Topic 2: cells and control

chromosomes

the nucleus contains your genetic information

• this is found in the form of chromosomes, which contain coils of DNA

• a gene is a short section of DNA that codes for a protein and as a result controls a characteristic - therefore each chromosome carries many genes

• there are 23 pairs of chromosomes in each cell of the body, as you inherit one from your mother and one from your father - resulting in 46 chromosomes in total in each cell

• sex cells (gametes) are the exception: there are half the number of chromosomes, resulting in 23 chromosomes in total in each gamete cell

• a diploid number of chromosomes is the amount found in body cells (46 in humans)

• a haploid number of chromosomes is half this amount (23 in humans), and is found in gametes (sperm and egg cells), which combine to form the diploid amount

what is the cell cycle

the cell cycle is a series of steps that the cell has to undergo in order to divide. mitosis is a step in this cycle - the stage when the cell divides. stage 1 = interphase, stage 2 = mitosis, stage 3 = cytokinesis

what happens in stage 1 of the cell cycle (Interphase)

in this stage the cell grows, organelles (such as ribosome and mitochondria grow and increase in number, the synthesis of proteins occurs, DNA is replicated (forming the characteristic ‘X’ shape) and energy stores are increased

what happens in stage 2 of the cell cycle (mitosis)

the chromosomes line up at the equator of the cell and cell fibres pull each chromosome of the ‘X’ to either side of the cell

what happens in stage 3 of the cell cycle (cytokinesis)

two identical daughter cells form when the cytoplasm and cell membranes divide

why is mitosis so important

cell division by mitosis in multicellular organisms is important in their growth and development, and when replacing damaged cells. mitosis is also a vital part of asexual reproduction, as this type of reproduction only involves one organism, so to produce offspring it simply simply replicates its own cells

what mitosis produce and why?

mitosis produces 2 daughter cells, each with identical sets of chromosomes to the parent cell. because the sets of chromosomes in the daughter cell’s nucleus are the same as in the parent cell’s nucleus, mitosis produces 2 genetically identical diploid daughter cells

how does growth occur in animals

growth occurs via cell division and differentiation. cell division occurs by mitosis, after which cells can differentiate to specialised forms, specially adapted to their function. for example, cells of the muscular system can bring about movement, and cells of the circulatory system are specialised to transport substances.

in animals, almost all cells differentiate at an early stage and then lose this ability. most specialised cells can make more of the same cell by undergoing mitosis (the process that involves a cell dividing to produce 2 identical cells). others such as red blood cells (which lose their nucleus) cannot divide and are replaced by adult stem cells (which retain their ability to undergo differentiation)

when does cell division happen in mature animals

mostly only to repair or replace damaged cells, as they undergo little growth

how does growth occur in plants

growth occurs by cell division and differentiation, but also by a unique process called elongation. plant cells can grow longer in a specific direction by absorbing water into their vacuoles, and this is controlled by substances called auxins

in plants, many types of cells retain the ability to differentiate throughout life. they only differentiate when they reach their final position in the plant, but they can still re-differentiate when it is moved to another position

how does cancer occur

cancer occurs as a result of small changes in cells, that lead to uncontrolled cell division. the group of cells that results from this uncontrolled cell division is called a tumour

how is growth of babies measured

percentiles charts, which ca tell us the rateat which an organism of interest is growing. the growth of babies can be measured by using mass, length or head circumference

being in a high percentile (eg. 90th of higher) can indicate a health problem

what is a stem cell

a stem cell is an undifferentiated cell which can undergo division to produce many more similar cells, of which some will differentiate to have different functions

types of stem cells

embryonic stem cells, adult stem cells, meristems in plants

embryonic stem cells

• form when an egg and sperm cell fuse to form a zygote

• they can differentiate into any type of cell in the body

• scientists can clone these cells (throgh culturing them) and direct them to differentiate into almost any cell in the body

• these could potentially be used to replace insulin-producing cells in those suffering from dibetes, new neural cells for diseases such as Alzheimer’s, or nerve cells for those paralysed with spinal cord injuries

adult stem cells

• if found in bone marrow they can form many types of cells including blood cells

meristems in plants

• found in root and shoot tips

• they can differentiate into any type of plant, and have this ability throughout the life of the plants

• they can be used to make clones of the plant - this may be necessary if the parent plant has a certain desireable features (such as disease resistance), for research or to save a rare plant from extinction

what is therapeutic cloning

therapeutic cloning involves an embryo being produced with the same genes as the patient

• the embryo produced could then be harvested to obtain the embryonic stem cells

• these could be grown into any cells that the patient needs, such as new tissues or organs

• the advantage is that they would not be rejected as they would have the exact same genetic make-up as the individual

benefits of research with stem cells

• can be used to replace damaged or diseased body parts

• unwanted embryos from fertility clinics could be used as they would otherwise be discarded

• research into the process of differentiation

problems of research with stem cells

• we do not completely understand the process of differentiation, so it is hard to control stem cellsto from the cells we desire

• removal of stem cells results in destruction of the embryo

• people may have religious or ethical objections as it is seen as interference with the natural process of reproduction

• if the grwoing stem cells are contaminated with a virus, an infection can be transfered to the individual

• money and time could be better spent on other areas of medicine

cerebral hemispheres

the most recognisable part of the brain, the two large cerebral hemispheres take up most of the skull and sit on the left and right-hand sides. together, these two parts are known as the cerebrum (not to be confused with the cerebellum) and perform a huge variety of functions, including consciousness, memory, intelligence as well as visual and sensory processing

cerebellum

the cerebellum is a large ‘lump like’ structure found at the bottom of the brain, on the rear side.

it is responsible for controlling fine movements of muscles, so we can move in complex ways. for example, when you catch a ball, your cerebellum) responsible for your co-ordination) will be highly active

medulla oblongata

the medulla oblongata, as the name suggests, is a small ‘elongated’ structure, and is found in the brainstem, at the base of the brain. it is responsible for maintaining basic autonomic (automatic) bodily functions, such as breathing, digestion, swallowing and sneezing

explain how the difficulties of accessing brain tissue inside the skull can be overcome by using CT scanning and PET scanning to investigate brain function

as doctors need to look inside the brain to examine brain tissue for injury and disease, they cannot cut the skull open and physically examine the brain as this is highly invasive - and often does not provide any clues about brain function. instead, they can use CT and PET scans to look inside the brain much more easily

how do CT scans work

CT scans fire X-ray radiation at the brain from several different angles to generate a 3D image of the brain. this is useful for examining bleeding within the skull, and damage to brain structures. this is usually not recommended for pregnant women and children, as it exposes the patient to higher doses of radiation than a normal x-ray

how do PET scans work

for PET scans, a radioactive ‘tracer’ is injected into the blood before the scan. the scan itself is sensitive to the tracer, so areas where the tracer builds up (which will also be areas with greater blood flow) will be highlighted more brightly on the resulting scan. this is useful for identifying cancerous tumours, as these use more blood than normal tissue

why is investigating brain functions and treating brain damage and disease difficult

• it is complex and delicate

• it is easily damaged

• drugs given to treat diseases cannot always reach the brain because of the membranes that surround it

• it is not fully understood which part of the brain does what

issues with cancerous tumours in the brain

cancerous tumours can form in the brain as in any other part of the body (recall that changes in cells leading to uncontrolled cell division is what results in a cancerous tumour)

these tumours can push against other strcutures and blood vessels in the brain, restricting their function. often, tumours can be buried deep in the brain or spinal cord, making them especially difficult to remove

what is the nervous system and how does it work

the nervous system allows us to react to our surroundings, and coordinate actions in rsponse to stimuli

1. receptor cells convert a stimulus into an electrical impulse

2. this electrical impulse travels along cells called sensory neurons to the central nervous system (CNS)

3. here, the information is processed and the apporpriate response is coordinated, resulting in an electrrical impulse being sent along motor neurones to effectors

4. the effectors carry out the responses (thismay be muscles contracting or glands secreting bormones)

what are automatic responses and why are they important

automatic responses which take place before you can think are called reflexes. they are important as they prevent the individual from getting hurt. this is because the information travels down a pathway called a reflex arc, allowing vital responses to take place quickly. this pathway is different from the usual response to stimuli because the impulse does not pass through the conscious areas of your brain

explain the steps of a reflex arc and an example

1. a stimulus is detected by receptors

2. impulses are sent along a sensory neuron

3. in the CNS the impulse passes to a relay neuron

4. impulses are sent along a motor neuron

5. the impulse reaches an effector resulting in the appropriate response

   examples of reflex arcs are: pupils getting smaller to avoid damage from bright lights, moving your hand from a hot surface to prevent damage

what are synapses

synapses are the gaps between two neurons

• when the impulse reaches the end of the first neuron, a chemical called a neurotransmitter is released into the synapse

• this neurotransmitter diffuses across the synapse

• when the neurotransmitter reaches the second neuron, it triggers the impulse to begin again in the next neuron. different neurotransmitters have different effects on the frequency and speed of the impulse in the second neuron

what does myelinated mean and what are the advantages of that

myelinated means the nerves are surrounded by a myelin sheath. the advantage of this is that it allows the nerve transmission or action potential to travel faster

how is myelin produced

produces by cells called Schwann cells

what is the eye

the eye is a sense organ containing receptors sensitive to light intensity and colour. it has many different structures within it. they are adapted to allow the eye to change its shape in order to focus on near or distant objects (a process called accommodation), and to dim light

what is the retina

layer of light sensitive cells found at the back of the eye. when light hits this, the cells are stimulated. impulses are sent to the brain, which interprets the information to create an image. the retina contains rod cells and cone cells, each of which convert light to nerve impulses destined for the brain. rod cells are more sensitive to light so they are better for seeing in low light, whereas cone cells allow colour vision

what is the cornea

the see through layer at the front of the eye. it allows light through and the curved surface bends and focuses light onto the retina

what is the iris

muscles that surround the pupil. they contract or relax to alter the size of the pupil. in bright light, the circular muscles contract and radial muscles relax to make the pupil smaller - avoiding damage to the retina.

in dim light, the circular muscles relax and the radial muscles contract to make the pupil larger - so more light can enter to create a better image

what are ciliary muscles and suspensory ligaments

hold the lens in place. they control its shape and allow us to focus on objects nearer or further away

what is the lens

transparent, curved surface on the front of the eye. the lens, like the cornea, reflects light onto the retina

what is the process of accommodation when focusing on a near object

to focus on a near object:

• the ciliary muscles contract

• the suspensory ligaments loosen

• the lens is then thicker and more curved - this refracts the light more

what is the process of accommodation when focusing on a distant object

to focus on a distant object

• the ciliary muscles relax

• the suspensory ligaments tighten

• the lens then becomes thinner - light is refracted less

examples of eye defects

short sightedness (called myopia), long sightedness (called hyperopia), cataracts and colour blindness

what is wrong in myopia

the lens is too curved, so distant objects appear blurry

what is wrong in hyperopia

the lens is too flat, so it cannot refract light enough

what does cataracts mean

cataracts means clouding of the lens of the eye. this can often occur congenitally from birth) but can also develop over time. it restricts vision and can be treated by replacing the lens of the eye in surgery

what is colour blindness

colour blindness is the inability to see certain colours. full colour blindness is rare, whereas specific colour blindness (eg.red-green) is very common. this usually occurs because people with the condition do not have enough cone cells in their retina. this is usually genetic

treatment methods

1. they can be treated with spectacle lenses - concave lenses to spread out the light to treat myopia and convex lenses to bring the rays together to treat hyperopia

2. contact lenses - work in the same way as glasses but allow activities such as sport to be carried out, hard or soft contact lenses last for different lengths of time

3. laser eye surgery - lasers can be used to either reduce the thickness of the cornea (so it refracts light less) to treat myopia or change its curvature (so it refracts light more strongly) to treat hyperopia

4. replacement lens - hyperopia can be treated by replacing the lens with an artificial one made of clear plastic (or adding the plastic on top of the natural lens). the risk include damage to retina or cataracts developing