Other
OCR Biology AS Level (6)
- Course
- Institution
- Book
Typed up notes
[Show more]
Connected book
Book Title:
Author(s):
- Edition:
- ISBN:
- Edition:
More summaries for
-
Document with link to ANKI flashcards for OCR A Level Biology A, year 2
-
Document with link to ANKI deck for AS level Biology content
-
Link to ANKI Flashcards for ALL of OCR-A, A Level Biology
Seller
Follow
dhilanraghwani
Content preview
Chapter 6: Cell division6.1: The cell cycle
Phases of the cell cycle
In eukaryotic cells the cell cycle has 2 main phases – interphase and mitotic place
Interphase
Cells do not divide continuously – long periods of growth and normal working separate divisions.
These periods are called interphase and a cell spends most of its time in this phase.
Interphase is sometimes referred to as the resting phase as cells are not actively dividing.
However, this is not an accurate description - interphase is actually a very active phase of the cell cycle,
when the cell is carrying out all its major functions such as producing enzymes or hormones, while also
actively preparing for cell division.
During interphase:
DNA is replicated and checked for errors in the nucleus
protein synthesis occurs in the cytoplasm mitochondria grow and divide, increasing in number in the
cytoplasm
chloroplasts grow and divide in plant and algal cell cytoplasm, increasing in number
the normal metabolic processes of cells occur
The three stages of interphase are:
G1
o the first growth phase: proteins from which organelles are synthesised are produced and organelles
replicate. The cell increases in size.
S - synthesis phase
o DNA is replicated in the nucleus.
G2 - the second growth phase
o the cell continues to increase in size, energy stores are increased, and the duplicated DNA is checked
for errors.
Mitotic phase
The mitotic phase is the period of cell division.
Cell division involves two stages:
o Mitosis - the nucleus divides.
o Cytokinesis – the cytoplasm divides and two cells are produced.
G0
G0 is the name given to the phase when the cell leaves the cycle, either temporarily or permanently.
There are several reasons for this including:
o Differentiation - A cell that becomes specialised to carry out a function is no longer able to divide. It
will carry out this function indefinitely and not enter the cell cycle again
o The DNA of a cell may be damaged, in which case it is no longer viable. A damaged cell can no longer
divide and enters a period of permanent cell arrest (G0). Most normal cells only divide a limited
number of times and eventually become senescent.
o As you age, the number of these cells in your body increases. Growing numbers of senescent cells
have been linked with many age-related diseases, such as cancer and arthritis.
A few types of cells that enter G0 can be stimulated to go back into the cell cycle and start dividing again, for
example lymphocytes in an immune response.
Control of the cell cycle
, It is vital to ensure a cell only divides when it has grown too the right size, the replicated DNA is error-free,
and the chromosomes are in their correct positions during mitosis.
This is to ensure the fidelity of cell division - those two identical daughter cells are created from the parent
cell.
Checkpoints are the control mechanisms of the cell cycle.
They monitor and verify whether the processes at each phase of the cell cycle have been accurately
completed before the cell is allowed to progress into the next phase.
Checkpoints occur at various stages of the cell cycle:
G1 checkpoint - This checkpoint is at the end of the G1 phase, before entry into S phase. If the cell satisfies
the requirements of this checkpoint it is triggered to begin DNA replication. If not, it enters a resting state
(G0)
G2 checkpoint - This checkpoint is at the end of G2 phase before the start of the mitotic phase. For this
checkpoint to be passed, the cell must check several factors, including whether the DNA has been replicated
without error. If this checkpoint is passed, the cell initiates the molecular processes that signal the beginning
of mitosis
Spindle assembly checkpoint: This checkpoint is at the point in mitosis where all the chromosomes should be
attached to spindles and have aligned. Mitosis cannot proceed until this checkpoint is passed.
6.2: Mitosis
The importance of mitosis
Mitosis is the term usually used to describe the entire process of cell division in eukaryotic cells.
It refers to nuclear division, an essential stage in cell division.
Mitosis ensures that both daughter cells produced when a parent cell divide are genetically identical
Each new cell will have an exact copy of the DNA present in the parent cell and the same number of
chromosomes.
Mitosis is necessary when all the daughter cells must be identical.
This is the case during growth, replacement, and repair of tissues in multicellular organisms such as animals,
plants, and fungi.
Mitosis is also necessary for asexual reproduction, which is the production of genetically identical offspring
from one parent in multicellular organisms including plants, fungi, and some animals, and in eukaryotic
single-celled organisms such as Ameoba species.
Prokaryotic organisms, including bacteria, do not have a nucleus and they reproduce asexually by a different
process known as binary fission
Chromosomes
Before mitosis can occur, all the DNA in the nucleus is replicated during interphase
Each DNA molecule is converted into two identical DNA molecules, called chromatids.
The two chromatids are joined together at a region called the centromere.
It is necessary to keep the chromatids together during mitosis so that they can be precisely manoeuvred and
segregated equally, one each into the two new daughter cells
During interphase DNA combines with proteins called histones to form a dense complex called chromatin.
The stages of mitosis
There are four stages of mitosis - prophase, metaphase, anaphase, and telophase.
They are described separately but in fact they flow seamlessly from one to another.
Each of these phases can be viewed and identified using a light microscope.
Dividing cells can be easily obtained from growing root tips of plants.
The root tips can be treated with a chemical to allow the cells to be separated – then they can be squashed
to form a single layer of cells on a microscope slide.
Stains that bind DNA are used to make the chromosomes clearly visible.
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller dhilanraghwani. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $9.02. You're not tied to anything after your purchase.
Can Stuvia be trusted?
4.6 stars on Google & Trustpilot (+1000 reviews)
53068 documents were sold in the last 30 days
Founded in 2010, the go-to place to buy study notes for 14 years now