2.6 Cell division, diversity and organisation - Biology OCR A

What is cytokinesis?

Cytoplasmic division following nuclear division, resulting in two new daughter cells

What is interphase?

Phase of cell cycle where the cell is not dividing; it is subdivided into growth and synthesis phases

What is mitosis?

Type of nuclear division that produces daughter cells genetically identical to each other and to the parent cell

What is the purpose of checkpoints in mitosis?

- To prevent uncontrolled division that would lead to tumours (cancer)

- To detect and repair damage to DNA

- Ensure the cycle cannot be reversed

- Ensures the DNA is only duplicated once during each cell cycle

Describe the M phase of the cell cycle (and check points)

-A checkpoint chemical triggers condensation of chromatin

- Halfway through the cycle, the metaphase checkpoint ensures that the cell is ready to complete mitosis

What events occur at the M phase of the cell cycle?

- Cell growth stops

- Nuclear division (mitosis) consisting of stages

->prophase, metaphase, anaphase and telophase

- Cytokinesis (cytoplasmic division)

Describe the G0 ( gap 0 ) phase of the cell cycle (and check points)

- A resting phase triggered during early G1 at the restriction point, by s checkpoint chemical

- Some cells (e.g. epithelial) do not leave this stage

What events occur at the G0 phase?

- In this phase, cells may undergo apoptosis (programmed cell death), differentiation or senescence

- Some types of cells (e.g. neurons) stay in this phase for a very long time or indefinitely

Describe the G1 ( gap 1 / growth phase ) phase of the cell cycle (and check points)

- A G1 checkpoint control mechanism ensure that the sell is ready to enter the S phase and begin DNA synthesis

What events occur at the G1 phase?

- Cells grow and increase in size

- Transcription of genes to make RNA occurs

- Organelles duplicate

- Biosynthesis (e.g. protein synthesis) including making the enzymes needed for DNA replication in the S phase

- The p53 (tumour suppresser) gene helps control this phase

Describe the S (synthesis / phase of interphase) phase of the cell cycle (and check points)

- Because the chromosomes are unwound and the DNA is diffuse, every molecule of DNA is replicated.

- There is a specific sequence to the replication of genes: housekeeping genes- these which are active in all types of cells, are duplicated first.

- Genes which are normally inactive in specific types of cells are replicated last.

What events occur at the S phase of the cell cycle?

- Once the cell has entered this phase, it is committed to completing the cell cycle

- DNA replicates

- When all chromosomes have been duplicated, each one consists of a pair of identical sister chromatids

- This phase is rapid, and because the exposed DNA base pairs are more susceptible to mutagenic agents, this reduces the chances of spontaneous mutations happening

Describe the G2 ( gap 2/ phase of interphase ) phase of the cell cycle (and check points) and the events that occur

- Special chemicals ensure that the cell is ready for mitosis by stimulating proteins that will be involved in making chromosomes condense and in formation of the spindle

What events occur at the G2 phase?

- Cells grow

What are chromatids?

Replicates of chromosomes

What is the significance of mitosis in the life cycle?

- Asexual reproduction

- Growth

- Tissue repair

What occurs during prophase?

- The chromosomes that have replicated during the S phase of interphase and consist of two identical sister chromatids, now shorten and thicken as the DNA supercoils

- The nuclear envelope breaks down

- The centriole in animal cells (normally found in a centrosome) divides and the two new daughter centrioles move to opposite poles of the cell

- Cytoskeleton protein (tubulin) threads form a spindle between these centrioles.

What occurs during metaphase?

- The pairs of chromatids attach to the spindle threads at the equator region

- They attach by their centromeres

What occurs during anaphase?

- The centromere of each pair of chromatids splits

- Motor proteins, walking along the tubulin threads, pull each sister chromatid of a pair in opposite directions towards opposite poles

- Because their centromere goes first the chromatids, now called chromosomes assume a V shape

What occurs during telophase?

- The separated chromosomes reach the poles

- A new nuclear envelope forms around each set of chromosomes

- The cell now contains two nuclei each genetically identical to each other and to the parent cell from which they arose

Describe what occurs during cytokinesis in an animal cell?

- The plasma membrane folds inwards and nips in the cytoplasm

Describe what occurs during cytokinesis in an plant cell?

- An end plate forms where the equatorial the spindle was, and new plasma membrane and cellulose cell wall material are laid down on either side along this end plate

What does haploid mean?

having only one set of chromosomes; represented by the symbol 'n'

What does pluripotent mean?

Ability of a stem cell to develop into many different cell types

What is meant by senescence?

The condition or process of deterioration with age

What are homologous chromosomes?

Matching chromosomes containing the same genes at the same places (loci). They may contain different alleles for some of the genes

What is meiosis?

Type of nuclear division that results in the formation of cells containing half the number of chromosomes of the parent cell

How does meiosis produce genetic variation?

- 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 assortment of genetic material

- Haploid gametes are produced, which undergo random fusion with gametes derived from another organism of the same species

What are the main stages of meiosis?

There are 2 divisions, in each division there are four stages

- In the first division there are: prophase 1, metaphase 1, anaphase 1 and telophase 1

- The cell then enters a short interphase before embarking on the second meiosis division

- In the second division there are: prophase 2, metaphase 2, anaphase 2 and telophase 2

- At the end of the second division cytokenisis may occur

Explain the events that occur during prophase 1 in meiosis

- the chromatin condenses and each chromosome supercoils. In this state, they can take up stains and can be seen with a light microscope

- the nuclear envelope breaks down, and spindle threads of tubulin protein form from the centriole in animal cells

- the chromosomes come together in their homologous pairs

- each member of the pair consists of two chromatids

- crossing over occurs where non-sister chromatids wrap around each other and may swap sections so that alleles are shuffled

Explain the events that occur during metaphase 1 in meiosis

- the pairs of homologous chromosomes, still in their crossed over state, attach along the equator of the spindle

- each attaches the a spindle thread by its centromere

- the homologous pairs are arranged randomly, with the members of each pair facing opposite poles of the cell. This arrangement is independent assortment

- the way that they line up in metaphase determines how they will segregate independently when pulled apart during anaphase

Explain the events that occur during anaphase 1 in meiosis

- the members of each pair of homologous chromosomes are pulled apart by motor proteins that drag them along the tubulin threads of the spindle

- the centromeres do not divide, and each chromosomes consist of two chromatids

- the crossed-over areas separate from each other resulting in swapped areas of chromosome and allele shuffling

Explain the events that occur during telophase 1 in meiosis

- in most animal cells two new nuclear envelope form around each set of chromosomes and the cell divides by cytokinesis. There is then a short interphase when the chromosomes uncoil

- each new nucleus contains half the original number of chromosomes, but each chromosome consists of two chromatids

- in most plant cells, the cell goes straight from anaphase into prophase 2

Explain the events that occur during prophase 2 in meiosis

- if the nuclear envelopes have reformed, then they now break down

- the chromosomes coil and condense, each one consisting of two chromatids

-the chromatids of each chromosome are no longer identical due to crossing over in prophase 1

- spindles form

Explain the events that occur during metaphase 2 in meiosis

- the chromosomes attach, by their centromere, to the equator of the spindle

- the chromatids of each chromosome are randomly arranged

- the way that they are arranged will determine how the chromatids separate during anaphase

Explain the events that occur during anaphase 2 in meiosis

- the centromeres divide

- the chromatids of each chromosome are pulled apart by motor proteins that drag them along the tubulin threads of the spindle, towards opposite poles

- the chromatids are therefore randomly segregated

Explain the events that occur during telophase 2 in meiosis

- nuclear envelope form around each of the four haploid nuclei

- in animals, the two cells now divide to give four haploid cells

- in plant cells, a tetrad of four haploid cells is formed

What is differentiation?

Process by which stem cells become specialised into different types of cell

What are epithelial cells?

Cells that constitute lining tissues

How are epithelial cells specialised?

- Squamous epithelial cells are flattened

- They have cilia

What is an erythrocyte?

red blood cell (RBC)

How are erythrocytes specialized?

- They are very small so have a large surface area to volume ratio - this means oxygen can diffuse across their membranes and easily reach all region inside the cell.

- Their biconcave shape also increases SA/V ratio

- They are flexible. A well developed cytoskeleton allows the erythrocytes to change shape so that they can twist and turn as they travel through very narrow capillaries

- Have no nucleus, mitochondria or endoplasmic reticulum and very little cytoplasm as most of their organelles are lost at differentiation - this provides more space for the many haemoglobin molecules housed inside them

- haemoglobin is synthesised within immature erythrocytes, whilst they still have their nucleus, ribosomes and rough endoplasmic reticulum

What are neutrophils?

Type of white blood cell that is phagocytic (can ingest microbes and small particles)

How are neutrophils specialised?

- Twice the size of erythrocytes and each contains a multilobed nucleus

- Attracted to and travel towards infection sites by chemotaxis

- Their function is to ingest bacteria and some fungi by phagocytosis

What is a stem cell?

Unspecialised cell able to express all of its genes and divide by mitosis

What is the need for cell differentiation and specialisation?

- Multicellular organisms are larger than single celled organisms and therefore have a smaller SA/V ratio.

- Single celled organism are small and have a large surface area to volume ratio (SA/V) so oxygen can diffuse across their plasma membrane, and waste products can diffuse out via the same membrane. No specialised cells are needed.

- This means that most of their cells are not in direct contact with the external environment. t

- They need specialised cells to carry out particular functions

What does embryonic cells enable once they differentiate?

- the proportions of the different organelles differs from those of other cells

- the shape of the cell changes

- some of the contents of the cell change

What is a genome?

The entire set of genetic material in an organism

How is spermatozoa specialised?

- many mitochondria for aerobic respiration. the ATP energy provides energy for the undulipodium (tail) to move and propel the cell towards the ovum

- Small but long and thin, can move easily

- Once the spermatozoa reaches the ovum enzymes are released from the acrosome (a specialised lysosome) it digest the protective layer of the ovum

- The head of the spermatozoa contains the haploid male gamete nucleus and very little cytoplasm

What are guard cells?

in leaf epidermis, cells that surround stomata

how are guard cells adapted and specialised for their function?

- light energy is used to produce ATP

- the ATP actively transports potassium ions from surrounding epidermal cells into the guard cells lowering water potential

- water is now enters the guard cells from neighboring epidermal cells into the guard cells by osmosis

- the guard cells swell but at the tips the cellulose cell wall is more flexible and more rigid where thicker, the tips bulge and the gap between them, the stoma enlarges

- as the stomata opens the air can enter the spaces within the layer of cell beneath the palisade cells

- gaseous exchange can occur and CO2 will diffuse into the palisade cells. as they will use it for photosynthesis (this will maintain a steep concentration gradient)

- oxygen produced during photosynthesis can diffuse out of the palisade cells into the air spaces and out through the open stomata

What are palisade cells?

closely-packed photosynthetic cells within leaves

How are palisade cells adapted for photosynthesis?

- long and cylindrical, so that they pack together closely but with little space between them for air to circulate CO2 so it can diffuse into cells

- have a large vacuole so that chloroplasts are positioned nearer the periphery of the cell, reducing the diffusion distance for CO2

- contain many chloroplasts which carry out photosynthesis

- contain cytoskeleton and motor proteins to move chloroplasts-nearer to the upper surface of the leaf when sunlight intensity is low but further down when it is high

What are root hair cells?

epidermal cells of young roots with long hair like projections

How are root hair cells specialised?

- Hair like projections greatly increase surface area for absorption of water and mineral ions

- Mineral ions are actively transported into the root hair cells lowering the water potential within them causing water to follow by osmosis down the water potential gradient

- Have specialised carrier proteins in the plasma membrane in order to actively transport the mineral ions

- They also produce ATP (needed for active transport)

What is a tissue?

group of cells that work together to perform a similar function / set of functions

what are the four main types of tissues in the body?

- epithelial or lining tissue

- connective tissues- hold structure together and provide support

- muscle tissue- made of cells that are specialised to contract and cause movement

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

What are the characteristics of epithelial tissue?

-made up almost entirely of cell

-theses cells form continuous sheets, adjacent cells are bound together by lateral contacts such as tight junctions and desmosomes

-there are no blood vessels within epithelial tissues, cells receive nutrients by diffusion from tissue fluid in the connective tissue

-some epithelial cells have smooth surfaces but some have projections e.g. cilia or microvilli

-epithelial cells have short cell cycles and divide up two or three times a day to replace worn or damaged cells

-epithelial cells are specialised to carry out its functions of projection, absorption, filtration, excretion and secretion

What are some examples of connective tissue?

blood, bone, cartilage, tendons, skin and ligaments

what does connective tissue consist of? and what does it do?

a non living extracellular matrix containing proteins and polysaccharides which separates the living cells within the tissue and enables it to withstand forces such as weight

what are three types of cartilage?

hyaline, elastic, fibrous

What does hyaline cartilage do? and where is it found?

- forms an embryonic skeleton, covers the ends of long bones in adults

- joins ribs to the sternum

- found in the nose, trachea (forming c shaped rings) and larynx

What does elastic cartilage make up?

- the outer ear (pinna)

- epiglottis (flap that closes over the larynx when you swallow)

Where is fibrous cartilage found?

in discs between vertebrae in the backbone (spine) and in the knee joint

Describe skeletal muscles

- packaged by connective tissue sheets

- joined to bones by tendons

- when contracted cause bones to move

Describe cardiac muscles

- makes up the walls of the heart

- allows the heart to beat and pump blood

Describe smooth muscles

- occurs in the walls of the intestine, blood vessels, uterus and urinary tracts

- propels substances along these tracts

Define meristem

area of unspecialised cells within a plant that can divide and differentiate into other cell types

Define organ

collection of tissues working together to perform a function/related function

Define Phloem

tissue that carries products of photosynthesis, in solution, within plants

Define xylem

tissue that carries water and mineral ions from the roots to all parts of the plant

Explain epidermal tissue

-equivalent to epithelial in animals

-consists of flattened cells that lack chloroplasts and form a protective covering over leaves stems and roots

-some of the walls have been impregnated with a waxy substance forming a cuticle

-this cuticle is important to plants that live in dry places and it prevents water loss

Explain vascular tissue (in both phloem and xylem)

- Vascular tissue is concerned with transport

- Xylem

-carry water and minerals from roots to all parts of the plant

- Phloem

- sieve tubes transfer the products of photosynthesis (mainly sucrose sugar) in solution, from leaves to parts of the plant that do not photosynthesize e.g. roots flowers and growing shoots

Explain meristematic tissue

- contains stem cells

- found at roots and shoot tips and in the cambium of vascular bundles

the cells in the meristem:

- 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

how do new cells arise at the meristem?

by mitosis

what cambium cells differentiate into xylem vessels

- lignin (a woody substance) is deposited in their cell walls to reinforce and waterproof them: however it also kills the cells

- the ends of the cells break down so that the xylem forms continuous columns with wide lumen to carry water and dissolved minerals

what do other cambium cells differentiate into (phloem)

- sieve tube elements lose most of their organelles and sieve plates develop between them

- companion cells retain their organelles and continue metabolic functions to provide ATP for active loading of sugars into the sieve tubes

what is the main function of the leaf?

photosynthesis

what is the main functions of the root?

-anchorage in soil

-absorption of mineral ions and water

-storage (e.g. of carbohydrates)

what is the main functions of stems?

-support

-holds leaves up so that they are exposed to more sunlight

-transportation of water and minerals

-transportation of products of photosynthesis

-storage of products (e.g. starch, sugars, polysaccharides)

what is the main function of a flower

sexual reproduction

Explain the organs involved in the digestive system and and example of the process carried out

organs/tissues

- oesophagus, stomach, intestine, liver and pancreas (glands)

example

- nutrition to provide ATP and materials for growth and repair

Explain the organs involved in the circulatory system and and example of the process carried out

organs/tissues

- blood vessels and heart

example

- transport to and from cells

Explain the organs involved in the respiratory system and and example of the process carried out

organs/tissues

- airways, lungs, diaphragm, intercostal muscles

example

- breathing and gaseous exchange excretion

Explain the organs involved in the urinary system and and example of the process carried out

organs/tissues

- kidneys, ureters and bladder

example

- excretion and osmoregulation

Explain the organs involved in the integumentary system and and example of the process carried out

organs /tissues

- skin, hair, nails

example

- waterproofing, protection, temperature regulation

Explain the organs involved in the musculo-skeletal system and and example of the process carried out

organs/tissue

- skeleton and skeletal muscles

example

- support, protection and movements

Explain the organs involved in the immune system and and example of the process carried out

organs/tissues

- bone marrow, thymus gland, skin, stomach acid and blood

example

protection against pathogens

Explain the organs involved in the nervous system and and example of the process carried out

organs/tissues

- brain, spinal cord and nerves

example

- communication, control and coordination

Explain the organs involved in the endocrine system and and example of the process carried out

organs/tissues

- glands that make hormones e.g. thyroid, ovaries, testes, adrenals

examples

-communication, control and coordination

Explain the organs involved in the reproductive system and and example of the process carried out

organs/tissues

- testes, penis, ovaries, vagina, uterus

example

- reproduction

Explain the organs involved in the lymph system and and example of the process carried out

organs/tissues

- lymph nodes and vessels

examples

- transport fluid back to the circulatory system and is also important in resisting infections

Define stem cells

unspecialised cell able to express all of its genes and divide by mitosis

explain some characteristics of stem cells

- undifferentiated cells capable of becoming any type of cell in the organism

- described as pluripotent

- able to express all their genes

- can divide by mitosis

- provide more cells that can differentiate into specialised cells for growth and tissue repair

what are the sources of stem cells

- embryonic stem cells

-> present in early embryo formed when the zygote begins to divide

- stem cells in the umbilical-cord blood

- adult stem cells

-> found in developed tissues e.g. blood, brain, muscle, bone, adipose (fat storage) tissue and skin

-> they act like a repair system because they are a renewing source of undifferentiated cells

- induced pluripotent stem cells

-> developed in laboratories by reprogramming differentiated cells to switch on certain key genes and become undifferentiated

How can stem cells be used in bone marrow transplants?

- stem cells are acquired from the bone marrow and are used to treat diseases of the blood and immune system

- they are also used to restore the patients blood system after a specific types of cancer

- the patients bone marrow cells can be obtained before treatment stored and then put back inside the patient after treatment

How can stem cells be used for drug research?

if stem cells can be made to develop into particular types of human tissue then new drugs can be tested first on these tissues rather than on animal tissues

How can stem cells be used to develop biology

- they can study how these cells develop to make particular cell types and can learn how each cell type functions and see what goes wrong when they are diseased

- they are trying to find out if they can extent the capacity that the embryos have for growth and tissue repair into later life

Can stem cells be used to treat diabetes

- Yes, studies have been done on mice with type 1 diabetes by programming iPS cells to become pancreatic beta cells

- Research is under way to develop such treatment for type 1 diabetes in humans

Can bone marrow stem cells be used to treat liver disease

Yes, they can be made into liver cells (hepatocytes) and used to treat it