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All known living organisms are composed of one or more cells. Cells are the units from which all living organisms are built. Some organisms (e.g. bacteria) have only one cell in the entire organism. Others are multi-cellular.
In this course, you will learn about the basic units of life, of how each cell is a self-contained and partially self-sufficient compartment designed to carry out a limited series of functions.
£299.00
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All known living organisms are composed of one or more cells. Cells are the units from which all living organisms are built. Some organisms (e.g. bacteria) have only one cell in the entire organism. Others are multi-cellular.
In this course, you will learn about the basic units of life, of how each cell is a self-contained and partially self-sufficient compartment designed to carry out a limited series of functions.
The word cell is derived from the Latin “cella” which means “small room”. Cells are the units from which all living organisms are built. Some organisms (e.g. bacteria) have only one cell in the entire organism. Others are multi-cellular. A human body can contain an estimated 100,000 billion cells. Each cell is a self-contained and partially self-sufficient compartment designed to carry out a limited series of functions. While the structure and function of cells is extremely variable, their basic structure is similar. All cells are bound by an outer membrane and contain cytoplasm and DNA.
A gene may be defined as a section of DNA that controls a hereditary characteristic.
The cell nuclei of all plants and animals carry hundreds or thousands of genes that control all the aspects of the plant or animals, but each gene controls only one particular factor. For example, Aberdeen Angus cattle all carry a gene that gives their black coat colour. They have no gene for white coats because a pure bred Angus has no white on its coat. In the same way, Hereford cattle all carry a gene for red coat and a white face but no gene for a black coat.
Generally speaking the more complex the animal the larger the number of genes to code for it. However this is not always the case, humans have approximately 20,000 – 25,000 genes while the black cottonwood tree contains over 45,000 and fruit flies have 14,000.
Aside from all the useful genes carried in DNA, there is also a great deal of what appears to be useless DNA that does not carry information. This is referred to as Junk DNA and there is disagreement about whether it is useful or not. Other sections of DNA that do not code for proteins are called Introns. Introns are sections of DNA that are transcribed to various types of RNA. During the process to mature RNA, the introns are ‘spliced’ out.
The nature of the base pairing which creates two antiparallel strands (opposite polarity) which are complementary, ensures that the DNA can be replicated with astounding accuracy. Each strand can produce an exact copy of its partner, thus a cell can replicate it’s DNA before replicating. As each strand replicates its partner strand, each daughter DNA is complete with one parent strand and a new one. Hence this is known as semiconservative replication.
The unique double helix allows DNA to accurately replicate itself. In order to do this, the helix can unzip down the middle creating two strands of DNA. Where the split starts is known as the ‘origin of replication’ or ‘replication origins’ or simply ‘origins’ in both eukaryotic and prokaryotic cells. The origins are marked by certain sequence of nucleotides which attract the initiator proteins. The sequence is usually rich in A-T base pairs as they contain less hydrogen bonds and therefore require less energy to split. As the initiator proteins attract other proteins to assist, the resulting separated DNA forms a bubble in the chain. As replicated continues the DNA chain continues to unwind creating a replication fork.
This DNA fork comprises of two strands: the parent 3’ – 5’ and the parent 5’ -3’. The daughter strand that forms on the parent templates are referred to as the leading strand (forms on the 5’ -3’) and the lagging strand (forms on the 3’ – 5’). These strands are read in different directions. The leading strand is replicated with the help of DNA polymerase which is an enzyme that finds the correct base pairs and then binds them to the DNA. DNA polymerase does this continuously.
DNA polymerase is extremely accurate in replication, in fact far more so that can be accounted for by the simplicity and stability of base pairs. DNA polymerase is able to proofread and correct mistakes by checking if the previously added nucleotide is correctly paired to the template strand, if it is not correct DNA polymerase will remove the offending nucleotide by severing the phosphate bond and start adding a new nucleotide again. This proof reading mechanism is the reason the DNA polymerase can only create DNA in the 5’ – 3’ direction and not the other because in the 3’ – 5’ direction it works as a exonuclease by degrading the phosphodiester bond.
The enzyme RNA Primase is bound at the initiation point. RNA Primase attracts RNA nucleotides which can bind nucleotides to the 3’-5’ parent strand. The RNA nucleotides function as ‘primers’ for the DNA nucleotides. The enzyme helicase is responsible for splitting the two strands by breaking the hydrogen bonds.
The lagging strand however is formed in fragments as the DNA polymerase cannot work in the 3’ – 5’ direction. To get around this RNA Primase adds more RNA primers which DNA polymerase can read. These fragments between the primers are called Okazaki fragments. Other enzymes can act degrade the RNA and act as exonucleases which can remove the RNA primers. The remaining gaps are filled in by DNA polymerase and DNA Ligase which adds the necessary phosphate to form the phosphate –sugar backbone.
Replication is essential and occurs before cell division to ensure that each daughter cell has the same genetic information as the parent cell. DNA replication is conversely regulated by the cell cycle in eukaryotes.
This course contains 10 lessons as follows:
Open the door to a career in –
An introductory yet challenging course designed for everyone wanting to learn more about biology.
Research and knowledge of cell biology plays an important part in many areas of life, including:
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