Download F212 2.6 Cell Division and Diversity

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2.6 CELL DIVISION, CELL DIVERSITY AND CELL DIFFERENTIATION
2.6.1: Cell cycle and regulation
By the end of this topic, you should be able to demonstrate and
apply your knowledge and understanding of:
 The cell cycle
 How the cell cycle is regulated
The eukaryotic cell cycle
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Cells reproduce by duplicating their contents
Two daughter cells are produced
Can be observed under a microscope
Cytokinesis refers to the splitting of the cytoplasm
Nuclear and cytoplasmic division = M phase
M phase takes up a short section of cell cycle
Interphase takes up the majority of the cell cycle
It contains check points and is where DNA is duplicated
Figure 1: The cell cycle
 G1 and G2 act as check points
 Purpose is to prevent uncontrolled cell division that would lead to
tumours
 Prevent cellular damage and repair DNA
 DNA is only duplicated once in each cycle and cannot be reversed
Phase of cell cycle and checkpoints
Events within the cell
Questions:
1. List the purpose of checkpoints to control the cell cycle?
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2. Epithelial cells lining the intestine divide two or three times a day. Liver
cells divide about once a year. In which parts of their cell cycle will liver
cells differ from intestine epithelial cells?
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2.6.2: Mitosis
By the end of this topic, you should be able to demonstrate and apply your
knowledge and understanding of:
 The main stages of mitosis
 Sections of plant tissue showing the cell cycle and stages of mitosis
 The significance of mitosis in life cycles
The significance of mitosis in the life cycle
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Asexual reproduction
Occurs in single celled protoctists and single celled fungi
Aphids sometimes produce eggs by mitosis
Allows growth in multicellular organisms
All cells are genetically identical
Allows tissue repair to muscle fibres and walls of blood vessels
Main stages of mitosis
Cytokinesis
 Once mitosis is complete, the cell splits into two so that each new cell
contains a nucleus
 Plasma membrane folds inwards and nips into the cytoplasm in animal
cells
 In plant cells, an end plate forms where the equator of the spindle was,
and new plasma membrane and cell-wall form
 Once plant cells become specialised, they cannot divide
Common mistakes made by students
 Students write that DNA is duplicated during mitosis. Incorrect, DNA is
divided during mitosis
 Students write that chromosomes move to poles during anaphase 2.
Incorrect, chromatids move to poles during anaphase
Questions
1. Explain why plant palisade mesophyll cells cannot undergo mitosis?
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2. State three reasons why mitosis is necessary in the cell cycle?
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3. Describe the difference between a chromatid and a chromosome?
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2.6.3: Meiosis
By the end of this topic, you should be able to demonstrate and apply your
knowledge and understanding of:
 The significance of meiosis in life cycles
 The main stages of meiosis
Key definitions
Haploid: Having only one set of chromosomes, represented by the symbol ‘n’
Homologous chromosomes: Matching chromosomes, containing the same
genes at the same places (loci). They may contain different alleles for some
of the genes
Meiosis: Type of nuclear division that results in the formation of cells
containing half the number of chromosomes of the parent cell
Homologous chromosomes
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Body cells there are 46 chromosomes
23 from the mother, 23 from father
Matching pairs are homologous
Genes come in alternative forms called alleles
Stages of meiosis
 Contains 2 rounds of division, each contains 4 stages
 In the first round of meiotic division you have prophase 1, metaphase
1, anaphase 1 and telophase 1
 In the second round of meiotic division you have prophase 2,
metaphase 2, anaphase 2 and telophase 2
 Round two takes place at a right angles plane compared to round one
 Cytokinesis occurs at the end of the second division.
Stages of Meiosis
How meiosis produces genetic variation/significance in life cycles
 Crossing over during prophase 1 shuffles alleles
 Independent assortment of chromosomes in anaphase 1 leads to
random distribution of maternal and paternal chromosomes of each
pair
 Independent assortment of chromatids in anaphase 2 leads to further
random distribution of genetic material
 Haploid gametes are produced, which can undergo random fusion with
gametes derived from another organism of the same species
Questions
1. Explain why sexual reproduction involves meiosis?
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2. Why is genetic variation good for populations of living organisms?
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3. What are the products of meiosis?
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2.6.4: Diversity in animal cells
By the end of this topic, you should be able to demonstrate and apply your
knowledge and understanding of:
 How cells of multicellular organisms are specialised for particular
functions
 The features and differentiation of stem cells
 The production of erythrocytes and neutrophils derived from stem cells
in bone marrow
Key Definitions
Differentiation: Process by which stem cells become specialised into different
types of cell
Epithelial cells: Cells that constitute linings of surfaces and cavities
Stem cell: Unspecialised cell able to express all of its genes and divide by
mitosis
The need for cell differentiation and specialisation
 Small organisms need a large surface area: volume ratio
 Obtain nutrients and gases through simple diffusion
 Multicellular organisms are larger and therefore have a smaller SA/V
ratio
 Need specialised cells to carry out functions
Differentiation
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Multicellular organisms start off as zygotes
Results when an ovum is fertilised by a spermatozoan
After several divisions an embryo forms
The embryonic cells differentiate as certain genes are switched off and
other genes may be expressed more
 The proportions of the different organelles differs from those of other
cells
 The shape of the cell and contents change
Specialised animal cells
 Erythrocytes – very small, 7.5 micrometres in diameter, large SA:V,
oxygen can diffuse across the membranes quickly
 They are flexible
 Neutrophils make up 50% of white blood cells in your body, they are
twice the size of erythrocytes and contain multi-lobed nuclei
 They travel towards infected sites via chemotaxis
 Ingest bacteria via phagocytosis
Spermatozoa
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Lots of mitochondria to carry out respiration
Long and thin
Can swim and move easily
Enzymes are released from acrosome
 Epithelial cells are thin and flat
Question
1. How are sperm adapted to their function?
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2.6.5: Cell diversity in plants
By the end of this topic, you should be able to demonstrate and apply your
knowledge and understanding of:
 How cells of multicellular organisms are specialised for particular
functions
Palisade cells
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Where photosynthesis occurs
Long and cylindrical so they are tightly packed together
Large vacuole to push chloroplasts nearer membrane
Contain cytoskeleton to move chloroplasts
Guard Cells
 Light energy is used to produce ATP
 ATP actively transports potassium ions from surrounding epidermal
cells by osmosis
 Water enters guard cells via osmosis from neighbouring cells
 Guard cells swell causing stomata to open, enabling gaseous exchange
Root hair cells
 Root hair cells are epidermal cells on the outer layer of young plant
roots
 Hair like projections increase surface area for absorption of water
 Have specialised carrier proteins for active transport of ions into cell
 Lowers water potential and triggers water absorption via osmosis
Xylem and phloem
 Xylem and phloem form the vascular tissue of plants
 Adapted for function e.g. xylem vessels are hollow
Question: Suggest why plants die if their roots are in waterlogged soil for
several days?
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2.6.6: Animal tissues
By the end of this topic, you should be able to demonstrate and apply your
knowledge and understanding of:
 The organisation of cells into tissues
Key Definitions
 Tissue: A group of cells that work together to perform a specific
function/set of functions
The four main types of tissue
 Epithelial – This is the lining of tissue
 Connective Tissue – These hold structures together and provide
support, e.g. blood, bone and cartilage
 Muscle Tissue – Made of cells that are specialised to contract and
cause movement
 Nervous Tissue – Made of cells specialised to conduct electrical
impulses
Epithelial Tissue
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Makes up vessels and respiratory systems
Adjacent cells are bound to each other
Cells receive nutrients by diffusion from tissue fluid
Some epithelial cells have smooth surfaces, some have projections
Function in absorption, filtration and excretion
Connective Tissue
 Widely distributed in the body
 It consists of a non-living extracellular matrix containing proteins
 Matrix separates living cells within the tissue and enables the
withstanding of forces e.g. weight
 Immature cells in cartilage are called chondroblasts
 They can divide by mitosis and secrete the extracellular matrix
 Hyaline cartilage forms the embryonic skeleton, covers the ends of
long bones in adults and found in nose/trachea
Muscle Tissue
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Muscle tissue is well vascularised (has many blood vessels)
Muscle cells are called fibres
Contain myosin and actin
Skeletal muscle is packaged by connective tissue
Cardiac muscle makes up the walls of the heart to pump blood
Question:
1. Cartilage tissue is not vascularised. Suggest how the cells in this tissue
may receive nutrients?
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2.6.7: Plant tissues and organs
By the end of this topic, all students should be able to demonstrate and apply
understanding of:
 The organisation of cells into tissues, organs and organ systems
 The production of xylem vessels and phloem sieve tubes
Key Definitions
 Meristem: Area of unspecialised cells within a plant that can be divide
and differentiate into other cell types
 Organ: Collection of tissues working together to perform a
function/related functions
 Xylem: Tissue that carries water and mineral ions from the roots to all
parts of the plant
Epidermal Tissue
 This is the equivalent to epithelial tissue in animals.
 It consists of flattened cells that apart from the guard cells, lack
chloroplasts and form a protective covering over leaves, stems and
roots.
 Some epidermal cells also have walls impregnated with a waxy cuticle
Vascular Tissue
 Vascular tissue is concerned with transport
 There are two sorts – xylem and phloem, both present in vascular
bundles
 Xylem vessels carry water and minerals from roots to all parts of the
plant
 Phloem sieve tubes transfer the products of photosynthesis, in
solution, from leaves to parts of the plant that do not photosynthesise,
such as roots, flowers and growing shoots
Meristemic Tissue
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Contains stem cells
It is found at root and shoot tips
These are called meristems
Cells in meristems have thin walls containing very little cellulose
 Do not have chloroplasts
 Do not have a large vacuole
 Can divide by mitosis and differentiate into other types of cells
The production of xylem vessels and sieve tubes
 New cells arise at meristem by mitosis
 Cambium differentiates into xylem and phloem
 Lignin reinforces walls of xylem and ensures it doesn’t break. It also
allows a continuous stream of water to pass up the xylem
 Cambium differentiates into phloem by forming sieve tube elements
or companion cells which maintain their organelles for metabolic
activity
Plant Organ
Leaf
Root
Stem
Flower
Main Functions
2.6.8: Organs and organ systems in animals
By the end of this topic, you should be able to demonstrate and apply
your knowledge and understanding of:
 The organisation of cells into tissues, organs and organ systems
 A number of organs working together make up an organ system
 The skin is the largest organ in the body
System
Digestive
Circulatory
Respiratory
Urinary
Integumentary
Musculo-skeletal system
Immune system
Nervous System
Endocrine System
Reproductive System
Lymph System
Organs and tissues
involved
Examples of life
processes carried out
2.6.9: Stem Cells and their potential uses
By the end of this topic, you should be able to demonstrate and apply your
knowledge and understanding of:
 The features and differentiation of stem cells
 The potential uses of stem cells in research and medicine
Key Definitions
Stem Cell: Unspecialised cell able to express all of its genes and divide by
mitosis
Stem Cells
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Undifferentiated
Can become any type of cell
Are able to express all of their genes
Can develop into specialised cells
Sources of stem cells
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There are different types of stem cells
Embryonic stem cells which are present in early development
Stem cells in umbilical cord blood
Pluripotent cells can also be produced
Potential uses of stem cells
 Stem cells from bone marrow are extensively used in bone marrow
transplants to treat diseases of the blood such as sickle cell anaemia
 Patients bone marrow cells can be extracted and reinserted after
treatment
 Can be used in chemical testing as opposed to using animals
Repair of damaged tissues or replacement of lost tissues
 Stem cells have been used to treat mice with type 1 diabetes
 Bone marrow stem cells can be sued to treat liver disease
 There is potential for treatment of Alzheimer and other diseases like
arthritis in the near future.
Questions