2.1.6 cell division, cell diversity and cellular organisation (OCR A-Level Biology)

What is the cell cycle? 

A highly ordered sequence of events that takes place in a cell, resulting in division of the cell, and the formation of two genetically identical daughter cells  

What is interphase? 

Period of growth and normal working 

What happens in interphase? 

DNA is replicated and checked for errors in the nucleus, protein synthesis, mitochondria and chloroplasts grow and divide, normal metabolic processes occur  

What are the stages of interphase? 

G1 (growth 1) proteins from which organelles are synthesised are produced and organelles replicate, the cell increases in size. S (synthesis) DNA is replicated in the nucleus. G2 (growth 2) cell continues to increase in size, energy stores are increased, and the duplicated DNA is checked for errors  

What is the mitotic phase? 

Period of cell division including mitosis (nucleus divides) and cytokinesis (cytoplasm divides and two cells are produced) 

What is G0? 

The phase when cells leave the cell cycle, either temporarily (e.g. lymphocytes in an immune response) or permanently 

Why do cells enter G0? 

Differentiation- a specialised cell that carries out a particular function is no longer able to divide and will carry out this function indefinitely (never enter the cell cycle again). DNA may have been damaged- cell is no longer viable, it can no longer divide and enters a period of permanent cell arrest. Senescent cells- majority of normal cells only divide a limited number of times and eventually become senescent (as you age the number of these cells increase, they have been linked to cancer and arthritis) 

Why is the cell cycle regulated? 

To ensure the cell only divides when it's at the correct size, the replicated DNA is error free (or repaired) and the chromosomes are in the correct positions. This ensures that two identical daughter cells are produced from the parent cell 

What are checkpoints? 

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 

Describe the checkpoints in the cell cycle 

G1 checkpoint= end of G1 phase to check for cell size, nutrients, growth factors and DNA damage before the cell enters S phase, if not satisfied the cell enters G0. G2 checkpoint= end of G2 phase to check for cell size, DNA replication without error and DNA damage before the mitotic phase, if satisfied the cell initiates mitosis. Spindle assembly checkpoint (also called metaphase checkpoint)= checks all chromosomes are attached to spindles and have aligned  

How do checkpoints work? 

Through the action of kinases- a class of enzyme that catalyse the addition of a phosphate group to a protein, phosphorylation changes the tertiary structure of checkpoint proteins (cyclins) and activates them 

What happens to chromosomes before mitosis? 

DNA is replicated in interphase where each chromosome is converted into two identical DNA molecules called chromatids. The two chromatids are joined at a region called the centromere to they can be precisely manoeuvred and segregated equally- one each to a daughter cell  

Describe prophase 

Chromatin fibres begin to coil and condense to form chromosomes that will take up stain and become visible under the light microscope. The nucleolus disappears. The nuclear membrane begins to break down. Protein microtubules form spindle shaped structures linking the poles of the cell. The fibres are necessary to move the chromosomes into the correct positions before division. In animal cells (and some plant cells) two centrioles migrate to opposite poles of the cell to help form to spindle. Spindle fibres attach to specific areas on centromeres and start to move the chromosomes to the centre of the cell.  

Describe metaphase 

Chromosomes are moved by the spindle fibres to form a plane in the centre of the cell (line up at the equator) called the metaphase plate and then held in position 

Describe anaphase 

The centromeres holding the pairs of chromatids divide. The chromatids are separated and pulled to opposite poles of the cell by the shortening of the spindle fibres (as sections of protein are removed) 

Describe telophase 

Chromatids have reached the poles and are now called chromosomes. The two new sets of chromosomes assemble at each pole and the nuclear envelope reforms around them. The chromosomes start to uncoil, and the nucleolus is formed. Cytokinesis (cell division) begins 

Describe cytokinesis 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 two cells 

Describe cytokinesis in plant cells 

Cannot form cleavage furrow due to cell wall. Vesicles from the Golgi apparatus begin to assemble in the same place where the metaphase plate was formed. The vesicles fuse with each other and the cell surface membrane, dividing the cell into two. New sections of cell wall form along the new sections of membrane (if cell wall formed before this, daughter cells would immediately undergo osmotic lysis from the surrounding water) 

How do you obtain dividing cells? 

Root tips are treated with a chemical to allow the cells to be separated then they can be squashed to form a single layer of cells (meristem) on a slide. The slide is stained with stains that bind to DNA to make the chromosomes clearly visible 

What do the stages of mitosis look like under the microscope? 

Accurate drawings shown. Prophase- chromosomes condense to defined units to allow organised and equal segregation of chromatids. Metaphase- line of chromosomes across centre of cell. Anaphase- V shape of chromatids pulled towards poles. Telophase- two separate groups of chromosomes 

Why is mitosis important? 

Growth, replacement and repair of tissues in multicellular organisms. Asexual reproduction (production of genetically identical offspring from one parent in multicellular organisms- no fusion of gametes) in plants, fungi and some animals. NB: prokaryotic cells no not have a nucleus, and they reproduce asexually by a different process called binary fission 

What makes mitosis important? 

It produces two daughter cells with an exact copy of the DNA present in the parent cell and the same number of chromosomes- genetically identical 

How does sexual reproduction work? 

Two sex cells (gametes), one from each parent, fuse to produce a fertilised egg. The zygote is the origin of all cells that the organism develops. Gametes must therefore contain half the standard number of chromosomes (haploid) and are formed by meiosis 

What is meiosis? 

Nucleus divides twice to produce four different daughter cells, each containing half the chromosomes number of the parent cell-haploid. It is reduction division 

What are homologous chromosomes? 

Pairs of chromosomes in a diploid organism that have the same genes in the same order/same loci. Each characteristic is coded for by two copies of a gene, one from each parent 

What are alleles? 

Different versions of the same gene, they have the same locus on a particular chromosome 

Why is meiosis important? 

Ensures genetic variation by independent assortment and crossing over. Also ensures that fusion of the gametes restores the correct number of chromosomes  

What is meiosis I? 

The reduction division where the number of cells is doubled but the number of chromosomes is not, this results in half as many chromosomes per cell (haploid)  

What is meiosis II? 

The second division, like mitosis, where the pairs of chromatids present in each daughter cell are separated, forming two more cells each. As a result, four haploid daughter cells are produced  

Describe prophase I 

Chromosomes condense, the nuclear envelope disintegrates, the nucleolus disappears, and spindle formation begins. The homologous chromosomes pair up, forming bivalents. Crossing over occurs 

What is crossing over? 

When moving chromosomes through the cytoplasm, they are brought together which results in entanglement and exchange of genes between homologous chromosomes at the chiasmata, resulting in a mixture of parental characteristics in offspring 

Describe metaphase I 

Homologous pairs of chromosomes (bivalents) assemble along the metaphase plate. The orientation of each homologous pair on the metaphase plate is random and independent of any other homologous pair. The maternal or 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. Independent assortment of homologous chromosomes and metaphase I results in genetic variation. 

Describe anaphase I 

Homologous chromosomes are pulled to the opposite poles and the chromated stay joined to each other. Sections of DNA on sister chromatids, which became entangled during crossing over, now break off and rejoin, sometimes resulting in an exchange of DNA. The points at which the chromatids break and rejoin are called chiasmata. When exchange occurs, this forms recombinant chromatids with genes being exchanged between them. The genes being exchanged may be different alleles of the same gene, meaning the combination of alleles on the recombinant chromatids will be different from the allele combination on the original chromatids. Genetic variation arises from this new combination of alleles. The sister chromatids are no longer identical. 

Describe telophase I 

The chromosomes assemble at each pole and the nuclear membrane reforms. Chromosomes uncoil. This cell undergoes cytokinesis and divides into two cells. The reduction of chromosome number from diploid to haploid is complete. 

Describe prophase II 

The chromosomes, which still consists of two chromatids, condense and become visible again. The nuclear envelope breaks down and spindle formation begins. 

Describe metaphase II 

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 more genetic variation produced. 

Describe anaphase II 

The sister chromatids of the individual chromosomes are pulled to opposite poles after division of the centromeres. 

Describe telophase II 

The chromatids assemble at the poles. The chromosomes uncoil and form chromatin again. The nuclear envelope reforms and the nucleolus becomes visible. Cytokinesis results in division of the cells forming 4 daughter cells in total. The cells will be haploid due to the reduction division. They will also be genetically different from each other and from the parent cell due to crossing over and independent assortment. 

What are the levels of organisation in multicellular organisms? 

Specialised cell. Tissue. Organ. Organ system. Whole Organism. 

How are erythrocytes specialised for a particular function? 

Red blood cells have a flat and bi concave shape, which increases their surface area to volume ratio. This is essential to their role of transporting oxygen around the body. In mammals, these cells do not have nuclei or many other organelles, which increases the space available for haemoglobin for more oxygen carriage. They are also flexible so that they can squeeze through narrow capillaries. 2microm thick and 8microm wide 

How are neutrophils specialised for a particular function? 

They play an essential role in the immune system. They have a multi lobed nucleus which makes it easier for them to squeeze through small gaps to get to the site of infection. The granular cytoplasm contains many lysosomes that contain enzymes used to attack pathogens and digest them. 10-14 microm  

How are sperm cells specialised for a particular function? 

Male gametes. Their function is to deliver genetic information to the female gamete, the ovum. They have a flagellum, so they are capable of movement and contain many mitochondria to supply the energy needed to swim. The acrosome on the head of the sperm contains digestive enzymes which are released to digest the protective layers around the ovum and allow the sperm to penetrate, leading to fertilisation. 40microm long 

How are palisade cells specialised for a particular function? 

Present in the mesophyll and contain many chloroplasts to absorb large amounts of light for photosynthesis. Rectangular shape so they can be closely packs to form a continuous layer. They have thin cell walls, increasing the rate of diffusion of carbon dioxide. They have a large vacuole to maintain turgor pressure. Chloroplasts can move within the cytoplasm to absorb more light. 25-75 microm long 

How are root hair cells specialised for a particular function? 

Have long extensions called root hairs, which increase the surface area of the cell to maximise the uptake of water and minerals from the soil. Has a vacuole containing cell SAP, which is a solution of ions and sugars and lowers the water potential. Thin cellulose cell wall. Acts as an anchor. 20-150 microm long 

How are guard cells specialised for a particular function? 

Pairs of guard cells on the surface of leaves form small openings called stomata. These are necessary for carbon dioxide to enter plants for photosynthesis. When guard cells lose water and become less swollen because of osmotic forces, they change shape, and the stoma closes to prevent further water loss from the plant. The cell wall of a guard cell is thicker on one side, so the cell does not change shape symmetrically as its volume changes. 15-20 microm long 

What is a tissue? 

A collection of differentiated cells that have a specialised function or functions, that work together.  

What are the four main categories of tissues in animals? 

Nervous tissue, which is adapted to support the transmission of electrical impulses. Epithelial tissue, which is adapted to cover body surfaces, internal and external. Muscle tissue which is adapted to contract. Connective tissue which is adapted to either holds other tissues together or as a transport medium. 

Describe squamous epithelium 

It is very thin as it is one cell thick. Made up of specialised squamous epithelial cells. It is present when rapid diffusion across a surface is essential. It forms the lining of the lungs and allows rapid diffusion of oxygen into the blood. 

Describe ciliated epithelium 

Made up of ciliated epithelial cells. The cells have hair like structures called cilia on one surface that move in a rhythmic manner. Ciliated epithelium lines for trachea, causing mucus to be swept away from the lungs. Goblet cells are also present, releasing mucus to trap any unwanted particles present in the air. This prevents particles such as bacteria from reaching the alveoli once inside the lungs. 

Describe cartilage 

A connective tissue found in the ear nose and between bones. It contains fibres of the protein's elastin and collagen. Cartilage is a firm, flexible connective tissue composed of chondrocyte cells embedded in an extracellular matrix. It prevents the ends of bones from rubbing together and causing damage. 

Describe muscle 

Tissue that can contract to move bones and therefore move different parts of the body. There are different types of muscle fibres. Such as skeletal muscle- consisting of several muscle fibres separated by connective tissue. The fibres contain myofibrils which contain contractile proteins 

What are some examples of tissues in plants? 

Epidermis tissue, which is adapted to cover plant surfaces. Vascular tissue, which is adapted for transport of water and nutrients. 

Describe plant epidermis 

A single layer of closely packed cells covering the surfaces of plants. It is usually covered by a waxy waterproof cuticle to reduce the loss of water. Guard cells are present and allow carbon dioxide in and out, and water vapour and oxygen in and out.  

Describe xylem tissue 

Vascular tissue responsible for 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 strengthened with a waterproof material called lignin, which provides structural support for the plants. 

Describe phloem tissue 

Vascular tissue in plants responsible for the transport of organic nutrients, particularly sucrose from leaves and stems, where it is made by photosynthesis, to all parts of the plants that is needed. It is composed of columns of safe tube cells separated by perforated walls called sieve plates. 

What is an organ? 

A collection of tissues working together that are adapted to form a particular function in an organism. 

What is an organ system? 

A number of organs working together to carry out a major function within the body. Such as the digestive system, the cardiovascular system and the gaseous exchange system. 

What is differentiation? 

The process of a cell becoming specialised. It involves the expression of some genes but not others in this cell's genome. 

What is a stem cell? 

A renewing source of undifferentiated cells which have the potential to differentiate. 

Why does stem cell activity need to be strictly controlled? 

If they do not divide fast enough, tissues are not efficiently replaced, leading to ageing. However, if there is uncontrolled division then they form masses of cells called tumours, which can lead to the development of cancer 

What is stem cell potency? 

The stem cell's ability to differentiate into different cell types- the more potent, the greater the number of different cell types it can differentiate into  

What does totipotent mean? 

Stem cells can differentiate into any type of cell e.g. fertilised egg (zygote) and can produce a whole organism. They can also differentiate into extra embryonic tissues such as amnion and umbilicus  

What does pluripotent mean? 

Stem cells can form all the tissue types but not whole organisms. They are present in early embryos and are the origin of many different tissue types  

What does multipotent mean? 

Stem cells can differentiate to a range of cells within a certain type of tissue e.g. haematopoietic stem cells in bone marrow give rise to various types of blood cell 

Why did multicellular organisms evolve? 

Evolved from unicellular organisms because groups of cells with different functions working together as one unit can make use of resources more efficiently

Why do erythrocytes need to be replaced? 

Due to lack of nucleus and organelles they have a lifespan of around 120 days 

Why do neutrophils need to be replaced? 

Have a lifespan of 6 hours 

How are blood cells replaced? 

Derived from stem cell colonies in the bone marrow 

What are the sources of animal stem cells? 

Embryonic stem cells: totipotent cells present at very early stages of embryo development, after a week a mass of cells (blastocyst) has formed, and stem cells are pluripotent. Tissue (adult) stem cells: found in specific areas such as bone marrow, are multipotent but there is some evidence to suggest they can be artificially triggered to become pluripotent. Umbilical cord: plentiful supply and non-invasive to take, can be stored and used by the individual in the future- would not be rejected by the umbilicus’ owner.  

What is the source of plant stem cells? 

Meristematic tissue in the tips of roots and shoots. Present as a layer of cambium between the xylem and phloem. Pluripotent.  

What are some uses of stem cells? 

Repair of damaged tissues: heart muscle repair after heart disease, repair of beta cells in islets of Langerhans in people with type 1 diabetes  

Treatment of neurological conditions: delaying death of dopamine producing cells in brain to delay progress of Parkinson’s, alleviate build-up of abnormal proteins in brain in Alzheimer's  

Developmental biology: the study of the changes that occur as multicellular organisms grown and develop from a single cell (e.g. fertilised egg) and why things go wrong 

Other: restoring movement to damaged spinal cords, reverse birth defects, treatment of burns, use in drug trials before being tested on animals and human 

What are the ethics of using stem cells? 

Removal of stem cells from embryos normally results in the destruction of the embryo. There are religious and moral objections as some people believe life begins at conception and therefore destruction of embryos is murder. There is also a lack of consensus as to when the embryo itself has rights, and who owns the genetic material that is being used for research. This controversy is holding back potential progress which could lead to successful treatment of incurable diseases.