cell division Flashcards
1
Q
What is the cell cycle?
A
The highly ordered sequence of events that takes place in a cell, resulting in division of the nucleus and the formation of two genetically identical daughter cells • 2 main phases in eukaryotic cells: interphase and mitotic phase
2
Q
What is interphase?
A
Growth period of the cell cycle, between cell divisions (mitotic phase). Consists of stages G1, S and G2. • The cell carries out all of it major functions e.g. producing enzymes or hormones, while actively preparing for cell division
3
Q
What happens during
interphase?
A
• 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 • Normal metabolic processes of the cell occur (some, including respiration, also occur throughout cell division)
4
Q
Describe the 3 stages of
interphase
A
G1 - 1st growth phase: proteins from which organelles are synthesised are produced, and organelles replicate. The cell increase in size S - Synthesis phase: DNA is replicated in the nucleus G2 - 2nd growth phase: cell continues to increase in size, energy stores are increased, and the duplicated DNA is checked for errors
5
Q
What is the mitotic phase?
A
Period of cell divine of the cell cycle. Consists of the stages mitosis (when the nucleus divides) and cytokinesis (the cytoplasm divides and two cells are produced)
6
Q
What is the G0 phase?
A
The phase when the cell leaves the
cell cycle either temporarily or
permanently
7
Q
List the reasons why a cell
may enter G0
A
• Differentiation - A cell that becomes differentiated can no longer divide; it carries out its function indefinitely and doesn’t enter the cell cycle again • Damaged DNA - 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, eventually becoming senescent • Ageing - As you age the number of senescent cells in the body increases, linking with diseases e.g. cancer and arthritis
8
Q
Give an example of a cell
that can leave G0 after
entering
A
Lymphocytes (white blood cells) can
go back into the cell cycle and start
diving again in an immune response
9
Q
What are checkpoints and
why are they important?
A
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 They ensure the fidelity of cell division - that two identical daughter cells are created from the parent cell
10
Q
Describe what happens at each checkpoint in the cell cycle: 1. G1 checkpoint 2. G2 checkpoint 3. Spindle assembly/ metaphase checkpoint
A
1. End of G1 before entry into S. Check for: cell size, nutrients, growth factors, DNA damage 2. End of G2 before start of mitotic phase. Check for: cell size, DNA replication, DNA damage 3. At the metaphase point in mitosis. Check for: chromosome attachment to spindle
11
Q
What is the importance of
mitosis?
A
• Ensures that the 2 daughter cells produced are genetically identical to each other and to the parent cell • Needed for growth; replacement and repair of tissues in multicellular organisms; and asexual reproduction in eukaryotes
12
Q
Define the following:
- Chromosomes
- Chromatin
- Chromatids
- Centromere
A
1. Structures of condensed and coiled DNA in the form of chromatin. They become visible under the light microscope when cells are preparing to divide 2. Uncondensed DNA in a complex with histones 3. 2 identical copies of DNA (a chromosome) held together at a centromere 4. Region at which 2 chromatids are held together
13
Q
What happens before
mitosis?
A
• All of the DNA in the nucleus is replicated during interphase • Each DNA molecule is converted into 2 identical DNA molecules called chromatids • The 2 chromatids are joined at a centromere • Chromatids must be kept together during mitosis so that the can be segregated equally, 1 in each of the 2 new daughter cells
14
Q
Describe the stages of
mitosis:
A
1. Early Prophase • Chromatin fibres coil & condense to form chromosomes that take up stain to become visible under light microscope • Nucleolus disappears & nuclear membrane begins to break down • The centriole divides and the 2 new daughter centrioles move to opposite poles of the cell • Protein microtubules form a spindle fibres that will form a spindle between these centrioles • Spindle fibres attach to specific areas on the centromeres and start to move the chromosomes to the centre of the cell • By the end of prophase the nuclear envelope has disappeared 2. Metaphase • Chromosomes are moved by the spindle fibres to form a plane in the centre of the cell, called the metaphase plate equator, and then held in position 3. Anaphase • Centromere of each pair of chromatids divide • Chromatids are separated and pulled to opposite poles of the cell by the shortening spindle fibres • Chromosomes assume a V shape 4. Telophase • Chromosomes reach the poles • New nuclear envelope forms around each set • Chromosomes start to uncoil and nucleolus is formed • Cytokinesis begins
15
Q
Describe cytokinesis in
animal cells
A
• Cleavage furrow forms around the middle of the cell • Cell-surface membrane pulled inwards by cytoskeleton until it is close enough to fuse around the middle forming 2 cells
16
Q
Describe cytokinesis in plant
cells
A
• Have cell walls so cleavage furrow can’t be formed • Vesicles from Golgi apparatus assemble in the same place as metaphase plate equator • Vesicles fuse with each other and cell surface membrane dividing the cell into 2 • New sections of cell wall then form along the new sections of membrane
17
Q
What is meiosis?
A
Form of cell division when the nucleus divides twice (meiosis I and meiosis II) resulting in a halving of the chromosome number and producing 4 haploid cells from one diploid cell. Reduction division: cell division resulting in the production of haploid cells from a diploid cell
18
Q
Why is meiosis needed?
A
To produce gametes that are haploid cells, so that when they fuse in fertilisation during sexual reproduction to produce a zygote, the zygote is a diploid cell
19
Q
What are homologous
chromosomes?
A
Matching pair of chromosomes, one inherited from each parent Alleles are different versions of the same gene As they have the same genes in the same positions, they will be the same length and size when they are visible in prophase. The centromeres will also be in the same positions
20
Q
What are the stages of
meiosis?
A
Meiosis I • The 1st division is the reduction division when the pairs of homologous chromes are separated into 2 cells • Each intermediate cell will only contain 1 full set of genes instead of 2, so the cells are haploid Meiosis II • The 2nd division is similar to mitosis • The pairs of chromatids present in each daughter cell are separated forming 2 more cells • 4 haploid daughter cells are produced in total
21
Q
Describe Prophase 1
A
• Chromosomes condense; nuclear envelope disintegrates, nucleolus disappears, spindle formation begins • The homologous chromosomes pair up, forming bivalents • Chromosomes are large DNA molecules and moving them through liquid cytoplasm as they are brought together results in chromatids entangling • This is called crossing over
22
Q
Describe Metaphase 1
A
• Pairs of homologous chromosomes still in their crossed over state assemble along the metaphase plate equator • Orientation of each homologous pair on the metaphase plate is random and independent of any other homologous pair • Independent assortment results in genetic variation
23
Q
Describe Anaphase 1
A
• Homologous chromosomes are pulled to the opposite poles and the chromatids stay joined together • Sections of DNA on ‘sister’ chromatids, which became entangled during crossing over, now break off and rejoin -sometimes resulting in DNA exchange • When exchange occurs, recombinant chromatids are formed, with genes being exchanged between chromatids • Results in swapped areas of chromosome and allele shuffling
24
Q
Describe Telophase 1
A
• The chromosomes assemble at each pole and the nuclear membrane reforms • Chromosomes uncoil • Cell undergoes cytokinesis and divides into 2 cells • reduction of chromosome number from diploid to haploid is complete
25
Describe Prophase 2
```
• The chromosomes, which still
consist of 2 chromatids, condense
and become visible again
• The nuclear envelope breaks down
and spindle formation begins
```
26
Describe Metaphase 2
```
• Individual chromosomes assemble
on the metaphase plate equator
• Due to crossing over, the
chromatids are no longer identical
soother is independent assortment
again and more genetic variation is
produced
```
27
Describe Anaphase 2
• Chromatids of the individual
chromosomes are pulled to
opposite poles after division of the
centromeres
28
Describe Telophase 2
```
• The chromatids assemble at the
poles
• The chromosomes uncoil and form
chromatin again
• Nuclear envelope reforms and the
nucleolus becomes visible
• Cytokinesis results in division of
the cells forming 4 daughter cells
in total
• Cells are haploid due to reduction
division
• Cells are genetically different to
each other and the parent cell due
to crossing over and independent
assortment
```
29
Describe Erythrocytes
```
• Red blood cells
• Flattened biconcave shape to
increase SA:V ratio, for
transporting O2 around the body
• In mammals don’t have a nuclei or
many other organelles to increase
space for haemoglobin
• Flexible to squeeze through
narrow capillaries
```
30
Describe Neutrophils
```
• Type of white blood cell
• Important role in immune system
• Multi-lobed nucleus making it
easier to squeeze through small
gaps to get to the site of infections
• Granular cytoplasm containing
many lysosomes that have
enzymes used to attack
pathogens
```
31
Describe Sperm Cells
```
• Male gametes
• Deliver genetic info to female
gamete (ovum or egg)
• Have a tail/ flagellum so they can
move
• Contain many mitochondria to
supply energy needed to swim
• Acrosome on the head contains
digestive enzymes that digest the
protective layers around the egg,
allowing the sperm to penetrate
and leading to fertilisation
```
32
Describe Palisade Cells
```
• Present in the mesophyll
• Contain chloroplasts to absorb
light for photosynthesis
• Rectangular box shapes that can
be closely packed to form a
continuous layer
• Thin walls to increase rate of
diffusion of CO2
• Large vacuole maintain turgor
pressure
• Chloroplasts can move within the
cytoplasm in order to absorb more
light
```
33
Describe Root Hair Cells
```
• Present at the surfaces of roots
near growing tips
• Have long extensions called root
hairs which increase the surface
area of the cell
• Maximises the uptake of water
and minerals from the cel
```
34
Describe Guard Cells
```
• Pairs of guard cells on surfaces of
leaves form small openings called
stomata
• Stomata are necessary for CO2 to
enter for photosynthesis
• When guard cells lose water, they
become less swollen (because of
osmosis), they change shape and
the stoma closes to prevent further
water loss
• The cell wall is thicker on one side
so the cell doesn’t change shape
symmetrically as its volume
changes
```
35
What are the 4 main
categories of tissues in
animals?
```
1. Nervous tissue: Adapted to
support the transmission of
electrical impulses
2. Epithelial tissue: Adapted to
cover body surfaces, internal
and external
3. Muscle tissue: Adapted to
contract
4. Connective tissue: Adapted
either to hold other tissues
together, or as a transport
medium
```
36
Describe the Squamous
| Epithelium
```
• Made up of specialised squamous
epithelial cels
• Flat appearance
• Very thin due to the flat cells that
make it up, and because it’s only 1
cell thick
• Present when rapid diffusion
across a surface is needed
• Lining of the lungs; allows rapid
diffusion of O2 into the blood
```
37
Describe the Ciliated
| Epithelium
```
• Made up of ciliated epithelial cells
• Cells have cilia on one surface that
move in a rhythmic manner
• Lines the trachea, causing mucus
to be swept away from the lungs
• Goblet cells are also present,
releasing mucus to trap any
unwanted particles present
• Prevents particles e.g. bacteria
from reaching the alveoli once
inside the lungs
```
38
Describe Cartilage
```
• Connective tissue found in the
outer ear, nose, at the end of (and
between bones)
• Contains fibres of the proteins
elastin and collagen
• Firm, flexible tissue composed of
chondrocyte cells embedded in an
extracellular matrix
• Helps prevent the ends of bones
rubbing together an causing
damage
• Many fish have skeletons made of
cartilage not bone
```
39
Describe Muscle
```
• A tissue that needs to be able to
contract in order to move bones,
which move different parts of the
body
• Skeletal muscle fibres - muscles
which are attached to bone;
contain myofibrils which contain
contractile proteins
```
40
What are the different types
| of tissue in plants?
• Epidermis tissue - Adapted to
cover plant surfaces
• Vascular tissue - Adapted for
transport of water and nutrients
41
Describe the Epidermis
```
• Single-layer of closely packed
cells covering the surfaces of
plants
• Usually covered by waxy,
waterproof cuticle to reduce water
loss
• Stomata are present here; allow
CO2 in and out, and water vapour
and O2 in and out
```
42
Describe the Xylem Tissue
```
• Vascular tissue responsible for the
transport of water and minerals
throughout plants
• Composed of vessel elements,
which are elongated dead cells
• Walls of cells strengthened with
lignin, providing structural support
```
43
Describe the Phloem Tissue
```
• Vascular tissue responsible for the
transport of organic nutrients (esp.
sucrose) from leaves and stems
where its made by photosynthesis
to the rest of the plant
• Composed of column of sieve
tube cells, separated by perforated
walls called sieve plates
```
44
Give 3 examples of organ
| systems in animals
```
• Digestive system: takes in food;
breaks down the large insoluble
molecules into small soluble ones;
absorbs nutrients into the blood;
retains water needed by the body;
removes undigested material from
the body
• Cardiovascular system: moves
blood around the body to provide
an effective transport system for
the substances it carries
• Gaseous Exchange system: brings
air into the body so O2 can be
extracted for respiration, and CO2
can be expelled
```
45
What are stem cells?
Undifferentiated cells with the
potential to differentiate into a
variety of the specialised cell types
of the organism
46
Describe cell division in
| stem cells
```
Stem cells can undergo cell
division many times, and are a
source of new cells necessary for
growth, development, and tissue
repair
• Once stem cells have become
specialised, they lose the ability to
divide and enter G0
• If they don’t divide fast enough,
tissues are not efficiently replaced,
leading to ageing
• If there’s uncontrolled division the
form tumours, which can lead to
cancer
```
47
What is meant by the term
| ‘potency’?
```
A stem cell’s ability to differentiate
into different cell types. The great
the number of cell types it can
differentiate into, the greater its
potency
```
48
Describe totipotent stem
| cells
```
• Can differentiate into any type of
cell
• A zygote and the 8 or 16 cells from
its first few mitotic divisions are
totipotent cells
• Eventually produce a whole
organism
• Can also differentiate into extraembryonic tissues like the aminos
and umbilicus
```
49
Describe pluripotent stem
| cells
```
• Can form all tissue types, but not
whole organisms
• Are present in early embryos and
are the origin of the different types
of tissue within an organism
```
50
Describe multipotent stem
| cells
```
• Can only form a range of cells
within a certain type of tissue
• e.g. Haematopoetic stem cells in
bone marrow, because they can
form the various types of blood
cell
```
51
What is differentiation?
The process of a cell becoming
differentiated. Involves the selective
expression of genes in a cell’s
genome
52
Why is differentiation
necessary in multi-cellular
organisms?
Cells have to specialise total on
| different roles in tissues and organs
53
Describe the replacement of
| red and white blood cells
```
• Erythrocytes (red blood cells) have
a short lifespan of 120 days
• The stem colonies in the bone
marrow produce 3 billion
erythrocytes per kilogram of body
mass per day to keep up with the
demand
• Neutrophils (white blood cells)
have a lifespan of 6 hours
• The stem colonies in the bone
marrow produce 1.6 billion per kg
per hour, which increases during
infection
```
54
What are the sources of
| plant stem cells?
```
Meristematic tissue (meristems) in
plants
• Found at the tips of roots and
shoots (apical meristems)
• Found in between phloem and
xylem tissues (in the vascular
cambium). Cell in this region
differentiate into different cells
present in xylem and phloem
Pluripotent nature of stem cells in
meristems continues throughout the
life of the plant
```
55
What are the sources of
| animal stem cells?
```
Embryonic stem cells
• Present at early stage of embryo
development and are totipotent
• After 7 days a blastocyst has
formed and the cells remain in a
pluripotent state in the foetus until
birth
Tissue (adult) stem cells
• Present throughout life from birth
• Found in specific areas e.g. bone
marrow
• Multipotent
• Stem cells can also be taken from
the umbilical cords of newborn
babies
```
56
List of some the potential
areas where stem cells
could be used
* Heart disease
* Type 1 diabetes
* Parkinson’s disease
* Alzheimer’d disease
* Macular degeneration
* Birth defects
* Spinal injuries
57
List some of the areas
where stem cells are already
used
```
• Burn treatment: Stem cells grown
on biodegradable meshes can
produce new skin for burn patients
faster than the normal process of
taking a graft from another part of
the body
• Drug trials: Potential new drugs
can be tested on cultures of stem
cells before animals and humans
• Developmental biology: Allows
more study of the changes that
occur as multicellular organisms
grown and develop, and why
sometimes things go wrong
```
58
What are the arguments
against the use of stem
cells?
```
• Religious and moral objections
• Many people believe that life
begins at conception, and so the
destruction of embryos is murder
• Lack of consensus as to when the
embryo itself has rights, and who
owns the genetic material that is
being used for research
```
