CB2- Combined Science


Mitosis (Pt 1)

Body cells normally have 2 copies of each chromosome- they're diploid cells, (1 from the mother & 1 from the father)

When a cell divides by mitosis it makes 2 cells identical to the original cell- nucleus of each new cell contains the same number of chromosomes as the original cell.

Body cells in multicellular organisms divide to produce new cells, process: Cell Cycle, the stage of this cycle when the cell divides = MitosisMulticellular organisms use mitosis to grow/ replace cells that have been damaged. 

Some organisms use Mitosis to reproduce- asexual reproduction.

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Mitosis (Pt 2)

Main Stages of the Cell Cycle

Interphase: Before it divides the cell has to grow & increase the amount of subcellular structures (e.g mitochondria and ribosomes), it then duplicates its DNA- so there's 1 copy for each new cell. DNA is copied & forms X-shaped chromosomes, each 'arm' of the chromosome is an exact duplicate of the other.

Prophase: Chromosomes condense, getting shorter and fatter, membrane around nucleus breaks down & chromosomes lie free in the cytoplasm.

Metaphase: Chromosomes line up at the centre of the cell.

Anaphase: Cell fibres pull chromosomes apart. The 2 arms of each Chromosome go to opposite ends of the cell.

Telophase: Membranes form around each of the sets of chromosomes. These become the nuclei of the 2 new cells- nucleus has divided.

Cytokinesis: Cytoplasm and cell membrane divide to form 2 separate cells. 

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Cell Division and Growth

Growth= increase in size/ mass. Plants and animals grow& develop due to these processes:

  • Cell Differentiation- the process of which cell changes to become specialised for its job. Having specialised cells allows multicellular organisms to work more efficiently.
  • Cell Division- by mitosis. 
  • Cell Elongation- plant cell expands, making the cell bigger & making the plant grow.
  • Growth in animals happens by cell division. Animals grow whilst they're young & when they reach full growth they stop growing. So when young, cells divide at a fast rate but once an adult most cell division is for repair- cells divide to replace old/ damaged cells, it means in most animals cell differentiation is lost at an early stage.
  • In plants, growth in height- mainly due to elongation- cell division usually happens at the tip of the roots and shoots(meristems). But plants often grow continuously, so plants continue to differentiate to develop new parts e.g roots and leaves.
  • The rate at which cells divide by mitosis is controlled by the genes. If there's a change in one of the genes that controls cell division- cell may start to divide uncontrollably. Can result in a mass of abnormal cells (tumour). If tumour invades and destroys surrounding tissue it is called cancer.
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Cell Division and Growth (pt2)

Growth charts are used to assess a child's growth over time, so an overall pattern in development can be seen and any problems are highlighted (obesity, malnutrition, dwarfism)

For Example, A baby's growth is regularly monitored after birth to make sure it's growing normally. 3 measurements are taken- length, mass and head circumference.These results are plotted on growth charts- the charts show a number of percentiles. 

Example: The 50th percentile shows the mass that 50% of babies will have reached at a certain age. 

Babies vary in size but doctors are likely to investigate if a baby's size is above the top percentile line or below the bottom percentile line, their increases/ decreases by 2 or more percentile lines over time, or if there's an inconsistent pattern. (e.g a small baby with a very large head.) 

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Stem Cells

Stem Cells- Undifferentiated cell.

Depending on what instructions they're given, stem cells can divide by mitosis to become new cells which then differentiate

Stem cells are found in early human embryos, embryonic stem cells have the potential to divide and produce any kind of cell at all. All the different types of cell found in a human have to come from those few cells in the early embryo. Stem cells are really important for the growth and development of organisms. 

Adults also have stem cells- only found in certain places such as bone marrow. They aren't as versatile as embryonic stem cells- can't produce any cell type at all only certain ones. In animals, adult stem cells are used to replace damaged cells e.g. to make new skin or blood cells.

Doctors already use adult stem cells to cure some diseases- sickle cell anaemia can sometimes be cured with a bone marrow transplant. (containing adult stem cells which produce new blood cells) Scientists experimented with extracting stem cells from very early human embryos and growing them. Under certain conditions stem cells can be stimulated to differentiate into specialised cells.

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Uses of Stem cells

Might be possible to use stem cells to create specialised cells to replace those which have been damaged by disease or injury. E.g. new cardiac muscles could be transplanted into someone with heart disease, this could be a potential for new cures. 

Before this can happen a lot of research needs to be done. There are many potential risks which scientists need to learn more about. For example:

Tumour development- stem cells can divide very quickly if scientists are unable to control the rate at which transplanted cells divide inside a patient a tumour may develop.

Disease transmission- viruses live inside cells if donor stem cells are infected with a virus and this isn't picked up the virus could be passed on to the recipient and so it makes them a sticker. 

Rejection- If the transplanted cells aren't grown using the patient's own stem cells, the patient's body may recognise the cells as foreign and trigger an immune response to try to get rid of them, the patient can take drugs to suppress this response but it makes them susceptible to diseases.

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In plants, the only cells that divide by mitosis are found in plant tissues called meristems. 

Meristem tissue is found in the areas of a plant that are growing e.g. the tips of the roots and shoots. 

Meristems produce unspecialised cells that are able to divide and form any cell types in the plant- they act like embryonic stem cells. But unlike human stem cells, these cells can divide and differentiate to generate any type of cell for as long as the plant lives. 

The unspecialised cells go on to form specialised tissues like xylem and phloem.

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The Nervous System

The nervous system is made up of neurones (nerve cells) which go to all parts of the body.

The body has lots of sensory receptors- a group of cells that can detect a change in your environment (stimulus). Different receptors detect different stimuli. For example, receptors in your eyes detect light, while receptors in your skin detect touch (pressure) and temperature change.

When a stimulus is detected by receptors the information is converted to a nervous (electrical) impulse and sent alone sensory neurones to the CNS (brain and spinal cord)

The CNS coordinates the response (decides what to do about the stimulus and decides what to do about the stimulus and tells something to do it) Impulses travel through the CNS across relay neurones. 

The CNS sends information to an effector (muscle/gland) along a motor neurone. Effector then responds accordingly- e.g. a muscle may contract/ a gland may secrete a hormone.

The time it takes you to respond to a stimulus is called your reaction time. 


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The Nervous System (Pt 2)

All neurones have a cell body with a nucleus. The cell body has extensions that connect to other neurons- dendrites and dendrons carry nerve impulses towards the cell body and axons carry nerve impulses away from the cell body. 

Some axons are surrounded by a myelin sheath. This acts as an electrical insulator- speeding up the electrical impulse.

 Neurones can be very long- also speeds up the impulse (connecting with another neurone slows impulse down, so 1 long neurone is much quicker than lots of short ones joined together).

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The Nervous System (Pt 3)

Sensory Neurone

  • One long dendron carries nerve impulses from receptor cells to cell body which is located in the middle of the neurone. One short axon carries nerve impulses from cell body to CNS.

Motor Neurone

  • Many short dendrites carry nerve impulses from the CNS to the cell body. One long axon carries nerve impulses from CND to cell body. One long axon carries nerve impulses from the cell body to effector cells/

Relay Neurone

  • Many short dendrites carry nerve impulses from sensory neurones to cell body. An axon carries nerve impulses from cell body to motor neurones.
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