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AS level biology
Difference between Prokaryotes and Eukaryotes
Prokaryotic cells are smaller and simpler organisms, whereas Eukaryotic
cells are more complex. The organelles found inside a Eukaryotic cell are:
Organelle Description Function
Nucleus A large organelle surrounded Chromatins is made from
by a nuclear envelope (double proteins and DNA. The pores
membrane) which contains allow substances to move
many pores. The nucleus between the nucleus and the
contains chromatins and are cytoplasm. The nucleolus
often a structure called makes ribosomes
Lysosome A round organelle surrounded Contains digestive enzymes,
by a membrane with no clear these are kept separate from
internal structure the cytoplasm by a
surrounding membrane but
can be used to digest
invading cells or to break
down worn out components
of the cell.
Vesicles A small fluid-filled sac, which Transport substances in and
is in the cytoplasm, it is out of the cell and between
surrounded by a membrane the organelle.
Ribosomes A very small organelle that The site where proteins are
floats free in the cytoplasm or made
is attached to the rough
endoplasmic reticulum
Endoplasmi There are two types of The smooth ER synthesises
c Reticulum endoplasmic reticulum these and processes lipids
are smooth ER (due to no The Rough ER folds and
ribosome's) and the rough ER, processes proteins that have
which is covered in ribosomes been made at the ribosomes
Golgi A group of fluid filled It processes and packages
Apparatus flattened sacs. Vesicles are new lipids and proteins. It
often seen at the edge of the also makes lysosomes
Centrioles Hollow cylinders, containing a Involves with the separation
ring of microtubules of chromosomes during cell
Mitochondri Are double bound membrane, The site of aerobic
a the inner one is folded to form respiration, and is where ATP
cristae and inside that is the is produced. They are very
matrix. It contains enzymes active and require a lot of
which are used for respiration energy

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There are many differences between Eukaryotes and prokaryotes, these
include the size of ribosomes, the size of the cell, and the cell walls involved;
most of the differences are listed below:
Eukaryotes Prokaryotes
Larger cells (2-200um) Extremely small cells less and 2.…read more

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Transcription occurs changing DNA to mRNA, where the mRNA leaves
the nucleus via pores in the nuclear envelope.
2. Proteins are made on ribosome's
3. The ribosomes sometimes attach to the rER where the protein made
moves through making the 3-D shape en route. (ribosomes on the rough
endoplasmic reticulum make proteins that are excreted or attached to
the cell membrane, whereas the ribosomes which are free in the
cytoplasm make proteins which stay in the cytoplasm)
4.…read more

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The Cell cycle and Mitosis
The cell cycle is a process that all body cells
from multicellular organisms use to grow and
divide. The cell cycle starts when a cell is
produced by cell division and ends with the cell
dividing to produce two identical cells. Mitosis
ensures genetic consistency.
The cell cycle consists of a period of cell growth
and DNA replication called interphase and a
period of cell division called mitosis.…read more

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The chromatids uncoil and come long and thin again.
They are now called chromosomes again.
The nuclear envelope forms around each group of
chromosomes, so there are two nuclei.
The cytoplasm divides and there are now two
daughter cells that are genetically identical to each
Mitosis is finished and the start of interphase begins.
Production of a gametes
Gametes are the male and female sex cells found in all organisms that
reproduce sexually.…read more

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The way the gametes divide is by meiosis; meiosis is a type of cell division that
happens in the reproductive organs to produce gametes. Cells that divide by
meiosis have the full number of chromosomes to start off with (23 pairs).
Without meiosis the number of chromosomes would be double the normal
1. The DNA replicates so there are two identical copies of each
chromosomes called chromatids
2. The DNA condense to form double armed
chromosomes, made from two sister
3.…read more

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Fertilisation is where male and female gametes fuse. It is a term to describe
the exact moment when the nuclei from the male and female gamete fuse.
Since each gamete contains half the required number of chromosomes the
fusion of the nuclei will create a complete set.
In mammals fertilisation occurs in the oviduct, and in flowering plants it
occurs in the embryo sac.
1. Sperm is deposited high up in the vagina, near the cervix.…read more

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Stem cells are cells
which are unspecialised and they can develop into any type of cell. Stem cells
divide by mitosis to become specialised; the process of which cells become
specialised is called differentiation.
In humans stem cells can be found in embryos where they can become
specialised into any cell. Also in some human tissues where they can
differentiate into specialised cells. An example is bone marrow.…read more

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Stem cells in Medicine
Stem cells could be used to treat some diseases, as they can develop into any
type of cell; scientists think they could be used to create a range of tissues
which can replace damaged cells.
Some stem cell treatment already exists, such as treatment for leukaemia,
where bone marrow transplants are given. Scientists are researching
treatments for injuries such as: spinal cord injuries, and heart disease.…read more

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They also provide evidence and advice to
governmental officials. There are no ethical objections to a person using
multipotent stem cells derived from adults. In the UK the HFEA regulates
research on human embryos.
Different genes are switched on and off. Such as the b-galactosidase which is
an enzyme, it breaks down the carbohydrate lactose when it is presented in
the surroundings.…read more


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