2.6 Cell division

4 types of tissue in muscle

-epithelial: lining tissue

-connective tissue: hold structure together and provide support e.g. bone and cartilage

-muscle tissue: made of cells specialised to contract, contain lots of mitochondria

-nervous tissue: made of cells specialised to conduct electrical impulses

epithelial

-made up of cells that are very close to each other and form a continuous sheet. adjacent cells are bound together by lateral contacts such as tight junctions and desmosomes

-no blood vessels within the tissues. nutrients arrive by diffusion from tissue fluid

-can be smooth or have projections (cilia/microvilli)

-short cell cycle, can divide every 2/3 days to replace worn tissue

connective tissue

-consists of non-living extracellular matrix containing proteins and polysaccharide

-this matrix separates the living cells within the tissue and enables it to withstand forces

-e.g. bone, blood, cartilage, tendons and ligaments

cartilage

-immature cells within the cartilage are called chondroblasts

-they secrete an extracellular matrix. once the matrix is synthesised these cells mature and become chondrocytes that then maintain the matrix

-hyaline cartilage forms the embryonic skeleton, covers the ends of long bones and is found as c-shaped rings in the trachea

-fibrous cartilage occurs in discs between vertebrae

-elastic cartilage makes up the outer ear and the epiglottis

-epiglottis cartilage is over the windpipe to stop food being swallowed into the lungs

muscle

-well vascularised (many blood vessels)

-these cells (fibres) are elongated and contain microfilaments made of actin and myosin

-skeletal muscle makes up the walls of the heart

-smooth muscle- occurs in walls of intestine and blood vessels propelling substances along

epidermal tissue

-equivalent to epithelial tissue in animals

-consists of flattened cells that apart from guard cells lack chloroplasts

-forms a protective covering

-some also have waxy substances in the wall to form the cuticle

vascular tissue

-transport tissue

-two types: xylem and phloem

-xylem carries water and minerals from root to all other parts

-phloem sieve tube transfers products in solution from a source to a sink

meristematic tissue

-contains stem cells

-this is found at root and shoot tips and in the cambium of vascular bundles

-the cells it is made of are called meristems

-these cells have thin walls with very little cellulose

-do not have chloroplasts

-do not have a large vacuole

how xylem and phloem derive from meristems

-when most plant cells mature they develop a large vacuole and rigid cellulose cell walls

-this stops the cell dividing

-when plants need to grow new ones arise at the meristems

-some cambium cells differentiate into xylem vessels

-lignin is deposited in the cell wall, killing the cell

-the ends of the cells break down making a continuous column

-other cambium cells differentiate into phloem sieve tubes or companion cells

-sieve tubes lose most of their organelles and sieve plates develop

-companion cells retain in the organelles and continue metabolic functions

plant organs

-an organ is a collection of tissues working together

-e.g. leaves, flowers

animal organ systems

-a group of organs working together

-e.g. digestive system, nervous system

stem cells

-before a cell has differentiated, or specialised, it is called a stem cell

-a stem cell is a cell that has not yet become a specialised cell

-they can: ~replicate many times ~has the potential to become different types of cells

stem cell sources

-umbilical stem cells- umbilical cord blood

-embryonic stem cells- aborted/miscarried embryos, cloned embryos

-adult stem cells- bone marrow, brain, skin, heart

induced pluripotent stem cells

developed by scientists by reprogramming differentiated cells to switch on certain key genes to allow them to become undifferentiated

adult stem cells

-we have stem cells in our body

-e.g. in bone marrow there are three types of stem cells, one for blood, one for bone and one for skin

-an advantage to using adult cells could be that tissues created from them won’t be rejected

multipotent

there is a limit to what the stem cells can become

embryonic stem cells

-these come from the inner cell mass of the blastocyst

-embryonic stem cells can differentiate into any kind of cell

pluripotent

there is no limit to what they can become

the Hayflick limit

-differentiated cells have a limit to the number of times they can divide

-this is what eventually caused old age

-the only two expectations are stem cells and cancer cells

the potential uses of stem cells in research and medicine

-treatment of neurological conditions, such as Alzheimer’s and Parkinson’s diseases- replacement of relay neurons in the brain and spinal cord

-research into development biology- stem cells are studied to better understand how embryos grow and mature and how certain tissues develop and diseases progress in these tissues

-drug research- cell lines and tissues derived from stem cells used to trial new drugs

cell cycle

mitotic phase: prophase, metaphase, anaphase, telophase

G1: first growth phase- growth and normal metabolic roles

S: synthesis phase- DNA replication

G2: second growth phase- growth and preparation for mitosis

cell cycle- cell growth and division

interphase: cell growth, synthesis of organelles, DNA copying and checking of genetic information

mitosis: chromosomes divide

cytokinesis: cytoplasm divided between the daughter cells

cancer

-caused by a groups of diseases, approx. 200 in total caused by a growth disorder in cells- caused because the genes that regulate mitosis have damaged so the cell as uncontrolled growth

-leads to a group of abnormal cells called a tumour forming and expanding

-cancer can form in any organ but some are more common than others

regulation of cell cycle

the cell cycle has two main checkpoints- G1/S - restriction point and G2/M checkpoints- as well as these main checkpoints other occur

the purpose of these checkpoints is: -to prevent uncontrolled division that would lead to tumours -to detect and repair damage to DNA

because the molecular events that control the cell cycle happen in a specific sequence, they also ensure that: -the cycle cannot be reversed -DNA is only duplicated once in each cycle

mitosis

-cell (nuclear) division in the parent cell

-mitosis produces daughter nuclei that have the same number of chromosomes as the parent cell and each other

-process by which new body cells are produced for: ~growth ~replacing damaged/old cells

-every different cell type in your body contains the same genes

interphase

-in between dividing

-at offset of mitosis the DNA replicates before cell division is visible in the S phase of interphase

-when the cell is not diving individual chromosomes are not visible

prophase

-once chromosomes have replicated during S phase of interphase they consist of 2 identical sister chromatids

-they then coil and contract becoming visible. shortening and thickening forming very tightly packed coils called supercoils

-nuclear membrane/nucleolus disappears

-centrioles in animal cells divide and move towards the opposite poles

-cytoskeleton protein threads form a spindle between these centrioles

-in plant cells, the tubulin threads are formed from the cytoplasm

metaphase

-once the chromatid pairs have lined up along the centre the nuclear membrane breaks down

-the chromatid pairs attach to the web of protein fibres by their centromeres

anaphase

-the centromeres then split and the chromatids separate

-the chromatids move along the spindle fibres to opposite ends. this happens as the centrioles contract

telophase

-the two separated exact copies of the original chromosomes group together

-new nuclear membranes develop and chromosomes uncoil

cytokinesis

-cytoplasm splits into two cells

-in animal cells, the plasma membrane folds inwards and nips in the cytoplasm

-cell plate form in plant cells

why are identical copies important?

growth- all multicellular organisms grow by producing more cells that are genetically identical to each other and to the parent cells

differentiation

-as all cells have the same DNA they change, or differentiate, to give specialised cells

-these cells must all be the same so copies identically

repair

-wounds heal when growth factors, secreted by platelets and macrophages, simulates the proliferation of endothelial and smooth muscle cells to repair injury

continuous process

when viewed under a microscope, you only get a snapshot of the process. the number at each stage is proportional to the time each cells spends undergoing each stage

meiosis

significance- sexual reproduction increases genetic variation as it is based on the combining of genetic material from two different individuals of the same species, by fertilisation

-genetic variation increases survival chances

-in most organisms, the body cells are diploid (have two copies of each chromosomes)

-gametes are haploid (only one set)

-occurs in diploid germ cells, the organism involved are called gonads

homologous chromosomes

-homologous pairs are always two chromosomes that determine the same characteristics that are not the same

-in humans our body cells have 46 chromosomes made up of 23 chromosomes from the mother (maternal) and 23 from the father (paternal)

-during the meiosis, the halving of the number of chromosomes is done in a manner which ensures that each daughter cells receives one chromosome for each pair

interphase (meiosis)

-at offset of meiosis the DNA replicates before cell division is visible

-because of this each chromosome consists of two identical sister chromatid joined at a centromere

prophase 1

-once chromosomes have replicated they coil and contract becoming visible shortening and thickening forming very tightly packed coils called supercoils

-they can take up stains and be seen with a light microscope

-the homologous chromosomes line up closely with all four chromatids coming into contact

-each member of the pairs has the same genes at the same loci (position)

-each member of a pair of homologous chromosomes consist of either two maternal or two paternal chromatids

-at contact points the non-sister chromatids wrap around each other and attach at points called chiasmata

-at these points the chromosomes break and reform, exchanging sections of DNA- this is called crossing over

-centrioles in animal cells divide and move towards the opposite poles

-cytoskeleton protein threads from a spindle between these centrioles

-the nucleolus disappears and the nuclear envelope disintegrates

-spindle fibres form, made of microtubules

metaphase 1

-the chromosomes line up across the equator of the cell and the spindle fibres attach to only one side of the centromere

anaphase 1

-the chromosomes move along the spindle fibres to opposite ends. this happens as the centrioles contract

-the chromosome is still intact

telophase 1

-nuclear membrane/nucleolus return

-each daughter cell consists of the correct number of chromosomes, some paternal and some maternal due to random assortment

cytokinesis

-the cell membrane divides to form the two new cells

meiosis 2

this happens in the plane at right angles to meiosis 1

prophase

-the nuclear membrane disappears

-new spindles start to form

metaphase 2

-the chromosomes arrange themselves on the equator of the spindle

-the spindle fibres bind to both sides of the centromeres

anaphase 2

-the spindle fibres contract and the centromere splits

-one sister chromatid moves to each pole of the cell

telophase 2

-nuclear membrane/nucleolus returns

-four cells are not complete

random assortment

-when the chromosomes line up during meiosis 1, independent assortment occurs as they line up in many ways

-this process is completely random either chromosome form the pair could be in any cell

-this guarantees that individuals produced from sexual reproduction are genetically different from each