Chapter 21 DNA Biology and Technology Mader, Sylvia S. Human Biology. 13 th Edition. McGraw-Hill, 2014

Chapter 21

DNA Biology and Technology

Mader, Sylvia S. Human Biology. 13th Edition. McGraw-Hill, 2014.

Points to Ponder• What are three functions of DNA?• Review DNA and RNA structure. • What are the 3 types of RNA and what are their functions?• Compare and contrast the structure and function of DNA and

RNA.• How does DNA replicate?• Describe transcription and translation in detail.• Describe the genetic code.• Review protein structure and function.• What are the 4 levels of regulating gene expression.• What did we learn from the human genome project and where are

we going from here?• What is ex vivo and in vivo gene therapy?• Define biotechnology, transgenic organisms, genetic engineering

and recombinant DNA.• What are some uses of transgenic bacteria, plants and animals?

What must DNA do?

1. Replicate to be passed on to the next generation

2. Store information

3. Undergo mutations to provide genetic diversity

21.1 DNA and RNA structure and function

DNA structure: A review• Double-stranded helix• Composed of repeating nucleotides made of

– a pentose sugar– phosphate – a nitrogenous base

• Sugar and phosphate make up the backbone while the bases make up the “rungs” of the ladder

• Bases have complementary pairing – cytosine (C) pairs with guanine (G) – adenine (A) pairs with thymine (T)


The Bases

Figure 2–22b, c

DNA structure


How does DNA replicate?

1. Two strands unwind by breaking the H bonds

2. Complementary nucleotides are added to each strand by DNA polymerase

3. Each new double-stranded helix is made of one new strand and one old strand (semiconservative replication)

**The sequence of bases makes each individual unique


DNA Replication• DNA helicases unwind the DNA and separates the

strands• DNA polymerase bind to the DNA and synthesizes

complementary antiparallel strands• DNA rewinds into double helix molecules

– New molecules contains one strand of the original DNA and one newly synthesized strand

DNA replication


Mutations• Cell has repair enzymes that usually fix errors in DNA replication• A replication error that persists is a mutation

– A permanent change in the sequence of bases that can cause a change in phenotype and introduce variability

• Most non-infectious disease, conditions, and disorders are due to mutations in the DNA that change the amino acids in the protein

• Point mutations – change in 1 base of DNA can be a silent mutation if the amino acids

is not changed

• Insertion mutation – addition of a base which changes the reading frame– whole protein after the mutation is wrong

• Deletion Mutation– removal of a base, alter reading frame, wrong protein is made

RNA structure and function• Single-stranded • Composed of repeating nucleotides • Sugar-phosphate backbone• Bases are A, C, G and uracil (U)• Three types of RNA

– Ribosomal (rRNA): • joins with proteins to form ribosomes

– Messenger (mRNA): • carries genetic information from DNA to the

ribosomes– Transfer (tRNA):

• transfers amino acids to a ribosome where they are added to a forming protein


RNA structure21.1 DNA and RNA structure and function

Comparing DNA and RNA

• Similarities: – Are nucleic acids– Are made of

nucleotides– Have sugar-phosphate

backbones– Are found in the

nucleus

• Differences:– DNA is double stranded

while RNA is single stranded

– DNA has T while RNA has U

– DNA has a deoxyribose sugar while RNA has a ribose sugar

– RNA is also found in the cytoplasm as well as the nucleus while DNA is not


Gene Expression

• DNA provides the cell with blueprints for synthesizing proteins

• DNA resides in the nucleus and protein synthesis occurs in the cytoplasm

1. mRNA carries a copy of DNA’s blueprint into the cytoplasm

2. Other RNA molecules (rRNA and tRNA) are involved in protein synthesis

Proteins: A review

• Composed of subunits of amino acids • Sequence of amino acids determines the shape

of the protein• Synthesized at the ribosomes• Important for diverse functions in the body

including hormones, enzymes and transport• Can denature causing a loss of function

21.2 Gene expression

Proteins: A review of structure

2 steps of gene expression

1. Transcription – DNA is read to make a mRNA in the nucleus of our cells

2. Translation – Reading the mRNA to make a protein in the cytoplasm


Overview of transcription and translation

Genetic code

• Made of 4 bases• Bases act as a code

for amino acids in translation

• Every 3 bases on the mRNA is called a codon that codes for a particular amino acid in translation

Step 1: Gene Activation

• Uncoils DNA

• Start and stop codes on DNA mark location of gene– Locates area for mRNA transcription

Step 2: DNA to mRNA

• Enzyme RNA polymerase transcribes DNA:– binds to promoter (start) sequence– reads DNA code for gene– binds nucleotides to form messenger RNA

(mRNA)– mRNA duplicates DNA coding strand, uracil

replaces thymine

Step 3: RNA Processing

• At stop signal, mRNA detaches from DNA molecule:– code is edited (RNA processing)– unnecessary codes (introns) removed– good codes (exons) spliced together– triplet of 3 nucleotides (codon) represents one

amino acid

1. Transcription

• mRNA is made from a DNA template

• mRNA is processed before leaving the nucleus

• mRNA moves to the ribosomes to be read

• Every 3 bases on the mRNA is called a codon and codes for a particular amino acid in translation


Processing of mRNA after transcription Modifications of mRNA:• One end of the RNA is capped• Introns removed• Poly-A tail is added

– important for the nuclear export, translation and stability of mRNA

Processing of mRNA after transcription

• mRNA Splicing– Cells use only certain exons rather then all to

form a mature RNA transcript– Result can be a different protein product in

each cell– Alternate mRNA splicing may account for the

ability of a single gene to result in two different proteins in a cell

2. Translation

1. Initiation - mRNA binds to the small ribosomal subunit and

causes 2 ribosomal units to associate

2. Elongation: polypeptide lengthens• tRNA picks up an amino acid • tRNA has an anticodon that is complementary to the

codon on the mRNA• tRNA anticodon binds to the codon and drops off an

amino acid to the growing polypeptide

3. Termination- a stop codon on the mRNA causes the ribosome to

fall off the mRNA


Overview of transcription and translation

Regulation of gene expression1. Transcriptional control (nucleus):

– chromatin density must decondense to allow for activation

– transcription factors DNA-bind proteins regulate activity of a gene

2. Posttranscriptional control (nucleus)– mRNA processing

3. Translational control (cytoplasm)– Differential ability of mRNA to bind ribosomes

4. Posttranslational control (cytoplasm)– changes to the protein to make it functional


What did we learn from the human genome project (HGP)?

• Genes are a sequence of DNA bases that is transcribed into RNA molecules and may be translated into protein

• Humans consist of about 3 billion bases (A,T,G,C) and 25,000 genes

• Human genome sequenced in 2003• Many polymorphisms or small regions of DNA that

vary among individuals were identified– Some have not physiological effects– Others contribute to the diversity of human beings and

possibly disease

• Genome size is not correlated with the number of genes or complexity of the organisms

What is the next step in the HGP?

• Functional genomics• Understanding how the 25,000 genes function• Understanding the function of gene deserts

(82 regions that make up 3% of the genome lacking identifiable genes)

• Comparative genomics• Help understand how species have evolved• Comparing genomes may help identify base

sequences that cause human illness• Help in our understanding of gene regulation

21.3 Genomics

New endeavors

• Proteomics – the study of the structure, function and interactions of cell proteins

• Can be difficult to study because:• protein concentrations differ greatly between cells• protein location, concentration interactions differ from

minute to minute

– understanding proteins may lead to the discovery of better drugs

• Bioinformatics – the application of computer technologies to study the genome

– May allow scientists to find cause-and-effect relationships between genetic profiles and disorders caused by multifactorial genes

21.3 Genomics

How can we modify a person’s genome?

• Gene therapy - insertion of genetic material into human cells to treat a disorder

– Ex vivo therapy – cells are removed for a person altered and then returned to the patient

– In vivo therapy – a gene is directly inserted into an individual through a vector (e.g. viruses) or directly injected to replace mutated genes or to restore normal controls over gene activity

• Gene therapy has been most successful in treating cancer

Ex vivo gene therapyRetrovirus -RNA virus that is replicated in a host cell via the enzyme reverse transcriptase to produce DNA from its RNA genome. - DNA is incorporated into the host's genome - Virus replicates as part of the host cell's DNA.

DNA technology terms

• Genetic engineering – altering DNA in bacteria, viruses, plants and animal cells

through recombinant DNA technology

• Recombinant DNA – contains DNA from 2 or more different sources

• Transgenic organisms – organisms that have a foreign gene inserted into them

• Biotechnology – using natural biological systems to create a product or to

achieve an end desired by humans

21.4 DNA technology

DNA technology

1. Gene cloning through recombinant DNA

2. Polymerase chain reaction (PCR)

3. DNA fingerprinting

4. Biotechnology products from bacteria, plants and animals

21.4 DNA technology

1. Gene cloning

• Recombinant DNA – contains DNA from 2 or more different sources that allows genes to be copies

– The gene of interest is inserted into a vector– Vector is typically a plasmid, small accessory rings of

DNA found in bacteria

• An example using bacteria to clone the human insulin gene:

– Restriction enzyme• cut the vector (plasmid) and the human DNA with the

insulin gene– DNA ligase

• seals together the insulin gene and the plasmid– Bacterial cells uptake plasmid and the gene is copied

and product can be made

21.4 DNA technology

2. Polymerase chain reaction (PCR)

• Used to clone small pieces of DNA

• Requires – DNA polymerase to

carry out DNA replication

– Nucleotides (phosphate, deoxyribose, and base)

• Important for amplifying DNA for analysis such as in DNA fingerprinting

21.4 DNA technology

3. DNA fingerprinting

• DNA Fragments are separated by their charge/size ratios

• Results in a distinctive pattern for each individual

• Often used for paternity or to identify an individual at a crime scene or unknown body remains

21.4 DNA technology

4. Biotechnology products: Transgenic bacteria

• Important uses:– Insulin– Human growth

hormone (HGH)– Clotting factor VIII– Tissue plasminogen activator (t-PA)– Hepatitis B vaccine– Bioremediation – cleaning up the

environment such as oil degradingbacteria

21.4 DNA technology

4. Biotechnology products: Transgenic plants

• Important uses:– Produce human proteins in their seeds such as

hormones, clotting factors and antibodies– Plants resistant to herbicides– Plants resistant to insects– Plants resistant to frost

• Corn, soybean and cotton plants are commonly genetically altered

• In 2001:– 72 million acres of transgenic crops worldwide– 26% of US corn crops were transgenic crops

21.4 DNA technology

4. Biotechnology products: Transgenic plants

21.4 DNA technology

4. Biotechnology products: Transgenic animals

• Gene is inserted into the egg that when fertilized will develop into a transgenic animal

• Current uses:– Gene pharming: production of pharmaceuticals

in the milk of farm animals– Larger animals: includes fish, cows, pigs, rabbits

and sheep– Mouse models: the use of mice for various gene

studies– Xenotransplantation: pigs can express human

proteins on their organs making it easier to transplant them into humans

Documents

Chapter 21 DNA Biology and Technology Mader, Sylvia S. Human Biology. 13 th Edition. McGraw-Hill, 2014