OCR A-Level Biology: Module 2.6 - Cell division

The cell cycle:

An ordered set of events, culminating in cell growth and division into two daughter cells.
It is the sequence of events that occurs between one cell division and the next.
The three main stages are interphase, mitosis and cytokinesis.

Stages of interphase:

G_1 (GAP 1).
S (synthesis).
G_2 (GAP 2).

G_0 (GAP 0/rest phase).

Stages of interphase - G_1 (GAP 1):

This is the first growth phase - an active stage. Proteins (from which organelles are synthesised) are produced and organelles replicate. The cell grows.

Stages of interphase - S (Synthesis):

DNA is replicated in the nucleus and 2 sister chromatids form from each chromosome.

Stages of interphase - G_2 (GAP 2):

This is the second growth phase. The cell continues to grow until mitosis, energy stores are increased, and the duplicated DNA is checked for errors. At this stage, the mitochondria divide (as well as chloroplasts in plants).

Stages of interphase - G_0:

The phase where the cell leaves the cycle, temporarily or permanently. It is a resting phase.
Reasons for this occurring are: Cell differentiation (when a cell becomes specialised and so is no longer able to divide, carrying out it's function indefinitely and not entering the cycle again), or damaged DNA (in which case it is no longer viable and enters permanent cell arrest).
3 things can happen during G_0: apoptosis (programmed cell death), deterioration (senescence [ageing]) or differentiation.

Cytokinesis:

Division of a cell.
Occurs at the end of mitosis.
The cell splits to form two identical daughter cells that are identical to the parent cell (they have a full set of chromosomes).

Checkpoints in the cell cycle:

The cell cycle is controlled by a series of checkpoints that occur during interphase and mitosis.
Each checkpoint ensures that the preceding stage is completed so the cycle can move on to the next stage.

The cell cycle checkpoints:

The G_1 checkpoint determines whether the cell goes through the cell cycle or does not divide, checking for nutrients, growth factors and DNA damage.
The G_2 checkpoint determines whether the cell is ready to enter mitosis, checking for correct cell size and DNA replication.
The metaphase checkpoint determines whether the chromosomes are attached to the spindle ready for mitosis.

Why is mitosis important?

All cells need to produce genetically identical daughter cells by mitosis for 3 main reasons: Asexual reproduction, growth, and tissue repair.

Chromosome structure:

Each chromosome has two chromatids joined by a centromere.
Genes on each chromatid are identical.

The 4 main stages of mitosis:

Prophase.
Metaphase.
Anaphase.
Telophase.

Mitosis - Prophase:

The chromatin condenses into chromosomes, so that the chromosomes are visible.
The chromosomes shorten and thicken.
The nuclear envelope breaks down.
The centriole divides and daughter centrioles move to the opposite poles of the cell.
The spindle forms.

Mitosis - Metaphase:

The pairs of chromosomes attach to spindle threads by their centromeres and line up along the equator of the cell.

Mitosis - Anaphase:

The spindle fibres contract.
The centromere of each pair of chromatids is split.
Each sister chromatid is pulled towards opposite poles.

Mitosis - Telophase:

The separated chromatids reach the poles.
A new nuclear envelope forms around each set of chromatids.
The cell now has two genetically identical nuclei.

After mitosis - Cytokinesis:

The cell splits to form two genetically identical daughter cells.
In animals, the membrane folds inwards.
In plants, the end plate is formed and cell wall material is laid down.
The chromatids then unravel back into chromatin.

Homologous chromosomes:

We have 46 chromosomes, 23 from the father, 23 from the mother.
These form matching pairs which have the same genes at the same places.
Diploid cells contain a full amount of chromosomes: 46 chromosomes in 23 pairs.
Haploid cells contain half the full amount of chromosomes: 23 chromosomes.
Karyotypes show the pairs of chromosomes of an organism (see image).

Features of Mitosis:

Produces cells with the diploid number of chromosomes.
Enables a species to colonise an area quickly using asexual reproduction - produces clones.
When out of control, a cancerous tumour may result.
Important for the repair of damaged tissue where new cells must be identical to the damaged cells, and is important for the growth of plants and animals.
The parent cell becomes 2 daughter cells - daughter cells are identical copies.
Daughter cells have the same number of chromosomes as the parent cell - DNA is copied exactly (if no mutations)

Features of meiosis:

Enables a species to adapt to a changing environment or to colonise new environments - produces new genetic combinations in individuals, increasing the variety of offspring and provides a vaired stock of individuals.
4 daughter cells are made, with the haploid number of chromosomes (half as many as the parent cell).
Cell division by reduction.
Happens only in gamete producing organs (testes, ovaries, pollen sacs, ovules).
Ensures that the number of chromosomes remains the same in sexually reproducing organisms.
Forms gametes.

Meiosis:

For the production of gametes (sex cells).
Produces cells that are haploid (half the amount of genetic information).
Important in providing variation in sexual reproduction.

Stages of Meiosis:

Starts with one diploid parent cell.
In meiosis I, pairs of chromosomes are split to form 2 haploid cells.
In meiosis II, chromosomes are split to form 4 haploid cells.

Meiosis - prophase I:

Chromosomes condense.
Nuclear envelope disintegrates.
Spindle fibres begin to form.
Homologous chromosomes pair up forming bivalents.
Crossing over occurs.

Meiosis - metaphase I:

Bivalents line up on the cell equator.
The spindle fibres attach to the centromeres.
Independent assortment occurs (maternal and paternal chromosomes can be either side of the equator).

Meiosis - anaphase I:

Homologous chromosomes are pulled by the spindle fibres to the poles.
This causes the genetic variation.

Meiosis - telophase I:

Nuclear envelopes reform.
Chromosomes uncoil.
The cells undergo cytokinesis.
These cells are now haploid cells.
In plants, the cell goes straight from anaphase I into prophase II.

Interphase 2:

Short phase between the two meiosis stages.
No DNA replication.
Chromosomes may remain visible.

Prophase II:

Beginning of second division.
Chromosomes re-condense.
Nuclear envelope breaks down again.
Spindle fibres reform.

Metaphase II:

Chromosomes line up on the equator and attach to spindle fibres.
Independent assortment occurs again.
More genetic variation is caused.

Anaphase II:

Chromatids are split apart by the spindle fibres.
Chromatids move to the poles of the cells.
(The same as anaphase in mitosis).

Telophase II:

Chromatids uncoil.
Nuclear envelopes reform.
The cell undergoes cytokinesis.
4 haploid daughter cells are produced.

Why is meiosis important?

Production of haploid cells.
Creates variation by independent assortment.
Creates variation by crossing over.

Independent assortment:

The chromosomes/chromatids (dependant on whether its meiosis I or II) are lined up along the equator, so each chromosome or chromatid could end up on either side (since they are pulled to opposite poles of the cell and will end up in different cells).

Crossing over:

The chromosomes crossover and swap some parts of DNA during meiosis. The chromatids that have swapped DNA are known as recombinant chromosomes, the ones that haven't are non-recombinant.

What is a stem cell?

An unspecialised cell that is capable of mitosis and able to differentiate.
Stem cells are capable of becoming any type of cell in an organism.
They are undifferentiated, so they have not become specialised and they are able to express all of their genes.

Where are stem cells found?

Embryos.
Umbilical cords.
Bone marrow.
Brain.
Skin.
Meristem (in plants).

Stem cell potency:

The ability to differentiate into different cell types.
Totipotent stem cells can develop into any type of cell.
Pluripotent stem cells can form tissue types but not whole organisms.
Multipotent stem cells can only form a range of cells within a certain type of tissue.

Animal stem cell types:

Embryonic - present in early embryos during the first few divisions when the zygote begins to divide.
Umbilical - taken from the blood in the umbilical cord of new-born babies.
Adult - found in developed tissues (e.g blood, brain, muscle).

Induced pluripotent stem cells (iPSCs):

Adult multipoint stem cells that have been genetically modified by inserting 4 genes. The genes make the differentiated cells behave like stem cells.
They are an ethical source of stem cells.
They may eliminate the chance of immune rejection as cells can be patient specific.

Using stem cells in treatment - repairing tissues:

Type 1 diabetes: stem cells used to regenerate insulin-producing cells in the pancreas.
Burns = stem cells can grow on biodegradable meshes that can produce new skin (regenerative medicine).
E.g to treat muscular degeneration, heart disease, birth defects and spinal injuries.

Using stem cells in treatment - Neurological conditions:

Parkinson's = symptoms (shaking and rigidity) are caused by the death of dopamine producing cells in the brain.
Alzheimers = brain cells are destroyed as a result of the build up of abnormal proteins.
Scientists can see if they can replace the faulty cells with stem cells or grow stem cells into tissues to test how negative new drugs are.

Using stem cells in developmental biology:

To study how stem cells develop to make particular cell types.
To look at what goes wrong when the cells are diseased.
To see if they can extend the capacity that embryos have for growth and tissue repair into later life.

The ethical concerns with using stem cells:

Removal of stem cells from embryos results in the destruction of embryos.
Many people believe life begins at conception therefore this is murder.
Religious objections.
There are questions such as whether the embryo has rights, and who owns the genetic material that is used for the research?

Cell differentiation:

The process of stem cells becoming specialised.
These cells acquire different sub-cellular structures to help them to carry out a particular function.
Adult stem cells are used to replace damaged cells.

Blood cells:

Stem cells in the bone marrow produce erythrocytes and neutrophils.
Erythrocytes (red blood cells) are responsible for carrying oxygen around the body. They have a lifespan of 120 days. When stem cells differentiate into erythrocytes, haemoglobin proteins have to be synthesised inside them.
Neutrophils are white blood cells that help to fight infection. They live for only 6 hours.

Meristem:

Where plant stem cells are located.
This tissue is found in places where growth is occurring, such as apical meristem in roots and shoots.
The vascular cambium is a layer of meristem tissue found between xylem and phloem that differentiates to allow the vascular tissue to grow as the plant grows.

Erythrocytes:

Primarily involved in the transportation of oxygen to body tissues from the lungs, and of CO_2 away from the tissues to be exhaled.
They have a biconcave disk shape to increase the surface area for picking up oxygen.
They do not have a nucleus, or any organelles (except those associated with protein synthesis), in order to make more space for haemoglobin (the protein that binds to oxygen).
Formed in bone marrow.

Neutrophils:

Ingest bacteria (and some fungi) by phagocytosis.
Have a multi lobed nucleus, which makes it easier for them to squeeze through small gaps to get to the site of infection.
Formed in bone marrow.
They contain many lysosomes that attack pathogens, allowing them to carry out phagocytosis (lysosomes contain digestive enzymes).

Squamous epithelial cells:

Surround surfaces that require a smooth flow of liquid.
They are flat, large and thin, and contain a rounded nucleus. They are also polar.

Ciliated epithelial cells:

Move things along surfaces, such as sweeping mucus and pathogens away from the lungs.
They have many cilia on their upper surface, beating in a rhythmic motion to sweep away mucus.
Goblet cells are present among the ciliated cells, producing mucus to trap pathogens.
Ciliated epithelial cells are also found on other surfaces where things need to be moved.

Sperm cells:

Need to reach an ovum and fuse with it to deliver their genetic material.
They are long and thin which allows them to move easily.
Contain many mitochondria to carry out aerobic respiration which gives them the energy to move.
The acrosome contains enzymes to digest the outer layer of an ovum.

Palisade cells:

Carry out photosynthesis.
They are long and cylindrical - they pack together closely, with a little space between them for gaseous exchange.
Have a large vacuole, which is in the centre of the cell so that chloroplasts are at the edge, reducing the diffusion distance for carbon dioxide.
Contain many chloroplasts (the organelles that carry out photosynthesis).

Root hair cells:

Absorb and actively transport mineral ions.
They have a hair-like projection that greatly increases their surface area.
They have special carrier proteins in the plasma membrane to enable active transport.
They have large amounts of mitochondria to provide more energy.
No chloroplasts since they are underground.

Guard cells:

Necessary for CO_2 to enter plants and to control how much water is lost from the plant.
The cell wall is thicker on one side so the cell does not change shape symmetrically as its volume changes.
The cell wall is more rigid where it is thicker and more flexible where it is thinner. The tips of the cell bulge and the gap between the guard cells enlarges.
When the guard cells are "open", CO_2 can enter the plant, and water can evaporate from the plant. When they are shut, the thick cell wall prevents water from escaping.

Tissue:

A group of specialised cells that work together to complete a specific function.

Epithelial tissue:

Covers and lines free surfaces in the body.
Used for protection, absorption, filtration, excretion and secretion.
It is made up almost entirely of cells, which are closely packed, to form continuous sheets.
Adjacent cells are bound together by lateral contacts (such as tight junctions and desmosomes).
It has no blood vessels - cells receive nutrients by diffusion from tissue fluid in the underlying connective tissue.
Some have smooth surfaces, others have projections such as cilia and microvilli.
They have short cell cycles; They divide up to 2 or 3 times a day to replace worn or damaged tissue.

Cartilage:

A connective tissue used for protection, shape and support - found at the ends of bones to stop them rubbing and chipping, and in places such as the outer ear, nose and spine.
Cartilage forms fish skeletons.
It is embedded in an extracellular matrix (fluid).
It is firm and flexible.
3 types: Hyaline, fibrous and elastic.
Hyaline cartilage forms embryonic skeletons, covers the ends of long bones in adults, joins ribs to the sternum (breast plate), and is found in the nose, trachea and larynx (voice box).
Fibrous cartilage occurs in discs between vertebrae in the spine and in the knee joint.
Elastic cartilage form the pinna (outer ear) and epiglottis (a flap that closes over the voice box when swallowing).

Muscle tissue:

Allows movement.
It is well vascularised (has many blood vessels).
Muscle cells are called fibres - they are elongated and contain special organelles called myofilaments (which are made of actin and myosin) that allow the muscle to contract.
Skeletal muscles cause bones to move when they contract; They are packaged by connective tissue sheets and joined to bones by tendons.
Cardiac muscle makes up the walls of the heart and allows the heart to beat and pump blood.
Smooth muscle occurs in the walls of the intestine, blood vessels, uterus and urinary tracts, and it propels substances along these tracts.

Xylem tissue:

Transports water and mineral ions from the roots to the leaves.
There are no end walls, making an uninterupted tube that allows water to pass up the middle easily.
Their walls are thickened with lignin to support the cell.
The cells are dead so there is no cytoplasm.

Phloem tissue:

Mainly transports assimilates (such as sugars and amino acids) both up and down the plant.
They are joined end to end to form sieve tubes.
The sieve parts have lots of holes to allow solutes to pass through.
They have companion cells that carry out metabolic reactions allowing them to stay alive.

Meristematic tissue:

Its main function is to trigger the growth of new cells in young seedlings at the tips of roots and shoots and forming buds.
Found in the tips of roots and shoots and in between the xylem and phloem.
It has thin walls containing very little cellulose.
Does not have chloroplasts or a large vacuole.

Organ:

A group of tissues that work together to perform a function.

Functions of the main plant organs.

Flower for sexual reproduction.
Leaf for photosynthesis.
Stem for support, holding up leaves so they are exposed to sunlight, transporting water and minerals, as well as the products of photosynthesis, and storing the products of photosynthesis.
Root for anchorage in soil, absorption of mineral ions and water, and for storage e.g storing carbohydrates.

Organ system:

A group of organs that work together to perform a function.

Skeletal system:

Provides structure; supports and protects internal organs.
E.g bones.

Muscular system:

Provides structure and allows movement; supports and moves trunk and limbs, moves substances through the body.
E.g muscles (skeletal, cardiac, and smooth).

Integumentary system:

Protects against pathogens; helps regulate body temperature.
E.g skin, hair, nails.

Circulatory system:

Transports nutrients and waste to and from all body tissues.
E.g heart, blood vessels, blood.

Respiratory system:

Carries air into and out of lungs, where gases (oxygen and CO_2) are exchanged.
E.g air passages, lungs.

Immune system:

Provides protection against infection and disease.
E.g Lymph nodes and vessels, white blood cells.

Digestive system:

Stores and digests food; absorbs nutrients and eliminates waste.
E.g Mouth, oesophagus, stomach, liver, pancreas, intestines.

Excretory system:

Eliminates waste; maintains water and chemical balance.
E.g Kidneys, ureters, bladder, urethra, skin, lungs.

Nervous system:

Controls and coordinates body movements and senses; controls consciousness and creativity, helps monitor and maintain other body systems.
E.g Brain, spinal cord, nerves, sense organs, receptors.

Endocrine system:

Maintains homeostasis; regulates metabolism, water and mineral balance, growth and sexual development, and reproduction.

Reproductive system:

Produces offspring.
E.g Ovaries, uterus and mammary glands (in females), testes (in males).