DNA REPLICATION
Why do bacterial cells not need telomerase? Why does telomerase have a built-in template?
(1) Telomerase is needed only to replicate the ends of a linear DNA. Because bacterial DNAs are circular, they lack ends and don't require telomerase. (2) Since telomerase works by extending one strand of DNA without any external template and because DNA synthesis requires a template, telomerase must contain an internal template to allow it to extend a DNA chain.
meiosis I: homologous chromosomes separate
1. interphase: duplication 2. early prophase I: nuclear envelope breaks down, centrioles move to poles, chromosomes condense 3. prometaphase I: microtubules begin to attach 4. metaphase I: homologous chromosomes line up along metaphasic plate 5. anaphase I: homologous chromosomes separate and begin to move toward poles 6. telophase and cytokinesis I: chromosomes get to poles, cleavage furrow forms, 2 separate cells created, no nucleus present
haploid number for humans
23
mistakes in meiosis: change of chromosome number
3 chromosomes at one spot: trisomy (at 21 this causes Down's syndrome) 1 chromosome at one spot: monosomy (missing a chromosome)
The leading strand is the daughter strand that has its _____ end pointed towards the replication fork and is therefore synthesized _____ .
3' ; continuously
diploid number for humans
46
DNA polymerases can only work in one direction, which is?
5' -> 3' because deoxyribonucleotides can only be added to the 3' end of a growing DNA chain (bcs thats the end w the free OH group)
At a specific area of a chromosome, the sequence of nucleotides below is present where the chain opens to form a replication fork: 3' C C T A G G C T G C A A T C C 5' An RNA primer is formed starting at the underlined T (T) of the template. Which of the following represents the primer sequence?
5' A C G U U A G G 3'
meiosis II: sister chromatids separate
7. prophase II: centrioles move to ends 8. metaphase II: sister chromatids line up at metaphasic plate with microtubules attached 9. anaphase II: sister chromatids are separated and begin to move toward poles 10. telophase and cytokinesis II: 2 new cells formed that are haploid and have nuclei present *happens 2x: once each for each cell that entered from meiosis I*
karyotype
A display of the chromosome pairs of a cell arranged by size and shape
Which letters are purines?
AG
RNA primers must be present on which strand during DNA synthesis?
Both leading and lagging strands, but spaced differently
Which letters are pyrimidines?
CUT
Separates the Double Stranded DNA
DNA Helicase
replication fork
DNA Helicase untwists the helix at locations called replication origins. The replication origin forms a Y shape, and is called a replication fork. The replication fork moves down the DNA strand, usually from an internal location to the strand's end
Attaches DNA nucleotides to an RNA primer
DNA Polymerase
Proofreads the growing DNA strands and replaces mismatched nucleotides
DNA Polymerase
The enzyme that can replicate DNA is called ___
DNA polymerase
What catalyzes DNA synthesis?
DNA polymerase This enzyme catalyzes DNA synthesis.
Why is the new DNA strand complementary to the 3' to 5' strands assembled in short segments?
DNA polymerase can assemble DNA only in the 5' to 3' direction Since DNA polymerase can assemble DNA only in the 5' to 3' direction, the new strand complementary to the 3' to 5' strand must be assembled in short 5' to 3' segments, which are later joined together by ligase.
What kinds of organisms require telomerase?
Eukaryotes contain linear chromosomes and therefore require telomerase to prevent loss of the ends of the chromosomes
Which of the following has the greatest potential as a cancer treatment?
Find a way to turn off the gene for making telomerase in cancer cells, since the cells would stop dividing when the telomeres were gone.
Why do histones bind tightly to DNA?
Histones are positively charged, and DNA is negatively charged.
Sliding Clamp
Holds DNA polymerase in place during strand extension in leading-strand and lagging-strnd synthesis
Which strand of DNA is synthesized toward the middle of the replication bubble?
Lagging. DNA polymerase can only work in the 5' to 3' direction.
The researchers used strands of DNA located at the ends of chromosomes (called telomeres) to classify the cells they studied. What assumption did they make about telomeres?
Longer telomeres indicate younger cells.
_____ are the short sections of DNA that are synthesized on the lagging strand of the replicating DNA.
Okazaki fragments
DNA Ligase
Once RNA primer is remove and replaces by DNA, this enzyme catalyzes the formation of a phosphodiester bond between adjacent fragments
Which part of a deoxynucleoside triphosphate (dNTP) molecule provides the energy for DNA synthesis?
Phosphate groups The potential energy stored in the bonds of the phosphates provides the energy for DNA synthesis.
DNA Polymerase
Polymerizes DNA monomers into DNA. Catalyzes DNA synthesis
Which of the following enzymes creates a primer for DNA polymerase?
Primase This RNA polymerase synthesizes an RNA primer antiparallel to the template DNA strand.
Which of the following statements about DNA synthesis is true?
Primers are short sequences that allow the initiation of DNA synthesis. When a primer is added to a single strand of DNA, DNA polymerase can start adding nucleotides to synthesize a complementary strand.
The synthesis of a new strand begins with the synthesis of a(n) _____.
RNA primer complementary to a preexisting DNA strand The synthesis of a DNA strand begins with the formation of an RNA primer.
Why does increasing doses of UV light cause increasing amounts of radioactive nucleotide to be incorporated in DNA
Radioactive nucleotide is incorporated into DNA in this study anytime there's DNA synthesis. Nucleotide excision repair requires DNA synthesis. Increasing UV doses cause increasing DNA damage and therefore corresponding increases in incorporation of radioactive nucleotides.
Prevents Strands of DNA from rejoining after being unwound
Single-stranded binding proteins
Is donor age or telemore length more important in determining how many divisoins a cell will undergo
Telomere length is important, and donor age makes little difference. The figure shows a strong relationship between telomere length and cell divisions but a weak relationship between age and cell divisions.
What would a strand look like if DNA polymerase I were defective or DNA ligase where defective
The leading strand would be continuous if either DNA polymerase I or DNA ligase were defective. However, on the lagging strand, if (a) DNA polymerase I were defective, there would be many unjoined Okazaki fragments that begin with RNA primers (because DNA polymerase I works to remove these primers); and if (b) ligase were defective, the lagging strand would have Okazaki fragments without RNA primers but with nicks separating each fragment (because ligase works to join Okazaki fragments).
A mutation that prevented DNA ligase from functioning would result in what effect on DNA synthesis?
The leading strand would continue to be synthesized, but lagging strand synthesis would be halted. DNA ligase seals the gaps between Okazaki fragments
Semiconservative replication
The parental strands of DNA separate, each one is used as a template for the synthesis of a new daughter strand. Each new daughter DNA molecule is one old and one new strand.
Which of the following statements about Okazaki fragments in E. coli is true?
They are formed on the lagging strand of DNA. While DNA is synthesized continuously on the leading strand, Okazaki fragments are formed on the lagging strand because DNA synthesis always proceeds in the 5' to 3' direction.
Which of the following enzymes is important for relieving the tension in a helix as it unwinds during DNA synthesis?
Topoisomerase This enzyme untwists the coils that occur in the DNA as it is being unwound into a single-stranded template.
Relieves Stress on the replication fork caused by unwinding
Topoisomerase II
True or false? Single-stranded DNA molecules are said to be antiparallel when they are lined up next to each other but oriented in opposite directions.
True When the 3' end of one DNA strand points in the same direction as the 5' end of the other DNA strand, the strands are said to be antiparallel.
A mutation that knocked out the proofreading function of DNA polymerase would result in __________.
a higher-than-normal rate of DNA synthesis errors. Without this proofreading function, incorrect nucleotides would be inserted about once every 100,000 bases instead of once every 10,000,000 bases.
sliding clamp
a protein ring that slides along DNA and holds the DNA polymerase III in place during replication
replisome
a work site located at the replication fork that synthesizes both leading and lagging strand at the same time leading is done linearly lagging is looped around during the process, keeping it efficient because the enzymes stay close
why is DNA replication necessary?
allows for sexual reproduction increases genetic variation maintains chromosome number
nucleotide excision repair
an example of DNA damage repair in which UV ray damage fixed. a damaged DNA strand would develop a bond between adjacent pyrimidine bases within the same strand, (thymine dimer) creates a kink in DNA structure. enzymes excise a stretch of nucleotides including the damage, and replace them in 5' -> 3' direction according to complementary strand, and re link the DNA with DNA ligase
mismatched nucleotides
are fixed by DNA polymerase which proofreads, removes, and replaces nucleotides exonuclease removes the incorrect nucleotide
topoisomerase
bind ahead of the replication fork breaks covalent bonds in DNA backbone
helicase
breaks H-bonds between bases binds at replication fork
damaged DNA
caused by UV light most common is a thymine dimer on one strand of DNA the kink/error is detected and cut out, and then replaced correctly by DNA polymerase ligase links the backbone back together
chromosomes when exiting meiosis
cells are haploid -- egg and sperm *when these fertilize they create the diploid cell that turns into an embryo and a human baby*
requirements for DNA replication
complementary base pairing antiparallel strands extension on 3' end with a free -OH group primer
After replication is complete, the new DNAs, called ---- are identical to each other.
daughter DNA
meiosis I: ___ cells enter and ____ cells exit
diploid haploid
meiosis turns a ____ cell into 4 ____ cells
diploid haploid
After DNA replication is completed, _____.
each new DNA double helix consists of one old DNA strand and one new DNA strand
nondisjunction
error in meiosis in which the homologous chromosomes fail to separate properly causes either an extra chromatid (n+1) or a missing chromatid (n-1)
what are shorter telomeres associated with?
fewer cellular divisions by the cell, not affected by age
DNA synthesis: DNA is opened
first step of synthesis helicase opens the double strand into single DNA strands, and single strand DNA-binding proteins (SSBPs) stabilize the strands to keep them from winding into a double strand topoisomerase relieves twisting build up pressure and keeps DNA from getting tightly wound
meiosis II: ____ cells enter and ____ cells exit
haploid haploid
genetic variation from meiosis: crossing over
happens in prophase only, it's when a break occurs in chromatid DNA during pairing of homologs and when synapsis occurs the chromatids are repaired with DNA from a non-sister chromatid required for the correct segregation of homologous chromosomes makes new combos of alleles
The first step in the replication of DNA is catalyzed by _____.
helicase
In a nucleosome, the DNA is wrapped around
histones.
question for hershey-chase experiment
how do we know DNA is the genetic material passed in the cell cycle?
genetic variation from meiosis: random alignment of homologs
humans have 23 pairs of chromosomes 2^23 = 8 million possible combinations of chromosomes depends on how the homologs line up before the cell is split
chromosomes when entering meiosis
in G1 there are homologous pairs made of 1 chromosome from mom and 1 from dad in S they replicate and create sister chromatids, and enter meiosis as such
DNA Polymerase I
in lagging strand synthesis, removes the RNA primer and replaces it with DNA
Which of the following damages cells and causes cell aging?
inflammation
The new DNA strand that grows continuously in the 5' to 3' direction is called the _____
leading strand.
The researchers found that telomeres were ______.
longer in individuals who exercised regularly
Replisome
macromolecular machine that copies DNA: includes DNA polymerse, helicase, primase, and other enzymes
lagging strand synthesis
makes DNA fragments called Okazaki fragments, which must be stitched together somehow: primers are removed by DNA polymerase I and replaced with DNA gaps between fragments are closed and a phosphodiester bond is formed by ligase, creating a single strand
2 chromosomal divisions of meiosis
meiosis I: parent cell is 2n, 2 daughter cells (made by separation of homologs) are n meiosis II: 2 daughter cells make 4 daughter cells that are all haploid (n) by separation of sister chromatids
eukaryotic chromosome origin of replication
multiple origins, which creates replication bubbles along the chromosome
do somatic cells have telomerase activity?
no
can haploid cells go through meiosis?
no, because their chromosomes cannot be halved and still be useful
bacterial chromosome origin of replication
one origin of replication on the circular chromosome
single-strand binding protein
prevents H-bonds between bases, binds after replication fork
The enzyme that synthesizes RNA primers for use in DNA replication is _______________. See Section 15.3 ( page 321) .
primase
hershey-chase experiment results
radioactive DNA in the pellet: was injected radioactive protein in the supernatant only: not injected
What are telomeres?
regions of DNA at the end of chromosomes which do not code for making proteins
During DNA replication, an open section of DNA, in which a DNA polymerase can replicate DNA, is called a _
replication fork
The action of helicase creates _____.
replication forks and replication bubbles A replication fork is the transition region between paired and unpaired DNA strands.
DNA synthesis: DNA is primed
second step of synthesis primase makes an RNA primer by laying down complimentary bases freely, which are comprised of A, U, C, and G
problem: replicating the ends
single stranded "overhangs" are at the end of a DNA strand because the ends cannot be replicated, and without a solution the important DNA there would get eaten away and cells would lose their functions
What are the repetitive DNA sequences present at the ends of eukaryotic chromosomes called?
telomeres Telomeres consist of many copies of a short DNA sequence that is bound by specific proteins.
the solution for replicating the ends
telomeres! end of DNA has repeats of TTAGGG sequence, which the cell has no need for this cap is created by telomerase, which uses an RNA template thus the end of the telomere that isn't double stranded gets eaten away, but nothing happens to the important DNA
An old DNA strand is used as a _____ for the assembly of a new DNA strand.
template An old DNA strand is used as a template for the synthesis of a complementary new strand.
what benefit would inhibiting telomerase activity in cancer cells have?
the cells could no longer replicate indefinitely, forcing them to eventually degrade and die
DNA synthesis: new DNA is made
third step of synthesis DNA polymerase III adds nucleotides to extending strand: it requires double stranded DNA, which is why we need primers DNA polymerase catalyzes DNA synthesis in the 5'-3' direction
why does this cancer drug that inhibits telomerase activity work best when a lipid group is added?
this makes the drug lipid soluble which means it can get through the membrane and into cancer cells
Part of DNA polymerase forms a ring- a sliding clamp like a ziploc bag
this surrounds the DNA, and another part grips the DNA strans in a way similar to hand classping rope. deoxyribo nucleotides additions catalyzed at an active site in a groove between the enzymes "thumb and fingers"
In a DNA double helix an adenine of one strand always pairs with a(n) _____ of the complementary strand, and a guanine of one strand always pairs with a(n) _____ of the complementary strand.
thymine ... cytosine This is referred to as specific base pairing.
hershey-chase experiment set up
used radioactively labeled P in DNA and S in proteins to test what heritable item is that viruses inject into DNA 1. grow viruses 2. infect E.coli 3. agitate cultures 4. pellet bacterial cells if DNA is what is injected, radioactivity will appear in the pellet if protein is what is injected, it will appear in the pellet and the supernatant
hershey-chase conclusion
viral genes consist of DNA
hershey-chase hypotheses
viral genes consist of DNA viral genes consist of protein
can haploid cells go through mitosis?
yes, because mitosis is just the replication of cells
do any organisms spend a majority of their life as a haploid cell?
yes, some fungi and yeasts do
