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Chapter 16 (Replication of DNA (Definitions (replicon: during DNA…
Chapter 16
Replication of DNA
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Before a cell undergoes nuclear division, either mitosis or meiosis, DNA is replicated.
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Definitions
replicon: during DNA replication, a short segment of DNA that has opened and where replication is occuring
primer RNA: during DNA replication, a short piece of RNA that is synthesized against open DNA and from which DNA polymerase can begin building a new molecule of DNA
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semiconservative replication: refers to the fact that during DNA replication, one new molecule is paired with one original molecule such that in every new chromosome, half the DNA is conserved from the pre-existing chromosome
replication fork: in DNA replication, the point at which the double helix opens and formation of new DNA occurs
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DNA, found within the nucleus, must be replicated in order to ensure that each new cell receives the correct number of chromosomes.
Step 1: Before DNA can be replicated, DNA must be “unzipped” into two single strands.
In order to unwind DNA, these interactions between base pairs must be broken. DNA helicase breaks the hydrogen bonding between base pairs to separate the strands into a Y shape known as the replication fork.
DNA has four bases called adenine (A), thymine (T), cytosine (C) and guanine (G) that form pairs between the two strands. Adenine only pairs with thymine and cytosine only binds with guanine.
Step 2: Once the DNA strands have been separated, a short piece of RNA called a primer binds to the 3' end of the strand. Primers are generated by the enzyme DNA primase.
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Step 4: Once both the continuous and discontinuous strands are formed, all RNA primers are removed from the original strands.
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Replication produces two DNA molecules, each with one strand from the parent molecule and one new strand.
Concepts
Definitions
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phenotype: the expression of alleles in the individual's size, shape, or metabolism
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Reproduction
Sexual
Combines alleles with others, possibly better adapted individuals = increases possibilities of own alleles survival
For example: Original plant may die in drought, but copies of its alleles survive in such progeny, due to protection of alleles from resistant parent. The second parent should benefit as well with not being quite superior in all defensive attributes.
Fitness and survival capacity are not governed by just one gene, but all genes. (includes proper construction of membranes, function of organelles, and production and transport of hormones)
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Each plant produces thousands of sex cells, each with unique genotype, which eventually fuse with other sex cells of other individuals.
For example: When plants bloom, they produce thousands or millions of pollen grains that are carried away and fertilize ovules on many other plants; simultaneously, its own ovules are receiving pollen from numerous plants.
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Asexual
Offspring identical to parents and siblings, therefore, same DNA and alleles.
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Mutations
Definitions
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inversion: a mutation in which a portion of the DNA double helix is excised, turned end for end, and ligated into place with reverse order
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insertion sequences: a small transposable element that contains only the genes coding for the enzymes necessary for the element's excision and insertion
transposon: a large transposable element that carries, in addition to the insertion sequence, other genes that code for proteins not directly associated with transposition
Effects
Depends on its nature, position, and extent.
If it occurs in spacer DNA between two genes, it may have no effect whatsoever.
Point mutations and small insertions and deletions in introns appear to be unimportant generally--they change a portion of hnRNA that will not be incorporated into mRNA.
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Causes
Mutagens cause mutations, such as certain chemicals, ultraviolet light, X-rays, and radiation from radioactive substances. Many are man made and are increasing in our environment.
Numerous classes of DNA repair enzymes exist, one of which can recognize abnormalities such as a G-U pair, and change it back to a G-C pair.
In meristematic cells that are replicating their DNA, DNA polymerase may arrive before the repair enzyme.
Deletions can be caused in several ways; one method is due to short regions of a self-complementary sequence.
Transposable elements
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Transposon mutagenesis offers the possibility of quickly correlating metabolic pathways, proteins, genes, and even promoters for virtually any aspect of an organism.
Insertion mutations can be caused by many methods because a variety of enzymes cut and rejoin DNA as part of repair processes. If a small piece of foreign DNA is present after cutting, it may accidentally be incorporated into the chromosomal DNA by DNA ligase. #
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