Module 11: DNA Replication and Cell Division Flashcards
How can a cell make more cells?
use a process known as cell division
Why does cell division occur?
- cell growth
- cell replacement
- cell healing
- cell reproduction
What are the requirements for cell division?
- after cell division, the two daughter cells that result must each receive all of the genetic material found in the single-parent cell
- the parent cell needs to be big enough to divide in two, so each daughter cell receives adequate cytoplasmic components
Prokaryotic cells divide by:
binary fission
Eukaryotic cells divide by:
mitosis and cytokinesis
Binary Fission
- process of cell division in prokaryotes
- DNA replication, circular DNA molecule
- Increase cell size
- division into two daughter cells, each daughter cell receives one copy of the replicated parental DNA
Steps of Binary Fission
- proteins bind the circular genome to the inner surface of the plasma membrane
- DNA replication starts at a certain spot on the molecule and travels around the circle in opposite directions
- two DNA molecules are produced, both of which are affixed to the cell membrane
- the two DNA attachment sites separate as the cell elongates during binary fission
- a constriction forms at the midpoint of the cell when it is about twice its original size and the DNA molecules are well-separated (cell wall separates into two daughter cells)
What happens at the location of constriction for binary fission
- a new membrane is created
- a new cell wall is created
- this produces two daughter cells that are identical to the parent cell
Eukaryotic Division
- eukaryotic cells reproduce by mitotic cell division
- the genome in eukaryotes are large and linear
- because it is in the cell nucleus of a eukaryote the genetic material is isolated from the other components of the cell
Eukaryotic Division VS Prokaryotic Division
3
GENOME
E: genome is large and linear
P: genome is small and circular
LOCATION OF DNA
E: nucleus
P: cytoplasm
MEMBRANE
E: nuclear membrane needs to be broken down and then restored for complete DNA distribution to the daughter cells
P: DNA is attached to cell membrane and cell growth allows for separation into daughter cells
What is a genome?
the genetic material of an organism
Examples of Genomes:
bacteria: bacterial genome
nucleus: nuclear genome
mitochondria: mitochondrial genome
chloroplast: chloroplast genome
Genome Size
genomes are measured in number of base pairs
- a thousand base pairs is a kilobase (Kb)
- million is a mega base (Mb)
- billion is gigabase (Gb)
** there is no relationship between genome size and organismal complexity amongst eukaryotes**
Bacterial Genome Organization
- bacterial genomes are circular
- forms a structure with multiple loops called a nucleoid
- the loops are bound together by proteins
Eukaryotic Genome Organization
- DNA in the nucleus is packaged differently than bacteria
- DNA is packaged with proteins to form a DNA protein complex called a chromatin
- forms a fiber that is 30 nm in diameter
- in order to form chromatin the eukaryotic DNA is first wrapped around a group of histone proteins to form a nucleosome
- the DNA strand wraps twice around each histone, making it look like beads on a string
- when mitosis or meiosis begins, the chromosomes will be fully condensed
- this forms the characteristic shape of the chromosomes we see in a karyotype
- chromosomes become visible only in cells about to divide
Stem cells
an undifferentiated cell that can undergo an unlimited number of mitotic divisions and differentiate into any of the large number of specialized cells
Somatic cells
a nonreproductive cell and the most common type of cell in the body of a multicellular organism
Germ cells
a reproductive cell that produces gametes (sperm or eggs)
In eukaryotes, cell division occurs through a series of stages known as the ____
cell cycle
What are the two distinct cell cycle stages?
- the time during which the parent cell divides into two daughter cells, M phase
- the time between two successive M phases, interphase
M phase
- parent cell divides into two daughter cells and consists of:
– separation of replicated chromosomes, mitosis
– division of the cytoplasm into two daughter cells, cytokinesis
How long is interphase?
lasts about 10-14 hours
What are two things the cell does in preparation for cell division?
- DNA replication in the nucleus
- increase the size of the cell
What are the four stages of interphase?
G1 phase:
- increase in cell size and protein content
- first “gap” phase
- preparing the cell for S phase
- synthesis and activation of regulatory proteins
S phase:
- the synthesis phase
- replication of DNA
G2 phase:
- second “gap” phase
- cell prepares for M phase
G0 phase:
- separate from G1 phase, no active preparation for cell division
- occurs in cells that do not actively divide, like liver cells
DNA replication is ___________
semiconservative
- two strands of parental DNA unwind and each strand is a template for synthesis of daughter strand
At the end of DNA replication, each new DNA molecule consists of:
One old parental strand and one newly synthesized strand
Helicase
- unwinds the parental double helix at the replication fork
- allows a single strand of DNA to be available for complementary base-pairs to be added by DNA polymerase
Single-strand binding protein
- binds to the single-stranded regions of the parental strands
- prevents the parental strands from coming back together
Topoisomerase
- works upstream of the replication fork
- changes the supercoiled state of DNA caused by the unwinding of the double helix at the replication fork
DNA polymerase
- adds bases to the nucleotide strand
- requires 4 deoxyribonucleotides
– dATP, dCTP, dGTP, dTTp - requires a DNA template & RNA primer strand with 3’-OH terminus
- can only synthesize DNA in a 5’ to 3’ direction, just like transcription
- most DNA polymerases can correct mistakes that may happen during replication
RNA primase
- synthesizes a short piece of RNA that is complementary to a sequence of the DNA parental strand
- is needed so the DNA polymerase can add DNA bases to the growing chain
Since newly synthesized DNA can be elongated only at the 3’ end, the two daughter strands use different replication mechanisms.
One strand grows______ the replication fork & synthesized _____, called the _______
One strand grows______ from the replication strand & synthesized _____ as _____, called the _______
toward, continuously, leading strand
away, discontinuously, fragements, lagging strand
Leading Strand
- has its 3’ end pointing toward the replication fork
- is synthesized as one long continuous polymer as the parental strand is unwound
Lagging Strand
- has its 3’ end pointing away from the replication fork
- it is synthesized in short, discontinuous pieces called Okazaki fragments
- a new short piece of the lagging strand is initiated at intervals as the parental DNA strand is unwound at the replication fork
- need to:
– add an RNA primer
– then have DNA polymerase extend the RNA primer
– then replace the RNA primer with DNA bases - the piecing of DNA is a necessity of DNA replication due to synthesis in one direction, 5’ to 3’ direction
RNA Primers
- the short RNA primers are added by an RNA polymers, ie RNA primer
- synthesizes a short piece of RNA complementary to the DNA
- once the primer has been synthesized, DNA polymerase takes over and elongates the primer adding DNA nucleotides
– until it hits the fragment in front of it
– a different DNA polymerase removes the primer and replaces it with DNA
DNA Ligase
when the replacement of the RNA primer with new DNA is complete, the fragments are joined together with an enzyme, DNA ligase
- completes the sugar-phosphate backbone of the new DNA
Synthesizing Leading & Lagging Strands
- the leading and lagging strands are synthesized at the same time
- this is accomplished by looping one of the strands of DNA, the trombone model
Proofreading
- most DNA polymerases can correct their own errors through proofreading
- hydrogen bonds temporarily hold the new nucleotide and the base across the way in the template strand, gives it an opportunity to check for errors, can easily remove if wrong
- DNA polymerase can correct errors because it detects the mispairing in hydrogen bond formation
- DNA polymerase activates a cleavage function:
– removes the incorrect nucleotide
– then inserts the correct one in its place
Prokaryotic Replication
- replication of circular DNA
- happens in most bacteria
- both mitochondrial and chloroplast DNA also replicate in this way since they are circular
Circular Chromosome Replication
two special features of replication of circular chromosomes
1. there is a single origin of replication
2. replication proceeds in both directions until the replication forks meet and fust on the opposite side, this completes one round of replication
Eukaryotes have linear DNA and there are multiple origins of replication, what does this result in?
multiple replication forks
Each replication for does what and has what two things?
proceeds bidirectionally (each replication bubble has 2 replication forks that move in opposite directions
- each replication fork has a leading and lagging strand
What happens when two replication bubbles meet?
DNA ligase seals the gap in the sugar-phosphate backbone (fuse to make one big bubble)
The leading DNA strand can be replicated all the way to where?
the end of the template DNA strand
What issue rises arises because DNA is linear?
- the lagging strand needs enough single-stranded template DNA to begin the next Okazaki fragment
– the final RNA primer is added about 100 bucleotides from the 3’ end of the template
– when the primer is removed, a section of template DNA remains unreplicated - each time the DNA is replicated one strand is shortened
- eventually the DNA would severely shorten after several cycles
Telomeres
the end of the linear chromosomes
- the repeated sequence and the number of repeats varies between species
- in humans the sequence is 5’-TTAGGG-3’
- repeated 1500-3000 times at each telomere
-the enzyme telomerase can extend the ends of the chromosome to address chromosome shortening
- in some cell types the missing nucleotides are replaced with the enzyme **
- this extends the ends of the chromosome to address chromosome shortening
Does every cell have telomerase activity?
no. teleomerase activity differs form one cell type to the next.
- almost inactive in adult somatic cells
What cells have fully active telomerase activity?
Stem cells and Germ (sex) cells that produce eggs or sperm
For how long can mitotic division occur before the telomeres are too short that the cell stops dividing?
can divide about 50 times. called the hayflick limit
Telomerase make up
- ribonucleoprotein, its a protein-RNA complex
- carries its won primer, template RNA
- has reverse transcriptase acitvity
– RNA –> DNA - adds nucelotides to 3’-OH end of lagging strand template to prevent shortening, it polymerizes deoxyribonucleotides directed by RNA template
- this RNA template is part of the enzyme
- it is complementary to the telomeric repeats
- after eleongation of 3’ end of leading template, synthesis of lagging strand can contnue form the new RNA primer
- results in an extra 3’ overhang which can form loops at the end of chromosomes
– protects from degradation
Chromatin can be looped and packaged to form what?
chromosomes
A cell with one copy of each chromosme is a
haploid
A cell with two copies of each chromomse is a
diploid
Whats needed for cell divison to proceed?
every chromosome in the parent cell mustbe duplicated so that each daughter cell recieves a full set of chromosomes
- this duplication takes place during S phase
After DNA replication there are two identical copies called:
sister chromatids
- even though the DNA in each hromosome is duplicated they do not seperate
- they stay side by side, held togteher at the centromere
- hard to see the chromsomes during interpahse, not condensed
When do chromosomes become condensed?
as the cell moves from G2 to the start of mitosis
What can be determined using a microscope?
each of the five stages of mitosis, depending on the postion of the chromosmes
mitosis=_______=_________
karyokinesis = nuclear division
M phase Stage 1
Prophase
- characerized by the appearance of visible chromosomes
- outside te nucleus the cell assembles a structure known as the mitotic spindle microtubules
- made up of mircrotubules that pull the chromomses apart into spereate daughter cells
- the microtubule-organizing ceners for animal cells are centrosomes
– are duplicated and begin to migrate to opposite poles
— plant cells also have microtubule-based mitotic spindles, but they lack centrosomes
M phase Stage 2
Prometaphase
- the nuclear membrane breaks down and the microtubules of the mitotic spindle attach to the chromosomes
- the microtubules grow and shrink to explore the region that was once the nucleus
- as the ends of the microtubule encounter chromosomes, they attach to the centromeres of the chromosomes
- each centromere is associated with two protein complexes called kinetochores
– one kinetochore on each side of the centromere
- each kinetochore is associated with one of the two sister chromatids
— forms the site of attachment for single microtubule
– arrangement is key for the separation of sister chromatids later in mitosis
– each sister chromatid is attached to a microtubule radiating from one of the poles of the cell
M phase Stage 3
Metaphase
- one of the most visually distinctive
- the spindle microtubules lengthen and shorten
- this pushes the chromosomes toward the center of the cell, align at the metaphase plate
- this is a single plane that is roughly equidistant from both spindle poles
M phase Stage 4
Anaphase
- the centromere divides and kinetochore microtubules shorten
– this pulls the chromatids apart toward the centrosome, ensures that one chromatid from each pair go to opposite poles
M phase Stage 5
Telophase
- complete set of chromosomes arrives at a spindle pole and cytosolic changes occur in preparation for the cells division
- the microtubules of the mitotic spindle break down and disappear
- the nuclear envelopes begin to reform around each set of chromosomes, creates two new nuclei
- once the nuclear envelope is reformed the chromosomes decondense
– they become less visible
- telophase marks the end of mitosis
Cytokinesis in Animal Cells
- next step in M phase is the division of the parent cells into two daughter cells
- cytokinesis begins with a ring of actin filaments that forms on the inner face of the cell membrane called the contractile ring
– forms at the equator of the cell perpendicular to the axis of what was the spindle - the ring contracts, pinches the cytoplasm of the cell
– divides it into two daughter cells
Cytokinesis in Plant Cells
- mitosis for the most part is similar in animal and plant cells but cytokinesis is different
- this is because plant cells have a cell wall
– need to construct new cell wall for division to occur - the plant cells form a structure called the phragmoplast in the middle of the cell during telophase
- the phragmoplast consists of overlapping microtubules
– these guide vesicles containing cell wall components to the middle of the cell - then during anaphase and telophase the vesicles fuse to form a new cell wall in the middle of the dividing cell called the cell plate
- the cell plate will fuse the original cell wall at the perimeter of the cell
– this completes cytokinesis
What is the goal of Meiosis?
produce daughter cells with exactly half as many chromosomes as the parent cell
What does it mean to go from diploid to haploid?
there is a reduction in chromosome number
Reduction in chromosome number is used in what? and to produce what?
used in sexual reproduction to produce sperm and egg cells, gametes
Why are not all gametes genetically identical?
because meiosis is a source of variability
Gametes will fuse during fertilization to form what?
a new organism
Once two gametes have fused to back a new organism what is their status of chromosomes?
they are now back to diploids, with a new genome
Meiosis has:
- one round of DNA synthesis
- two rounds of cell division, meiosis 1 and meiosis 2
- by the end of meiosis 1, homologous chromosomes separate
- by the end of meiosis 2 sister chromatids separate
Meiosis 1 - Prophase 1
- the DNA replication is the same as mitosis, before prophase 1 of meiosis 1
- each chromosome has become two sister chromatids held together at the centromere
- homologous chromosomes pair with each other and become physically connected along their length, called synapsis
- once synapsis is complete there is a four-stranded structure that includes:
– two pairs of sister chromatids - the whole unit is known as a bivalent
– chromatids attached to different centromeres, non-sister chromatids
After bivalent has happened, crossing over happens at crosslike structures called…
chiasma (s)
chiasmata (pl)
What does chiasma allow?
homologous chromosomes of maternal origin and paternal origin to undergo an exchange of DNA segments
- a random process that results in completely unique chromosomes when meiosis is complete
- no nucleotides are gained or lost in this process
- crossing over also helps hold together the bivalents for metaphase 1
Meiosis 1
- the nuclear envelope breaks down and he meiotic spindles attach to kinetochores during prophase 1
- at the end of the first meiotic division, there’s anaphase 1 and telophase 1
– the homologous chromosomes have separated, the resulting cells are haploid (only 1 one of each sister)
Meiosis 2
- there is no DNA synthesis between the two meiotic divisions
- the process of meiosis 2 resembles that of mitosis
– except that the nuclei in prophase 2 have the haploid number of chromosomes
— no the diploid number - sister chromatids will separate during this stage of meiosis
— results in gametes - meiosis 2 is often called the equational division
– cells in meiosis 2 have the same number of chromosomes at the beginning and end
Mitosis vs Meiosis
- sister chromatids separate from one another in meiosis 2, this is like mitosis
- this similarity suggests that meiosis evolved from mitotic cell division
- similarity in meiotic division among eukaryotes suggests that meiosis evolved in the common ancestor of eukaryotes
Explain how cytoplasmic division in meiosis is unequal in females and males:
Females
- unequal cytoplasmic division resulting in:
– one cell with most of the cytoplasm, the oocyte (egg)
– three polar bodies with only small amounts of cytoplasm
Males
- equal cytoplasmic division
– most of the cytoplasm is eliminated
- Combining of a sperm and egg during fertilization restores the diploid state
- the production and joining of gametes increase genetic diversity
It is important to control the cell cycle. What are two events that are critical?
- initiation of DNA replication, G1/S transition
- initiation of mitosis, G2/M transition
- cells must have regulatory mechanisms to progress through the cycle
- without these mechanisms, the cell may undergo cell death or divide uncontrollably -> cancer development
Through the cell cycle there is cyclic activity of protein kinases, what do they do?
their combined activity controls cell cycle progression
What are the proteins and Kinases that control cell cycle activity?
Cyclins
- regulatory proteins subunits of specific protein kinases
– their levels rise and fall with each turn of the cell cycle
CDKs
- kinases that phosphorylate other proteins whose actions are necessary for the cell cycle to progress
– always present in the cell
– active only when bound to appropriate cyclin
There are several different cyclins and CDK, act at specific steps of the cycle.
During G1 phase:
- the levels of cyclin D and E rise and activate CDKs
- it prepares the cell for S phase
– for example, it activates transcription factors that lead to the expression of DNA polymerase
There are several different cyclins and CDK, act at specific steps of the cycle.
During S phase:
- cyclin A levels increase activating CDKs that initiate DNA synthesis
There are several different cyclins and CDK, act at specific steps of the cycle.
During G2 phase:
- levels of cyclin B rise activating CDKS that initiate multiple events associated with mitosis
– breakdown of nuclear envelope in prometaphase
– formation of mitotic spindles
At various stages of the cell cycle, checkpoints make sure the cell is ready to move on to the next phase.
What are the three major stages in the cell cycle?
- DNA Damage Checkpoint
- checks for damaged DNA before it enters S phase - DNA Replication Checkpoint
- checks for the presence of unreplicated DNA at the end of G2 before the cell enters mitosis - Spindle Assembly Checkpoint
- check that all of the chromosomes are attached to the spindle before the cell progresses with mitosis
If DNA is damaged by radiation, what happens?
- it activates a protein kinase that phosphorylates the p53 protein
- when p53 is phosphorylated its levels in the nucleus rises, this activates the transcription of several genes
- one of which is an inhibitor that blocks the activity of the G1/S cyclin-CDK complex, arrests the cell at the G1?S transition
– gives the cell time to repair the DNA damage
What is p53 often called?
Guardian of the genome
- if issue, will stop cycle and fix before duplicate
What happens if the damage in the DNA is not repaired in time?
Once p53 is activated the DNA needs to be repaired quickly because phosphorylated p53 also:
- stimulates transcription of the bax gene which codes for the Bax protein
- represses transcription of the Bcl-2 gene which codes for the Bcl-2 protein
- in a healthy cell there is a balance of Bax & Bcl-2 , makes Bax/Bcl-2 dimers
- this will shift the overall concentrations of Bax & Bcl-2 which results in the formation of Bax/Bax dimers
The increase of Bax/Bax dimers activates what?
the pathway for programmed death
- apoptosis
– this results in a controlled and orderly disintegration of the cell
What is Apoptosis important for?
important in the developing embryo, allows for remodeling of tissue
- ex, developing hands look like paddles until cells are selectively killed
in the adult
- maintenance of tissue size, eliminates excess cells to balance cell proliferation
- elimination of specialized cells, ex removal of activated T lymphocytes that are no longer required
- elimination of genetically damaged cells, cells that are irreparable and need to be removed
The loss of the ability to undergo apoptosis is a step to becoming what?
cancerous
What is cancer?
uncontrolled cell division
Cancer causing genes are known as…
oncogenes
Where was cancer first discovered?
in viruses
- also altered versions of proto-oncogenes, normal genes important in cell division
– have the potential to become mutated oncogenes
What are tumor suppressor genes?
inhibit cell division
- p53 protein is an example
The key is that cell division is regulated by…
- proto-oncogenes that promote cell division
- and tumor suppression genes that inhibit cell division
- this is a counterbalance system, must agree for cell division to take place
- balance between cell division, promotion and inhibition
Key features of cancer cells:
- ability to divide on their own without growth signals
- resistance to inhibitory or cell death signals
- ability to invade tissues, metastasis
- promote new blood vessel formation, angiogenesis
