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Genetics (Replication of DNA (Before a cell undergoes nuclear division,…
Genetics
Replication of DNA
Before a cell undergoes nuclear division, DNA is replicated during S phase of the cell cycle.
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Primer RNA acts as a substrate for the DNA-synthesizing enzyme, DNA polymerase
This method of replication, in which each strand of DNA acts as the template for making the complementary strand, is semiconservative replication because each resulting double helix contains one new molecule and has conserved one old one.
A replication fork is when DNA uncoils and then separates, it has a forked appearance.
New pieces of DNA are ligated, attached to each other with covalent bonds.
Mutations
Causes of Mutations
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Insertion sequences are only a few thousand base pairs long and contain the genes that code for the enzymes actually involved in cutting the insertion sequence out and splicing it into DNA somewhere else.
A transposon is like an insertion sequence except that it may be much longer and carries genes that code for proteins not associated with transposition.
Effects of Mutations
The effect and significance of a mutation depend on its nature, its position, and its extent.
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Mutations in promoter regions can completely inactivate a gene or cause it to be active in the wrong time or place.
Somatic Mutations
Mutations can occur at any time in any cell, but if they happen in cells that never lead to sex cells, they are called somatic cells.
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DNA Repair Processes
Most mutations are deleterious and occur frequently enough to be a significant problem, it is selectively advantageous for organisms to have DNA repair mechanisms that recognize and remove mutations.
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Mutation is any change, however large or small, in DNA.
The smallest mutation, affecting the least amount of DNA, is a point mutation in which a single base is converted to another base by any of various methods.
Monohybrid Crosses
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Test Crosses
We can only see an organism's phenotype, to discover its genotype, we must perform crosses, and the test cross is the most useful.
When traits with incomplete dominance are studied, the genotype of any plant is easy to determine from its phenotype.
The genotype can be revealed by a test cross, a cross involving the plant in question and one that is homozygous recessive for the trait being studied.
Multiple Alleles
A gene may have many alleles, not just two as in the examples discussed so far.
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Dihybrid Crosses
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Crossing-Over
Independent assortment can also occur if two genes are located far apart on the same chromosome such that crossing-over occurs between them during prophase 1, after homologous chromosomes have paired and a synaptonemal complex is formed.
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Dihybrid cross is one in which two genes are studied and analyzed simultaneously, rather than just one, as in a monohybrid cross.
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