside cells

DNA being injected into bacterium

A) In one experiment, bacteria were infected with virus particles that had been labeled with a radioisotope of sulfur (35S). The sulfur had labeled only viral proteins. The viruses were dislodged from the bacteria by whirling the mixture in a kitchen blender. Most of the radioactive sulfur was detected in the viruses, not in the bacterial cells. Thus, the viruses had not injected protein into the bacteria.

Hershey Chase Experiments

Virus DNA labeled with 32P

Labeled DNA being injected into bacterium

32P remains inside cells

B) In another experiment, bacteria were infected with virus particles that had been labeled with a radioisotope of phosphorus (32P). The phosphorus had labeled only viral DNA. When the viruses were dislodged from the bacteria, the radioactive phosphorus was detected mainly inside the bacterial cells. Thus, the viruses had injected DNA into the cells—evidence that DNA is the genetic material of this virus.

Hershey Chase Experiments

C) Detail of Alfred Hershey and Martha Chase’s publication describing their experiments with bacteriophage. “Infected bacteria” refers to the percentage of bacteria that survived the blender. The micrograph on the right shows three bacteriophage particles injecting DNA into an E. coli bacterium.

Chapter6

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