✨Module 2: Cell division Flashcards
What is the cell cycle?
Sequence of events that lead to the division of the cell and the formation of two genetically identical daughter cells.
A chromosome consists of …
A DNA molecule has …
A gene has …
Lots of DNA molecules.
Lots of genes.
Lots of base pairs.
Describe the structure of a chromosome.
Made of DNA and proteins. DNA is wrapped around proteins called histones. This results in a structure called chromatin. Chromatin is then coiled into chromosomes.
What is a chromatid?
An arm of a chromosome. Two arms are held together by the centromere. Aka sister chromatids.
What are telomeres?
Regions of structural DNA at each end of a chromosome. The chromosome will become shorter after replication. Telomeres protect the genes on the chromosome.
Non-coding telomeres at the end of chromosomes prevent the coding DNA from being lost in cell division.
What is the difference between homologous chromosomes and sister chromatids?
Homologous chromosomes are pairs of chromosomes that contain the same genetic information, and they are present in diploid cells. Sister chromatids are identical copies of a single chromosome that are produced during replication, and they are present in both diploid and haploid cells.
What are the 2 phases of the cell cycle in eukaryotes?
Interphase and the mitotic phase.
What are the 3 growth phases in interphase?
G1, S, G2
Interphase is referred to as the resting phase as cells are not actively dividing. Why is this an inaccurate description?
Interphase is a very active stage of the cell cycle. It involves:
=> DNA is replicated and checked for errors.
=> Cell gets bigger and makes more organelles.
=> Mitochondria grow and divide, increasing in number.
=> Protein synthesis occurs in the cytoplasm.
What happens in the G1 and G2 phase?
(stands for Gap 1/2 phase)
G1 - the cell grows and makes new organelles for the daughter cells. (chromosomes are NOT duplicated here).
G2 - cell growth continues, duplicated DNA is checked for errors.
What happens in the S phase (synthesis phase)?
DNA is replicated in the nucleus. We will have 92 chromosomes, making it a tetraploid. The new copies of chromosomes are called homologous chromosomes.
There is also checking of genetic material in this phase.
Why is the S phase important?
Ensures the two daughter cells can each get the full chromosome set.
What happens after interphase?
The M/mitotic phase - nucleus divides and chromatids separate.
What is the G0 phase?
When the cell leaves the cycle, either temporarily or permanently.
Describe the reasons why a cell may enter the G0 phase.
=> Cell becomes specialised to carry out a particular function so doesn’t need to divide.
=> DNA may be damaged so it’s no longer able to divide. This cell enters cell apoptosis/permanent cell arrest.
=> Cells become senescent (grow old).
Some cells that enter G0 can be stimulated to go back into the cell cycle such as lymphocytes in an immune response.
Explain why checkpoints are important in the cell cycle.
It ensures the cell has grown to the right size, replicated DNA is error free.
Having checkpoints reduce the risk of copying any damaged DNA into the newly replicated cell.
What does the G1, G2, spindle assembly checkpoint check for?
G1 - this checkpoint is before the S phase. Checks fault in DNA structure, cell size. Ensures entry into the M phase is stopped until repaired. if not, the cell enters G0 phase.
G2 - end of the G2 phase. Checks for cell size, DNA replication and DNA damage.
Spindle assembly checkpoint - checks whether the chromosomes (centromeres) have attached to the spindles and have aligned (in metaphase). Mitosis cannot proceed until this checkpoint is passed.
What is the difference between the centrosome and the centromere?
Centrosome - moves to the opposite poles of the cell and controls the formation of the spindle in mitosis. It contains a pair of centrioles.
Centromere - 2 chromatids attach here and attaches to the spindle.
The importance of mitosis?
Growth and repair of cells, asexual reproduction in plants and fungi (also some animals like amoeba).
How to observe mitosis in a plant?
Growing root tips (or shoot tips) can be treated with a chemical to allow the cell to be separated. Then they can be squashed to form a single layer of cells on a microscopic slide. Stains that bind to DNA make chromosomes visible.
These growing root or shoot tips are called meristems. meristems are totipotent, so they can differentiate into various cells.
Explain what happens in each stage of mitosis.
Prophase - chromosomes condense and become visible. Nucleolus disappears. Centrioles (centrosome) move to opposite poles of the cell, which produce spindle fibres that extends across the cell. Nuclear membrane breaks down.
Later prophase - nuclear membrane disappears completely, spindle microtubules attach to the centromeres and start to move the chromosomes to the centre.
Metaphase - chromosomes line up along the equator/metaphase plate and the spindle fibres attach to the centromeres of each chromosome.
Anaphase - the centromere DIVIDES and the sister chromatids are pulled to the opposite poles of the cell by the spindle fibres shortening. This splits the chromosome into two V-shaped sister chromatids.
Telophase - chromatids reached opposite poles of the cell, so are now known as daughter chromosomes. The nuclear envelope reforms around the chromosomes - one on each pole of the cell. Therefore the cell now has two nuclei, each with a complete set of identical DNA. Nucleolus reforms. Spindle fibres break down. A cleavage furrow begins to form (the point at which the cytoplasm and cell membrane will divide).
What happens in cytokinesis?
Cytoplasm divides to form 2 separate daughter cells.
What happens to animals in cytokinesis?
A cleavage furrow forms at the cells equator by a ring of actin. The cytoskeleton pulls the cell surface membrane inwards and fuses, forming 2 cells.
What happens to plants in cytokinesis?
They have cell walls so they can’t form a cleavage furrow. Instead, a cell plate develops across the equator (formed by vesicles from the Golgi). These fuse with each other, dividing in 2.
What does the mitotic index tell us?
The number of visible chromosomes. It is calculated by:
Cells going through mitosis divided by all cells in the sample.
What is the difference between mitosis and meiosis?
Mitosis - cell division that occurs in cells called somatic cells. Somatic cells are the majority of dividing cells in an organism.
Meiosis - cell division process only in gametes. Gametes are the sperm and the egg, which play a role in sexual reproduction.
What is meiosis and what is its purpose?
Type cell division that gives rise to genetic variation. The main role of meiosis is production of haploid gametes and are genetically different.
How is genetic variation achieved in meiosis?
- Crossing over of chromatids to form bivalents and chiasmata.
- Independent assortment of chromosomes.
Define homologous chromosomes.
Pairs of chromosomes that have the same size and gene loci (position of gene). But they may carry different alleles of those genes.
Each chromosome from each parent.
Difference between Meiosis 1 and Meiosis 2?
Meiosis 1 - reduction division (halves in chromosome number). Diploid to haploid. But a different website says: forms 2 diploid daughter cells.
Meiosis 2 - similar to mitosis to form 4 haploid daughter cells.
Explain what happens in the Meiosis 1.
Before this, the cell must go through interphase to replicate DNA.
Prophase 1 - chromosomes condense, nuclear membrane breaks down, centrioles move to poles and forms the spindle. The homologous chromosomes pair up to form bivalents. The non-sister chromatids ‘cross over’ to form chiasmata. This produces a new combination of alleles.
Metaphase 1 - bivalents align on cell equator and attach to spindle fibres by centromeres. Independent assortment: homologous chromosomes line up randomly and when separated, each gamete receives a random combo of maternal and paternal chromosomes. Different alleles facing the poles leads to genetic variation.
Anaphase 1 - spindle fibres contract to move the homologous CHROMOSOMES to the opposite poles of the cell.
Telophase 1 - chromosomes assemble at each pole and nuclear membrane reforms. Chromosomes uncoil and then undergoes cytokinesis.
Explain what happens in Meiosis 2.
There is no interphase before Meiosis 2.
Prophase 2 - chromosomes recondense and become visible again. Nuclear membrane breaks down and centrioles form spindle fibres.
Metaphase 2 - chromosomes attach to spindle fibres and aligns along the equator. There is more independent assortment.
Anaphase 2 - spindle fibres contract to move the individual CHROMATIDS to opposite poles after the division of centromeres.
Telophase 2 - chromosomes uncoil and form chromatin again. Nuclear envelope reforms. Cytokinesis results in the division of cells forming 4 haploid daughter cells in total.
How could you observe meiosis in plants?
Anther from a flower should be used, prepare a squash slide, use stain, observe using microscope.
How can meiosis bring about genetic variation?
=> Random arrangement of chromosomes during line up.
=> Chromatids crossing over before first division.
How can random fertilisation bring about genetic variation?
Gametes are haploid. Due to meiosis, every gamete has different DNA. So when 2 fuse, they produce genetically different offspring.
How are palisade cells/plant cells adapted for their function?
Chloroplasts can move within cytoplasm for maximum absorption of light. The cells are rectangular so they can be closely packed to form a continuous layer. Large vacuole to maintain turgor pressure.
Describe the 4 categories of tissue.
- Nervous tissue - supports transmission of electrical impulses.
- Epithelial tissue - covers body surfaces, internal and external.
- Muscle tissue - contracts and relaxes.
- Connective tissue - hold other tissues together like cartilage.
What is cartilage and describe its structure.
A flexible, firm, CONNECTIVE tissue found between bones to prevent them rubbing together. Contains chondrocyte cells (which is responsible for cartilage formation) embedded in an extracellular matrix. Contains elastin and collagen.
What is muscle and describe its structure.
A tissue that contracts to move bones. Skeletal muscle fibres contain myofibrils which contain contractile proteins.
Describe the 2 types of tissues in plants.
- Epidermis tissue - covers plant surfaces.
- Vascular tissue - transports water and nutrients.
What is the epidermis and describe its structure.
Single layer of closely packed cells covering the surface of plants. Usually covered by a waxy cuticle to prevent water loss. Stomata are present in the epidermis => allows CO2, water, O2 in and out.
What is the xylem tissue and how it is adapted for its function.
Type of vascular tissue that transports water and minerals throughout plants. The tissue is composed of elongated dead cells.
Xylem cells have no cytoplasm or end walls, meaning they form a tube through which water can pass freely to allow water transport. Waterproof lignin strengthens the cell walls, helping to support the plant.
What is phloem tissue and how is it adapted for its function.
Type of vascular tissue that transports sucrose all around the plant to where it is needed. Made of living cells (as opposed to xylem vessels, which are made of dead cells) that are supported by companion cells. Cells are joined end-to-end and contain holes in the end cell walls (sieve plates) forming tubes which allow sugars and amino acids to flow easily through (by translocation Sieve tubes. - specialised for transport and have no nuclei. colums of sieve tube cells separated by walls called sieve plates.
Describe the function of the vital organs in the digestive system.
Digestive system is a group of organs working together.
=> Pancreas produces digestive enzymes.
=> Stomach contains HCL for digesting food.
=> Liver produces bile to aid digestion of fats.
=> Small intestine digests and absorbs soluble food
=> Large intestine absorbs water from undigested food, producing faeces.
What are stem cells and where are they found?
Undifferentiated cells that can divide and differentiate to perform a particular function. They’re found in bone marrow.
Once stem cells have become specialised they lose the ability to divide, entering the G0 phase.
Stem cells can …
Self-renew or differentiate.
Why does the activity of stem cells need to be strictly controlled?
=> If they don’t divide fast enough, tissues aren’t replaced, leading to tissue degeneration.
=> Uncontrolled division can form a tumour as stem cells can self renew. More likely for a mutation that would lead to cancer.
Define potency.
The stem cell’s ability to differentiate into different cell types. The greater the number of cell types, the higher the potency.
What are multipotent stem cells?
Adult stem cells that diff into a small number of cells in a certain tissue type. Bone marrow stem cells are multipotent.
What are unipotent stem cells?
Divide and differentiate into one type of cell. Stem cells in testes => sperm.
What is the difference between totipotent and pluripotent stem cells?
Totipotent stem cells - can divide into any type of cell until it creates an entire organism. Totipotent cell come from an embryo.
Pluripotent stem cells - can divide into any type of body cell but cannot develop into an entire organism. Pluripotent cells come from the inner cell mass of the blastocyst.
What 2 types of cell are produced in the bone marrow?
Haematopoietic stem cells differentiate into erythrocytes and neutrophils. These multipotent stem cells can be used to treat leukemia, lymphoma.
Where can we harvest animal stem cells from?
Umbilical cord of newborn babies. Invasive surgery isn’t needed, tissues cultured from these stem cells wouldn’t be rejected in a transplant to the owner.
Where can we harvest stem cells in a plant?
Apical meristems. Tips of roots and shoots.
They are also present between phloem and xylem tissues called the vascular cambium. This stem cells differentiate into xylem vessels and sieve tubes in the phloem.
Cell therapy.
Cell reprogramming.
Therapeutic cloning.
Replacing damaged cells with functional cells.
Changing adult cell into a pluripotent cell. (discovered by Japanese scientist)
Cloned embryo would be allowed to develop to the blastocyst stage, then the inner mass cells would be harvested and put into culture. This results in pluripotent embryonic stem cells genetically identical to parent.
Describe the advantages of using stem cells.
=> Potential to treat a wide range of diseases and can repair damaged tissue and neurological conditons.
=> If your own stem cells are used, risk of rejection is reduced.
=> Potential new drugs can be tested on cultures of stem cells before being tested on animals and humans.
=> Developmental biology.
=> Embryos left over from fertility treatment so they are discarded anyway.
Describe the disadvantages of stem cells.
=> Cultured stem cells can become infected with viruses which could be passed to the patient.
=> Religious beliefs where people believe life begins at conception and destruction of embryos is murder.
=> Risk of mutation in cultured stem cells that could lead to cancer in a patient.
