Cell Division
Why do cells need to divide?
They divide because as a cell grows, its volume increases faster than its surface area, making nutrient absorption and waste removal insufficient
What haopens when a cells surface area is not enough to support its volume?
The cell divides into two smaller cells to restore a favorable surfsce area-to-volume ratio
What energetix processed occur mainly at the cell surface?
The proton motive force and ATP synthesis.
Why is cell division essential in multicellular eukaryotes?
For development, growth, repair, replacement, and call specialisation
How quickly do mammalian cells divide?
Every 12-24 hours
How quickly do bacterial cells divide?
Every 20-30 minutes
What role does cell division play in unicellular organisms?
It reproduces the entire organism
What type of cell division created genetically identical daughter cells?
Mitosis creates genetically identical daughter cells
What is the exception to genetically identical cell division?
Meiosis, which produces sperm and egg cells.
What do cells need before they can divide?
A complete copy of genetic material and division machinery.
In which phase does the cell physically divide?
In the M phase (mitosis or meiosis)
When is DNA replicated?
DNA is replicated during the S phase.
What does the cell need befote DNA replication can begin?
Enough replication machinery and nucleotides that are prepared in G1 phase
What are eukaryotic chromosomes made of?
They are made of Chromatin, a combination of DNA and proteins
How many chromosome sets do somatic cells have?
2 Sets (diploid)
How many chromosome sets do gametes have?
One set (haploid)
How many chromosomes do humans have?
46 chromosomes (23 pairs)
What does diploid mean?
Having chromosomes in pairs
Are all eukaryotes diploid most of the time?
No, some eukaryotes are mostly haploid and be ome diploid only briefly, some plants alternate between haploid and diploid stages
Do all somatic cells in an organism have the same chromosome number?
Yes,all somatic cells have the same kind and number of chromosomes.
What happens to chromosomes before a eukaryotic cell divides?
Chromosomes replicate and condense before a eukaryotic cell can divide.
What are sister chromatids?
They are identical copies of a chromosome joined together after DNA replication
What holds sister chromatids together?
Cohesins.
What is a centromere?
The narrow region where sister chromatids are most tightly attached
What must chromosomes do before a cell divides?
They must replicate and then seperate correctly.
What controls the timing of cell division?
The cell cycle, which includes key regulatory checkpoints.
What are the two major parts of the cell cycle?
Interphase and Mitotic (M) phase
What happens during interphase?
The cell grows, performa normal functions and copies its DNA and organelles
How much of the cell cycle is spent in interphase
About 90%
What are the subphases of interphase?
G1, S & G2
In which phase are chromosomes duplicated?
Chromosomes are duplicated in the S phase
What do G1 and G2 phases have in common?
Both involve cell growth.
Whatxis the G0 Phase?
A long or permanent non-dividing state that some cells enter from G1
What happens to chromosomes during interphase?
They unwind into long thin strands of DNA.
What is a replication origin?
A replication origin is a specific DNA sequence where replication begins
Why do eukaryotes have many replication origins?
To allow fast replication of large chromosomes
What is a replication bubble?
A replication bubble is a region where DNA strands have seperated and replication occurs
What enzyme seperates DNA strands during replication?
Helicase seperates DNA strands during replication
What type of bonds foed helicase break?
Helicase breaks weak hydrogen bonds betweem base pairs
What is meant by semiconservative replication?
Semiconservative means a new DNA molecule has one old strand and one newly made strand
Which end of a DNA strand can DNA polymerase add nucleotides to?
Only the 3' end.
In which direction is a new DNA strand built?
5' -> 3'
What does DNA polymerase need becore it can start adding nucleotides?
An RNA primer
What is the leading strand?
The strand synthesised continuously from the origin towards the replication fork
What is the lagging strand?
The strand sunthesised discontinuously in short fragments
Wjat are the short fragments on the lagging strands called?
Okazaki fragments
In which direction is the lagging strand synthesised?
From the replication fork back towards the origin
Why must the DNA double helix unwind during replication?
To expose botn strande as templates
What must the DNA molecule do as it unwinds?
Rotate, about one full turn every 10 base pairs
Why is rotation a problem for large chromosomes?
A long chromosome would require millions of rotations, creating intense tension.
What enzyme relieves the stress caused by DNA unwinding?
Topoisomerase.
Where does topoisomerase act during replication?
At the replication form to reduce twisting strain
Are cells allowed to divide indefinitely?
No each cell is limited to a certain number of divisions before programmed cell death.