2.1.6 cell division etc Flashcards
What are the levels of organisation in multicellular organisms?
specialised cells -> tissues -> organs -> organ systems -> whole organism
Erythrocyrtes (red blood cells)
- Have bioconcave shape which increases their surface area to volume ratio. This is essential in their role of transporting oxygen around the body.
- In mammals these cells do not have nuclei which increases available space for haemoglobin.
- They are flexible which allows them to squeeze through capilaries.
Neutrophils (a type of white blood cell)
- They have a multi lobed nucleus which makes it easier for them to reach and destroy pathogens to fight infection.
- The granular cyptoplasm contains many lysosomes which contain enzymes used to attack pathogens.
Sperm cells (male gametes)
- Their function is to deliver genetic information to the female gamete.
- They have their own tail so they are capable of movement
- Contain many mitochondria which supply the energy needed to swim towards the egg
- The acrosome contains many digestive enzymes, which are released to digest the protect layers around the ovum (the egg)
Palisade cell
- Present in the mesophyll
- Contain chloroplasts to absorb large ammounts of light for photsynthesis
- Regular box shapes which form a continuous layer.
- They have thin cell walls which increases the rate of diffusion of CO2
- They have a large vacuole to maintain turgor pressure.
- Chloroplasts can move within the cyptoplasm to absorb more light.
Root hair cells
- Present at the surface of roots near the growing tips.
- Have long extensions called root hairs which increase the surface area of the cell; this maximises the uptake of water and minerals from the soil.
Guard cells
- Pairs of guard cells form stomata, these are required for carbon dioxide to enter plants for photsynthesis.
- When guard cells lose water and become less swollen as a result of osmotic forces, they change shape and the stoma closes to prevent further water loss from the plant.
- The cell wall is thicker on one side so the cell does not change shape symmetrically as its volume changes.
Squamous epithelium
- Very thin due to the flat cells that make it up and also because it is only one cell thick.
- It is present when rapid diffusion across a surface is essential.
- Forms the lining of the lungs and allows rapid diffusion of oxygen into the blood.
Cilitated epithelium
- Made up of celiatated epithelial cells.
- These cells have a hair like structures called cilia on one surface that have a ryhthmic movement.
- They line the trachea e.g. sweep away mucas from the lungs.
- Goblet cells are present which release mucas to trap any unwanted particles present in the air. This prevents them from reaching the alveoli
Cartilage
- Connective tissue found in the outer ear, nose and at the ends of or between bones.
- Cartalige is a flexible connective tissue composed of chondrocyte cells embedded in a extracellular matrix.
- Cartilage prevents ends of bones from rubbing together and being damaged.
Muscle
- Muscle is a tissue that needs to be able to contract so it can move different parts of the body.
- There are different types of muscle fibres. Skeletal muscle fibres (muscles which are attatched to the bone) contain myofibrils which contain contractile proteins.
Epidermis
- A single layer of closely packed cells covering the surfaces of plants.
- Covered by a waxy waterproof layer to prevent waterloss
- Guard cells are present in the epidermis
Xylem tissue
- Responsible for the transport of water and minerals throughout plants.
- The tissue is composed of vessel elements, which are elongated dead cells.
- The walls of these cells are stregthened with a waterproof material called lignin which provides structural support for the plants.
Phloem tissue
- A vascular tissue responsible for the transport of organic nutrience.
- Composed of collumns of sieve tube cells seperated by perforated walls called sieve plates.
What are the 4 categories of tissues in animals?
- Nervous tissue
- Epithelial tissue (adapted to cover body surfaces)
- Muscle tissue (adapted to contact)
- Connective tissue (adapted to either hold tissues together or as a transport medium)
What are the 3 specialised plant tissues?
epidermis, xylem and phloem
What are the 4 specialised animal tissues?
Squamous epithelium, cilitated epithelium, cartilage, muscle
What is differentiation?
The process of a cell becoming specialised
What is potency?
The stem cells ability to differentiate into different cell types.
What is a stem cell?
Undifferentiated cells in all multicellular organisms which can undergo cell division and differentiate into a variety of different specilaised cells.
Totipotent stem cells
- These cells can differentiate into all cell types of both embryo placenta.
Pluripotent stem cells
- Can form all tissue types but not whole organisms
Multipotent stem cells
Can produce cells of closely related tissues
Unipotent stem cells
Can only produce one cell type but have the property of self renewal (e.g. skin cells)
Replacement of red and white blood cells
- Mammalian erythrocytes transport oxygen around the body. Due to the lack of nucleus and organelles they only have a short lifespan of around 120 days so therefore they need to be replaced constantly.
- The stem cell colonies in bone marrow produce approx 3 billion erythrocytes per kilogram of body mass per day to keep up with the demand.
- Neutrophils have an essential role in the immune system, they live only 6 hrs and have large colonies in the bone marrow.
Sources of animal stem cells
- Embryonic stem cells = present at the very early stage of embryo developement and are totipotent. After about 7 days a mass of cells have formed and are now in a pluripotent state.
- Adult stem cells = these cells are present throughout life and birth. They are found in specific areas e.g. bone marrow. They are multipotent.
Sources of plant stem cells
- Present in meristematic tissue (meristems) in plants. This is found wherever growth is occuring in the plant.
- Meristematic tissue is also located between the phloem and xylem tissue, this is called the vascular cambium. Cells originating from this region differentiate into different cells present in the xylem and phloem tissues
Categories of human stem cells in research
- Embryonic stem cells (obtained from IVF)
- Adult stem cells (found in adult tissues e.g. bone marrow)
- Cord blood cells (found in the umbilical cord)
- Foetal stem cells (from amniotic fluid or miscarriage/abortion)
- Induced pluripotent stem cells (iPS), pluripotent stem cells generated directly from adult non stem cells.
Potential use of stem cells
- Heart disease = muscle tissue repaired
- Type 1 diabetes
- Parkinsons disease
- Alzheimers disease
- Macular degeneration
- Birth defects
- Spinal injuries
Chromosomes
- Before mitosis can occur all the DNA in the cell is replicated during interphase. So each chromosome is converted into 2 identical molecules called chromatids.
- The chromatids are joined together at the centromere.
Phases of the cell cycle
In eukaryotic cells the cell cycle has 2 different main phases - interphase and mitotic (division) phase.
Interphase
- A cell spends the majority of its life in this phase.
- During the interphase DNA is replicated and checked for errors in the nucleus, protein synthesis occurs in the cyptoplasm, mitochondria grow and divide, chloroplasts grow and divide in plants, the normal metabolic processes of cells occur.
The 3 stages of interphase are:
- G1 = the first growth phase: proteins from which organelles are synthesised are produced and organelles replicate.
- S = Synthesis phase: DNA is replicated in the nucleus
- G2 = The second growth phase: cell continues to increase in size, energy stores are increased and the duplicated DNA is checked for errors.
Mitotic phase
- The period of cell division
- Cell division involves 2 stages: mitosis ( the nucleus divides) and cytokinesis (cyptoplasm divides and two cells are produced)
G0 stage
- G0 is the name given to the phase when the cell leaves the cycle, either temporarily or permanently.
The reasons for cells to go into this phase:
- Differentiation = A cell that becomes specialised to carry out a specific function is no longer able to divide.
- DNA of cell may be damaged. a damaged cell can no longer divide and enters a period of permanent cell arrest
- Senescence ageing of cells
What are checkpoints in cell division?
They are the control mechanisms of the cell cycle
What are the checkpoints of the cell cycle?
- Metaphase checkpoints = to ensure correct attatchment of spindle fibres to spindle fibres.
- Restriction checkpoint = commits cells to dividing if all requirements are met (cell size, nutrients, DNA damage)
What is mitosis?
- Mitosis is nuclear division
- The seperation of identical sister chromatids into 2 daughter cells.
- Mitosis is needed for asexual reproduction
Chromosomes
- Before mitosis can occur all of the DNA in the nucleus is replicated during the interphase. Each chromosome is converted into 2 identical chromatids.
- The chromatids are joined together at a region called the centromere
What are the four stages of mitosis?
Prophase, metaphase, anaphase, telophase
Prophase
- Chromatin fibres begin to condense and coil to form chromosomes. The nucleolus dissapears and membrane begins to break down.
- Protein microtubules formed spindle shaped structures linking the poles of the cell. The fibres which form the spindle are neccesary to move the chromosomes into the correct positions before division.
- In animal cells and some plant cells, two centrioles move to opposite poles of the cell. The centrioles are cylindrical bundles of proteins that help in the formation of the spindle.
- Spindle fibres attatch 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 dissapeared.
Metaphase
- Chromosomes are moved by the spindle fibres to form a plane in the centre of the cell, called the metaphase plate, and then held in position.
Anaphase
- Centromeres holding together the pairs of chromatids in each chromosome divide.
- The chromatids are seperated are seperated and pulled to opposite poles of the cell by the shortening spindle fibres.
Telophase
- In the telophase chromatids have reached the poles and are now called chromosomes.
- The new sets of chromosomes assemble at each pole and the nuclear envelope reforms around them, the chromosome starts to uncoil and nucleolus is formed
Cytokinesis
- Cytokinesis is the actual division of the cell into two seperate cells, begins during telophase.
In animal cell:
- In animal cells a cleavage furrow forms around the middle of the cell.
- The cell surface membrane is pulled inwards by the cytoskeleton until it is close enough to fuse around the middle, forming 2 cells.
In plant cells:
- Plant cells have cell walls so it is not possible for cleavage furrow to form.
- Vessicles from the golgi begin to assemble in the same place as where the metaphase plate was formed. The vessicles fuse with eachother and the cell surface membrane, dividing the cell into 2.
- New sections of the cell wall then form along the new sections of membrane.
The importance of mitosis
- Mitotic division of chromosomes ensures an equal quantity of DNA to each daughter cell, this DNA is genetically identical for both daughter cells.
- Growth of multicellular organisms by cell division, tissue repair and replacement
- Increase in number of immune cells
- Asexual reproduction
- Cloned plants grow from meristematic tissues of parent plant e.g. tubers, runners, bulbs.
Sexual reproduction (diploids, gametes etc)
- Normal cells have 2 chromosomes of each type (diploid) one inherited from each parent. During mitosis the nucleus divides once following DNA replication, resulting in 2 genetically identical diploid daughter cells.
- In sexual reproduction 2 sex cells (gametes), one from each parent, fuse to produce a fertilised egg. This egg is the zygote. So gametes only contain half (diploid) of the standard number of chromosomes in a cell.
- In meiosis the nucleus divides twice to produce 4 daughter cells ( the gametes), each gamete contains half the chromosome number of the parent cell so it is haploid.
Homologous chromosomes
Each nucleus contains 2 full sets of genes, a pair of genes for each characteristic. Therefore each nucleus contains matching sets of chromosomes, called homologous chromosomes. Each chromosome in a homologous pair has the same genes at the same loci.
Alleles
- Different versions of the same gene, the different alleles of a gene will all have the same locus.
Prophase 1
- Chromosomes condense, nuclear envelope disintergrates, the nucleolus dissapears and spindle formation begins.
- The difference between this and mitosis is that the homologous chromosomes pair up, forming bivalents.
- The movement of chromosomes through the liquid cyptoplasm results in chromatids entangling, this is called crossing over.
Metaphase 1
- The same as metaphase in mitosis except the homologous pairs of chromosomes assemble along the metaphase plate instead of the individial chromosomes.
- The orientation of each pair on metaphase plate is random and independent on any other homologous pair.
- The maternal of paternal chromosomes can end up facing either pole, this is called independent assortment and can result in many different combinations of alleles facing the poles.
Anaphase 1
- Homologous chromosomes are pulled to the opposite poles and the chromatids stay joined to eachother.
- Sections of DNA on ‘sister’ chromatids, which became entangled during crossing over, now break off and rejoin which sometimes results in an exchange of DNA. The points at which chromatids break and rejoin are called chiasmata.
- When this exchange occurs this forms recombinant chromatids, this causes genetic variation as there will be a different combination of alleles.
Telophase 1
- Essentially the same as telophase in mitosis. The chromosomes assemble at each pole and the nuclear membrane reforms. Chromosomes uncoil.
Prophase 2
- The chromosomes, which still consist of 2 chromatids, condense and become visible again. The nuclear envelope breaks down and spindle fibre formation begins.
Metaphase 2
- Differs from mp1 as the individual chromosomes assemble on the metaphase plate
- Due to crossing over, the chromatids are no longer identical so there is independent assortment again and moregenetic variation.
Anaphase
- Results in the chromatids of individual chromosomes being pulled to opposite poles after division of centromeres.
Telophase 2
- Chromatids assemble at the poles, they uncoil and form chromatin again.
- Nuclear envelope reforms and the nucleolus becomes visoble.
- Cytokinesis results in the division of cells forming 4 daughter cells in total. The cells will be haploid due to reduction division, they will be genetically different from eachother and from the parent cell, due to the process of crossing over and independent assortment.
