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