Chapter 16

Primase

Provides an RNA primer to start new DNA strand synthesis

Nuceloid

Region where DNA is "supercoiled" in bacterium

Leading strand

Replicated continuously in the 5' - 3' direction -Daughter strand elongates toward the replication fork

What happens to chromatin at interphase?

Some chromatin in organized into a 10-nm fiber, but much is compacted into a 30-nm fiber, through folding and looping.

What does chromatin do in preparation of cell division?

The chromatin must pack much more tightly into pairs of chromatin

Why Must a Cell Copy Its DNA?

-Because it contains more information than RNA or Protein -Because it is the only biological molecule that can copy itself

Single Strand Binding Protein (SSB)

-Binds to and stabilizes the single stranded DNA -Binds after the replication fork -Prevents H-bonds between bases

Prokaryotic DNA replication

-Circular genome -Single DNA molecule -1 origin of replication

Eukaryotic DNA replication

-Linear genome -Multiple linear chromosomes -Multiple origins of replication on each chromosome

Lagging strand

-Replicated discontinuously by DNA Polymerase III in short segments called Okazaki fragments. Begins with RNA. Polymerase III adds DNA in a 5' to 3' direction, then polymerase I binds to the strand and removes RNA and replaces it with DNA, but it cannot connect the last pieces so an enzyme called DNA ligase joins the pieces together at the 5' end. -Daughter strand elongates away from the replication fork

Helicase

-Unwinds DNA to create two single strands -Binds at the replication fork -Breaks H-bonds between bases

Chromatin

A complex of DNA and protein, found in the nucleus of eukaryotic cells. -Fit into the nucleus through an elaborate, multilevel system of packing.

How do the leading and the lagging strands differ? A) The leading strand is synthesized towards the replication fork, while the lagging strand is synthesized away from the replication fork. B) The leading strand is synthesized at twice the rate of the lagging strand. C) The lagging strand is synthesized continuously, whereas the leading strand is synthesized in short fragments that are ultimately stitched together. D) The leading strand is synthesized by adding nucleotides to the 3' end of the growing strand, whereas the lagging strand is synthesized by adding nucleotides to the 5' end

A) The leading strand is synthesized towards the replication fork, while the lagging strand is synthesized away from the replication fork.

Polymerase III

Assembles new DNA strand in the 5' - 3' direction

Formation of the Replication Fork

At the origin of replication the two strands of DNA separate serving as templates for making new strands. -Result is a replication bubble, growing in both directions, forming two replication forks

If one strand of a DNA molecule has the sequence of bases 5'ATTGCA3', the other complementary, antiparallel strand of DNA would have the sequence: A) 5'TAACGT3' B) 5'TGCAAT3' C) 5'UAACGU3' D) 3'UAACGU5' E) 5'UGCAAU3'

B) 5'TGCAAT3'

How does DNA correct replication errors?

Both DNA Pol I and DNA Pol III have 3 'to 5' exonuclease activity which allows them to remove the mistaken nucleotide as soon as it is added

Eukaryotic cells have multiple origins of replication on their chromosomes. Why do you think that prokaryotic cells have only one origin of replication on their chromosomes? A) They lack the proteins necessary to replicate at multiple origins. B) They don't replicate their DNA. C) They have less total DNA to copy. D) They have more time to copy their DNA.

C) They have less total DNA to copy.

Polymerase I

Can remove RNA nucleotides as it adds new DNA in a 5' - 3' direction

Diploid cells

Cell that contains two sets of chromosomes

Proposed models for DNA replication

Conservative, Semiconservative, and Dispersive

If DNA pol I was non-functional, how would that affect the leading strand during DNA synthesis in a eukaryotic cell? A) It would have no effect in a eukaryotic cell B) DNA synthesis would never even start on the leading strand C) The leading strand would consist of many Okazaki fragments that can not be connected D) The leading strand would not be able to connect to the lagging strand approaching it from the next origin of replication E) The leading strand would be made entirely of RNA instead of DNA

D) The leading strand would not be able to connect to the lagging strand approaching it from the next origin of replication

Replication Fork

DNA is unwound, DNA polymerases build new strands of DNA. Because strands in a DNA double helix run in opposite directions, the new strands must be made in different ways (leading vs lagging strand)

Nucleosomes

DNA wrapped around histones

Without the activity of the primase enzyme what would happen during DNA replication? A) The DNA double helix would not unwind B) Okazaki fragments would not be joined together C) The leading strand would form, but the lagging strand would not D) The lagging strand would form but the leading strand would not E) Neither strand would form because new DNA cannot be synthesized without a primer

E) Neither strand would form because new DNA cannot be synthesized without a primer

DNA polymerases

Enzymes that catalyze the elongation of a new DNA at a replication fork -Most require a primer and a DNA template strand -Add nucleotides only to the free 3' end, therefore, a new DNA strand can only elongate in the 5' to 3' direction.

How do eukaryotes protect chromosome ends?

Eukaryotic chromosomal DNA molecules have special repetitive, non-coding nucleotide sequences at their ends called telomeres -They do not prevent the shortening of DNA molecules but they do postpone the erosion of genes near the ends of DNA molecules

Which model do we support and why?

Experiments by Matthew Meselson and Franklin Stahl at Cal Tech supported the semiconservative model. They labeled the nucleotides of the old strands with a heavy isotope of nitrogen (N15), while any new nucleotides were labeled with a lighter isotope (N14)

Histone proteins

Main protein component of chromatin associated with linear DNA molecules -positively charged

Telomerase

Maintains telomere length in germline cells to protect the health of gametes

Why do cells replicate their DNA?

In preparation for cell division

What is a result of dense packing of heterochromatin?

It makes it difficult for the cell to express genetic information coded in these regions.

Ligase

Joins two DNA strands together at their ends

Euchromatin

Loosely packed chromatin, present during interphase and condenses prior to mitosis

Dispersive model

The parental double helix is broken into double-stranded DNA segments that act as templates for the synthesis of new double helix molecules. The segments then reassemble into complete DNA double helices, each with parental and progeny DNA segments interspersed.

Conservative model

The two parental DNA strands are back together after replication has occurred. One daughter molecule contains both parental DNA strands, and the other daughter molecule contains DNA strands of all newly-synthesized material

Semiconservative model

The two parental DNA strands separate and each of those strands then serves as a template for the synthesis of a new DNA strand. Two DNA double helices are formed, both of which consist of one parental and one new strand.

Heterochromatin

Tightly packed chromatin, a few regions of chromatin are highly condensed into these during interphase.

Topoisomerase

Travels ahead of the replication fork "nicking" and swiveling the DNA to relieve torsional strain on the molecule -induces (-) supercoils in the DNA to counteract the (+) supercoils induced by unwinding -breaks covalent bonds in DNA backbone