chapter 16: overview dna replication

8/9/2019 Chapter 16: Overview DNA Replication

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Pgs 300- 307

CHAPTER 16

DNA Replication:A Closer Look

y DNA replication is fundamentally similar for prokaryotes and eukaryotesGetting Started: Origins of Replication

y Origins of replication- special sites where the replication of a DNA molecule beginsy Proteins that initiate DNA replication recognize nucleotide sequence and attach DNA un zipping,

or separating it into 2 strands

y Opening up the replication bubbley Bacteria have a single origin, where as a eukaryotic chromosome may have many replication

origins

y Replication fork- a y- shaped region where the new strands of DNA are elongating at the end ofa replication bubble

Elongating a New DNA Strand

y DNA polymerase- enzymes which catalyze the elongation of new DNA at a replication forky DNA polymerase add nucleotides to the end of a growing strand, each nucleotide that is added

is actually a nucleoside triphosphate

y Nucleoside- sugar and a base


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AntiparallelElongation

y The 2 ends of DNA are differenty The 2 ends of DNA in a double helix are triparallel, meaning they are oriented in opposite

directions to each other

y DNA polymerases add nucleotides only to the free 3 end of a growing DNA strandy A new DNA strand can only elongate in the 5- 3y DNA pol III simply nestles in the replication fork on that template strand and continuously adds

nucleotides to the complementary strand

y Leading strand- DNA strand made by this mechanismy To elongate the other new strand of DNA in the 5- 3 direction DNA pol III must work along the

other template strand, the DNA made in this direction is called the lagging strand

y Segments of the lagging strand are called Okazaki fragmentsy DNA ligase- eventually joins the backbones of theOkazaki fragments forming a single new DNA

strand

Priming DNA Synthesis

y DNA polymerases cannot initiate the synthesis of a polynucleotide: they can only add nucleotideto the 3 end of an already existing chain

y Primer- the initial nucleotide chain is a short chain that is based pair with a templatestrand(DNA or RNA)

y An enzyme called primase- can start an RNA chain from scratchy For synthesis of a lagging strand, each Okazaki fragment must be primed separately


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1. Primase joins RNA nucleotides intoa primer

2. DNA pol III adds DNA nucleotides tothe primer, forming an Okazaki

fragment

3. After reaching the next RNA DNApol

III falls off

4. After the 2nd fragment is primed,DNA pol III adds DNA nucleotides

until it reaches the first primer and

falls off

5. DNA pol I replaces the RNA withDNA adding to the 3 end of

fragment 2

6. DNA ligase forms a bond btw thenewest DNA and the adjacent DNA

of fragment 1

7. The lagging strand in this ration isnow complete

OtherProtein That Assist DNA Replication

y Helicase is an enzyme that untwists the double helix at the replication forks, separating the 2parental strands and making them available as template strands

y The untwisting causes tighter twisting and strain ahead of the replication fork, topoisomerasehelps relieve this strain.


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The DNA Replication Machine as a Stationary Complex

The various proteins that participate in DNA replication form a single large complex, a DNAreplication machine

Proofreading and Repairing DNA

y DNA polymerases proofread newly made DNA replacing any incorrect nucleotidesy In mismatch repair of DNA- repair enzymes correct errors in base pairingy Reactive chemicals, radioactive emissions, X- rays, and ultraviolet light can change nucleotides in

ways that can affect encoded genetic info

y Usually a segment containing DNA is cut out by a DNA cutting enzyme- a nuclease, and thenreplace= nucleotide excision repair


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Replicating the Ends of DNA molecule

The ends of eukaryotic chromosomal DNA get shorter with each round of replication Telomeres- nucleotide sequences which eukaryotic chromosomal DNA molecules, they also

postpone the erosion of genes near the ends of DNA molecules

If the chromosomes of germ cells became shorter in every cell cycle, then essential genes wouldeventually be missing from the gametes they produce

An enzyme called telomerase catalyzes the lengthening of telomeres in germ cells Viruses use enzymes, ribosomes, and small molecules of host cells to synthesize progeny viruses


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Pgs 337- 339

General Features ofViral Reproductive Cycles

y Viruses can reproduce only within a host celly Each type of virus can infect only a limited range of host cells called its host rangey Viruses identify their host cells by a lock- and- key fit btw proteins on the outside of the virus

and specific receptor molecules on the surface

y A viral infection begins when the genome of a virus makes its way into a host celly Once inside the viral genome takes command of its hosty The host provides the nucleotides for making viral nucleic acids

y A simplified viral reproductive cycle

Reproductive Cycle of Phages

y Phages are the best understood of all viruses and they gothrough two alternative reproductive mechanisms: the lytic

cycle and the lysogenic cycle

The Lytic Cycle

y A phage reproductive cycle that culminates in death of thehost cell is known as a lytic cycle

y The lytic cycle also produces new phages and digests thehosts cell wall, releasing the progeny viruses

y A phage that reproduces only by a lytic cycle is a virulent phage

yhen phage

DNA


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successfully enters a bacterium, the DNA is recognized as foreign and cut up by

restriction enzymes

The Lysogenic Cycle

y The Lysogenic cycle replicates the phage genome w/ out destroying the hosty Phages capable of using both modes of reproducing w/in a bacterium are called

temperate phages

y The lytic and lysogenic cycles of phage P, a temperate phage

y Viral DNA is known as a prophageReproductive Cycles of Animal Viruses

y The nature of the genome is the basis for the common classification of animal viruses


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Pg 342 HIV reproduction

y The retrovirus that causes AIDS(acquired immunodeficiency syndrome)y HIV(human immunodeficiency virus) and other retroviruses are enveloped viruses that contain 2

identical molecules of single- stranded RNA

y After HIV enters a host cells its reverse transcriptase molecules are releasedy Then the DNA integrates itself, becoming a provirus, and never leaves the hosts genome,

permanent


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Pgs 345- 346

Viral Diseases in Plants

More than 2,000 types of viral diseases of plants are known Common symptoms of viral infection include spots on leaves and fruits, stunted growth, and

damaged flowers or roots

y Vertical transmission- in which a plant inherits a viral infection from a parent, can occur inasexual propagation or sexual reproduction

y Plant viruses spread disease in two major modes- Horizontal transmission, entering throughdamaged cell walls and vertical transmission, inheriting the virus from a parent

Viroids and Prions: The Simplest Infectious Agents

y Viroids - are circular RNA molecules that infect plants and disrupt their growthy Prions- are slow-acting, virtually indestructible infectious proteins that cause brain diseases in

mammals


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Pgs 346- 348

Concept 18.3: Rapid reproduction, mutation, and genetic recombination contribute to the genetic

diversity of bacteria

Bacteria allow researchers to investigate molecular genetics in the simplest true organismsThe Bacterial Genome and Its Replication

The bacterial chromosome is usually a circular DNA molecule with few associated proteins This dense region of DNA is called the nucleoid, it is not bounded by membrane like the nucleus

of a eukaryotic cell

In addition to the chromosome many bacteria have plasmids, smaller circular DNA moleculesthat can replicate independently of the bacterial chromosome

Bacterial cells divideby binary fission

Mutation and Genetic Recombination as Sources of Genetic Variation

y Bacteria reproduce rapidly= new mutations quickly increase a populations geneticdiversity

y Genetic diversity from recombination of the DNA from two different bacterial cells


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Mechanisms of Gene Transfer and Genetic Recombination in Bacteria

y Transformation- the alteration of a bacterial cells genotype and phenotype by the uptake ofnaked, foreign DNA from the surrounding environment

y Transduction- Phages carry bacterial genes from one host cell to anothery Conjugation- direct transfer of genetic material between bacterial cells that are temporarily

joined


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R Plasmids and Antibiotic Resistance

y Genes conferring the type of resistance to destroy certain antibiotics are generally carried byplasmids known as R plasmids

y R for resistancey The theory of natural selection, fraction of the bacterial pop carrying genes for antibiotic

resistance will go up

y Medical consequences are also predictable: resistant strains of pathogens, making treatmentharder

y R plasmids, like F plasmids, have genes that encod sex pili and enable plasmid transfer from onebacterial cell to another

chapter 16: overview dna replication

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