Posts tagged biology
Posts tagged biology
· Both are enclosed by plasma membranes
· Both have DNA
· Both contain ribosomes
· Both are filled with cytoplasm
· Size – Eukaryotic cells are larger and much more complex
· Nucleus – prokaryotic cells don’t have a nucleus, eukaryotic cells do.
· Eukaryotic ribosomes are much larger than prokaryotic ribosomes
· Organelles: the cytoplasm of prokaryotic cells does not contain any organelles
· Eukaryotic are linear, prokaryotic are circular
· Mitochondria are only in eukaryotic cells
· Prokaryotic cells have flagellum
· Eukaryotic cells contain membrane bound organelles
· Prokaryotic cells contain plasmids
· Eukaryotic cells have chromatin and the DNA is in chromosomes
· Prokaryotic cells have pili
The first stage of cell division is interphase which is divided into 3 phases; G1, S and G2. The cell cycle starts with G1 (Gap phase 1) during which the cell grows larger. This is followed by phase S (synthesis) during which the genome is replicated. Finally, G2 (gap phase 2) is the second growth phase which separates the newly replicated genome and marks the end of interphase.
The fourth stage is mitosis which is divided into prophase, metaphase, anaphase and telophase. During mitosis the spindle fibers attach to the chromosomes and pull sister chromatids apart. This stage separates the two daughter genomes. Finally, cytokinesis is the last stage during which the cytoplasm divides to create two daughter cells. In animal cells the cell is pinched in two while plant cells form a plate between the dividing cells.
Interphase is an active period in the life of a cell during which many metabolic reactions occur such as protein synthesis, DNA replication and an increase in the number of mitochondria and/or chloroplast.
During prophase the spindle microtubules grow and extend from each pole to the equator. Also chromosomes super coil and become short and bulky and the nuclear envelope breaks down.
During metaphase the chromatids move to the equator and the spindle microtubules from each pole attach to each centromere on opposite sides.
During anaphase the spindle microtubules pull the sister chromatids apart splitting the centromeres. This splits the sister chromatids into chromosomes. Each identical chromosome is pulled to opposite poles.
During telophase the spindle microtubules break down and the chromosomes uncoil and so are no longer individually visible. Also the nuclear membrane reforms. The cell then divides by cytokinesis to form two daughter cells with identical genetic nuclei.
Mitosis is divided into four stages; prophase, metaphase, anaphase and telophase. During prophase, the chromosomes become visible under a light microscope as they super coil and therefore they get shorter and more bulky. The nuclear envelope disintegrates and the spindle microtubules grow and extend from each pole to the equator. At metaphase the chromatids move to the equator. The sister chromatids are two DNA molecules formed by DNA replication and are therefore identical. These sister chromatids are then separated in anaphase as the spindle microtubules attaches to centromere and pulls the sister chromatids to opposite poles. As the sister chromatids separate they are called chromosomes. This means that each pole has the same chromosomes (same genetic material). Finally the microtubules break down, the chromosomes uncoil and the nuclear membrane reforms. The cell then divides into two daughter cells with genetically identical nuclei.
Option E - Neurobiology and behaviour
When we look at an object, light rays pass through the pupil and are focused by the cornea, lens and humours. The image is both upside down and in reverse; the wrong way round.
Once the light reaches the retina, the photoreceptors are stimulated. They send impulses to the bipolar neurones and ganglion cells. The axons from the ganglion cells travel to the visual areas of the cerebral cortex of the brain.
The brain then turns the image the right way up and around.
There are many visual illusions where if you look at a certain spot, it appears white, whereas other spots of the image are grey when they should be white too, such as in the Hermann Grid illusion. The reason for this is because the grey spots are in our peripheral vision where there are less light sensitive cells than there are in the fovea, which is in the centre of the retina. This sends the message that the area is grey, rather than white. The reason the grey spots disappear when we look directly at them is because we are then using our retinas, where there is a higher conentration of light sensitive cells and therefore the message of white is sent.
Contralateral (meaning oppositite side) processing is the means by which both sides of our brain work together to form a complete image. The optic chiasma is the point where all nerve fibres converge. Basically, some nerve fibres bring in information from only the RIGHT half of each visual field (eye). These fibres converge at the optic chiasma and pass to the left side of the brain. The same in reverse; nerve fibres that bring in information from the left half of each visual field converge at the optic chiasma and pass through to the right side of the brain. From here, this information is used in conjunction with other stimuli such as colour, form and motion to rebuild a complete image in the cerebral cortex.