2.1.6 - Cell division Flashcards
What is the cell cycle ?
The sequence of events that takes place in a cell to enable growth and repair
What are the two main phases of the cell cycle ?
- Interphase
- Mitotic phase
What does a cell do during interphase ?
- Cells carry out their major functions
- Protein synthesis occurs in the cytoplasm
- Organelles grow and divide in the cytoplasm
- Normal metabolic processes (some continue throughout cell division e.g. respiration)
- DNA is replicated and checked for errors in the nucleus
What are the three stages of interphase ?
- G1 - growth phase 1
- S - synthesis phase
- G2 - growth phase 2
What occurs in the G1 phase of interphase ?
- Proteins for organelle synthesis are produced ; helicase, polymerase and ligase enzymes are produced
- Organelles are replicated
- The cell increases in size
What occurs in the S phase of interphase ?
Chromosomes are duplicated in the nucleus
What occurs in G2 of interphase ?
- Cell continues growth
- Energy stores are increased
- Cell double checks duplicated chromosomes for errors and makes any necessary repairs
What is G0 of the cell cycle ?
- Phase when cells leave the cell cycle either temporarily or permanently.
- A few types of cells that enter G0 can be stimulated to go back into the cell cycle and start dividing again
What can be stimulated to go back into the cell cycle after G0 ?
Lymphocytes in an immune response
Why might a cell leave the cell cycle?
- The cell has differentiated
- The DNA of the cell is damaged and therefore becomes senescent.
- Age. The number of senescent cells increases with age and has been linked with age related diseases.
What is senescence ?
Senescence is the gradual deterioration of functional characteristics in living organisms
What occurs in the G1 checkpoint ?
- Checks that chemicals needed for replication are present
- Checks for any damage to DNA before S phase
- Checks for cell size
- Checks that the nutrients required for replication are present
What occurs in the G2 checkpoint ?
- Checks for cell size
- Checks for DNA replication
- Checks for any errors in the DNA and repairs mistakes
What occurs in the spindle assembly checkpoint ?
- Checks if spindle fibres are connected to the chromosomes properly
- Checks that the chromosomes have aligned ready for metaphase
What is the spindle assembly checkpoint also known as ?
It is also known as the metaphase checkpoint - mitosis can not proceed until this checkpoint is passed
What are the two parts of the mitotic stage of the cell cycle ?
- Mitosis - The nucleus divides
- Cytokinesis - The cytoplasm divides and two cells are provided
Define chromatid
Two identical copies of DNA (a chromosome)
Define sister chromatid
Two identical DNA molecules joined by a common centromere
Define chromatin
Uncondensed DNA which is in complex with histone proteins
Define chromosomes
Structures of condensed and coiled DNA molecules in the form of chromatin
Define homologous pairs
- A pair of chromosomes, one maternal and one paternal
- Does not necessarily have the same alleles
Define centromere
- Region at which two sister chromatids are held together
- Point where mitoticspindle fibresattach to pullsister chromatidsapart duringcell division.
Define centrioles
Component of the cytoskeleton made of microtubules and associated proteins, involved in the development of spindle fibres
What are centrosomes ?
Centrosomes are pairs of centrioles and they are the sub-cellular region which organise the cell’s microtubules
Why are centrioles important in cell division ?
- During mitosis the spindle fibres start at the centrosomes
- Spindle fibres are responsible for the movement of chromosomes within the cell
Define spindle fibres
A structure made of microtubules and associatedproteins that assemble from the centrosome to provide thestructure that moves chromosomes.
What is mitosis ?
Mitosis is the division of the nucleus into two genetically identical nuclei
What are the four stages of mitosis, in order ?
- Prophase
- Metaphase
- Anaphase
- Telophase
PMAT
Why is mitosis important ?
- It ensures thetwo daughter cellsproduced are genetically identical
- Each cell produced has anexact copy of the DNA present in the parent cell, and the same number of chromosomes
- Growth, repair and replacementof cells in multicellular organisms
- Asexual reproductionby Eukaryotic organisms
Why do single celled organisms use mitosis ?
To reproduce asexually
What is mitosis in bacteria called ?
Binary fission
How is DNA packaged ?
- Wrapped around histone proteins to form chromatin
- Coiled around more proteins to form chromosomes
Why does DNA wrap around histones ?
- Makes it more compact
- Makes it easy to access
What happens in prophase ?
- Chromatinfibres condense into chromosomes
- Nuclear membrane begins to break down andnucleolus disappears
- Each duplicated chromosome appears astwo identical sister chromatids joined at their centromeresand, in some species, all along their arms by cohesins
- Themitotic spindlebegins to formfrom centrosomes and the microtubules from them.
What happens in early metaphase ?
- Thenuclear envelope fragments
- Microtubules extending from each centromere can now invade the nuclear area
- The chromosomes have become evenmore condensed
- Each of the two chromatids of each chromosome now has akinetochore
- Centrioles taken to opposite poles for formation of the spindle fibres
- Some of the microtubules attach to the kinetochores
- Non-kinetochore fibres interact with those from the opposite pole of the spindle.
What happens in late metaphase ?
- Chromosomes are moved by spindle fibres toline up along the equatorial plateof the cell known as themetaphase plate
What happens in anaphase ?
- Cohesin proteins are cleaved allowing the sister chromatids to be pulledapartby spindle fibres
- Daughter chromosomes move towards the poles of the cell as their kinetochore microtubules (spindle fibres) shorten
- Thecell elongatesas the non-kinetochore microtubules (spindle fibres) lengthen
- Results in two poles of cell having equivalent, and complete, collections of chromosomes.
What happens in telophase ?
- Two daughter nuclei formin the cell - nuclear membrane forms
- Nucleolireappears
- Chromatids are now calledchromosomes whichbecome less condensed and more distinct
- Remaining spindle fibres are depolymerised
- Mitosis is now complete.
What is cytokinesis ?
Cytokinesis is the physical process of cell division, which divides the cytoplasm of a parental cell into two daughter cells
What occurs in cytokinesis ?
- Thecytoplasm dividesand thetwo identical daughter cellsare produced
- Starts in late telophase
- Characterised by the formation of a cleavage furrow, which pinches the cell in two, in animal cells.
What happens in cytokinesis of animal cells ?
- A cleavage furrow forms
- Microfilaments form a ring around the edge of the plasma membrane pullingthe membrane inwards pinching the cell in two
- The plasma membrane fuses in the middle to form two cells
What happens in cytokinesis of plant cells ?
- Vesicles carrying cell membrane and cell wall components assemble along the plane of division
- The cell membrane first forms as vesicles fuse
- Cell walls form alongside
Define diploid
Normal chromosome number, two chromosomes of each type inherited from each parent
Define haploid
Half the normal number of chromosomes, one chromosome of each type
Define gametes
Haploid sex cell produced by meiosis in organisms that reproduce sexually
Define zygote
The initial diploid cell formed when the nuclei of gametes fuse during the fertilisation stage of sexual reproduction
Define reduction division
- The first cell division in meiosis, the process by which germ cells are formed
- Cell division resulting in the production of haploid cells from a diploid cell
Define meiosis
Meiosis is a form of cell division where the nucleus divides twice resulting in a halving of chromosome number and producing 4 haploid cells
How many cell divisions occur in meiosis ?
2
What occurs in meiosis I ?
- The first division, reduction division, when the pairs of homologous chromosomes are separated into two cells
- Cells are haploid, contain one full set of genes instead of two
What are the stages of meiosis I ?
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I
What happens in prophase I ?
- During prophase I the duplicated chromosomes condense, and homologous chromosomes pair up to form bivalents
- The bivalents are connected lengthwise by proteins and crossing over can occur as chromatids entangle
- A chiasma occurs at each point where a crossover has ocurred
- The nuclear envelope fragments as in mitosis
Define bivalent
Homologous chromosomes that are associated in pairs physically held together with at least one crossover
Define chiasmata
- An x-shaped region in a homologous pair which denotes where sections of DNA have become entangled and crossed over, break and rejoin
- Singular chiasma
What is crossing over ?
A genetic rearrangement between two non-sister chromatids involving the corresponding segments of DNA molecules.
How does crossing over lead to genetic variation ?
The genes exchanged can be different alleles so the recombinant chromosomes have different sets of alleles to the original chromatids
Define recombinant chromatids
Chromatids with a combination of DNA from both homologous chromosomes formed by crossing over at chiasmata.
What happens in metaphase I ?
- Homologous pairs line up with one chromosome in each pair facing each pole
- Orientation of homologous pairs on the midline is random and independent of any other homologous pair
- This is called independent assortment
Define independent assortment
The random arrangement of bivalents at the metaphase plate.
What can independent assortment lead to ?
Variation
How can independent assortment lead to variation ?
Different alleles can face the poles e.g. the maternal or paternal chromosomes can therefore end up at either pole during anaphase.
What happens in anaphase I ?
- Breakdown of proteins responsible for sister chromatid cohesion along chromatid arms allows homologs to separate
- Spindle fibres pull homologous chromosomes to opposite poles
- Random assortmentof alleles aspairs move apart
What happens in telophase I ?
- Starts with each half of the cell having a complete haploid set of duplicated chromosomes
- Each chromosome is composed of two sister chromatids; one or both chromatids include regions of non-sisterchromatid DNA
- Nuclear envelope reforms
When does cytokinesis happen ?
Cytokinesis occurs at the same time as telophase I forming two haploid daughter cells (which are not identical )
What occurs in meiosis II ?
- The second division
- The pairs of chromatids present in each daughter cell are separated, forming two more cells
- Four haploid cells are produced
What are the stages of meiosis II ?
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II
What happens in prophase II ?
- The chromosomes, which still consist of two chromatids, condense and become visible again
- The nuclear envelope fragments once again, and spindle fibres form and start to move to the chromatids
What happens in metaphase II ?
- Individual chromosomes line up along the metaphase plate as in mitosis
- Independent assortment occurs again, leading to more genetic variation following metaphase II
What happens in anaphase II ?
- It results in the chromatids of the individual chromosomes being pulled to opposite poles
- Breakdown of the protein holding the sister chromatids together at the centromeres allows this division
What happens in telophase II ?
- Is essentially the same as in mitosis, resulting in the nuclear membrane reforming around chromosomes at each pole
- Cytokinesis occurs simultaneously and results in the division of the cells forming genetically different haploid daughter cells in total
What two processes occur in meiosis to introduce genetic variation ?
- Crossing over
- Independent assortment
Besides meiosis, what are the two ways in whichvariation can be introduced ?
- Random fertilisation
- Gene mutation
How does random fertilisation introduce variation ?
- Each gamete has a unique combination of genes
- Any of the numerous male gametes can fertilise the egg
When does random fertilisation occur ?
When two of the nuclei of two gametes fuse to form a zygote
How does gene mutation introduce variation ?
Bases with a gene can be altered during DNA replication
Define cell
A single repeating unit of an organism
Define tissue
A collection of differentiated cells that have a common function
Define organ
A group of tissues that work together to carry out a common function
Define organ system
A group of organs that carry out a common function
Define specialised
Having particular structure to serve a particular function
Define differentiated
- The process by which a less specialised cell develops a distinct form and function
- A cell specialising
Why do multicellular organisms need specialised cells ?
- Complex communication in a multicellular organism
- Specialised functions need specialised cells
- To maximise efficiency of processes like absorbing nutrients
What features does an erythrocyte have to suit it for its function ?
- Biconcave disk, maximises SA to carry oxygen
- No nucleus, more room for haemoglobin
- Flexible, can squeeze through narrow capillaries
What features does a neutrophil have to suit it for its function ?
- Multi lobed nucleus, makes it easier for neutrophils to squeeze through small gaps
- Granular cytoplasm, contains many lysosomes which contain enzymes used to attack pathogens
What features does a sperm cell have to suit its function ?
- Mitochondria, supplies the energy needed to swim
- Tail, allows for movement to occur
- Acrosome, contains digestive enzymes which digest protective layers around the ovum
What features does a root hair cell have to suit its function ?
Root hairs, increase SA:V ratio to maximise the uptake of water/minerals from the soil
What features does a palisade cell have to suit it for its function ?
- Rectangular, packs closely to form a continuous layer
- Chloroplasts are able to move in cytoplasm, so can absorb more light
- Lots of chloroplasts
What features does a guard cell have to suit it for its function ?
- Cell wall thicker on inside than the outside, so cell does not change symmetrically as its volume changes
- Gap in the centre, allows gases in and out
What are the 4 main categories of tissues in animals ?
- Nerve tissues, adapted to support the transmission of electrical analysis
- Epithelial tissues, adapted to cover body surfaces, internal and external
- Muscle tissues, adapted to contract
- Connective tissues, adapted either to hold other tissues together or as a transport medium
What features does squamous epithelium have to suit its function ?
- Made up of squamous epithelial cells
- Very thin due to the flat cells that make it up, 1 cell thin
- Forms the lining of the lungs, allows for rapid diffusion
What features does ciliated epithelium have to suit its function ?
- Made up of ciliated epithelial cells
- Have cilia that line the trachea causing mucus to be swept away from the lungs
- Have goblet cells, release mucus to trap unwanted particles
What features does cartilage have to suit its function ?
- Contains elastin and collagen
- Firm flexible tissue, composed of chondrocyte cells embedded in an extracellular matrix
- Prevents the ends of bones from rubbing together and causing damage
What features do muscles have to suit their function ?
Skeletal muscle fibres contain myofibrils which contain contractile proteins
What are two tissues in plant cells ?
- Epidermis tissue
- Vascular tissue
What features does the epidermis have to suit its function ?
- Covered by a waxy, waterproof cuticle to reduce the loss of water
- Stomata are present, allow for the movement of CO2, O2 and water vapour
What features does the xylem tissue have to suit its function ?
- It is a vascular tissue that is responsible for transport of water and minerals throughout plants
- Composed of elongated dead cells, the walls of these cells are strengthened with lignin, provides structural support
What features does the phloem tissue have to suit its function ?
- It is a vascular tissue that is responsible for the transport of nutrients from leaves and stems to parts of the plant where it is needed
- Composed of columns of sieve tube cells separated by perforated walls called sieve plates
Define undifferentiated
An unspecialised cell originating from mitosis or meiosis
Define potency
The ability of a stem cell to differentiate into different cell types
Define totipotent
A stem cell that can differentiate into any type of cell and form a whole organism
Define pluripotent
A stem cell that can from all types of tissue but not a whole organism
Define multipotent
Can only form a range of cells from a certain cell lineage
What are stem cells ?
Undifferentiated cells with the potential to differentiate into a variety of the specialised cell types of an organism
What are the characteristics of stem cells ?
- Can repeatedly undergo cell division
- Not adapted to a specific function
- Can give rise to specialist cells
What are stem cells used for in the body ?
- Growth
- Development, of an embryo
- Tissue repair
What happens after they differentiate ?
They can no longer divide and leave the cell cycle into the G0 phase
What happens if stem cells do not divide fast enough ?
The tissues are not efficiently replaced, leading to ageing
What happens if stem cells divide too fast ?
They can form masses of cells called tumours which can lead to the development of cancer
Where are the sources of stem cells in animals ?
- Embryonic stem cells
- Adult stem cells
- Umbilical cords of newborn babies
Where are the sources of stem cells in plants ?
Meristematic tissues
Where are meristematic tissues found ?
- In the tips of roots and shoots
- In phloem and xylem tissues, called the vascular cambium
How do cells become specialised ?
- All the cells have the same DNA
- Genes are switched on and off by epigenetics
- Different proteins are produced in the cell
- So the cell can undergo a specialised function due to these proteins in the cell
What determines the function of the cell ?
The proteins within it
Give an example of where totipotent cells can be found in a human
In a zygote
Give an example of where pluripotent cells are found in a human
Blastocyst
Give an example of where multipotent cells are found in a human
Blood stem cells
What can totipotent cells turn into ?
- A whole organism
- Extra embryonic tissues such as the amnion and the umbilicus
What can pluripotent cells turn into ?
Different types of tissues within an organism
What can multipotent cells turn into ?
Various types of cell from the lineage of the tissue they are from
How long do erythrocytes live for ?
120 days
How long do neutrophils live for ?
6 hours
What supplies erythrocytes and neutrophils ?
Bone marrow
