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Genetics (Replication of DNA (Next, one strand of DNA double helix is cut…
Genetics
Replication of DNA
before a cell undergoes nuclear division, either mitosis or meiosis
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replication doubles the amount of DNA and each gene exists in at least two copies,
one on each of the two chromatids,
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as replication begins
chromatin first becomes less compact, opening sufficiently to allow entry of the necessary replicating factors
DNA does not release histones; instead, Nucleosomes structure remains intact.
Next, one strand of DNA double helix is cut and the two strands separate from each other in a short region, forming a small "Bubble" called a replicon pic
with the helix opened, free nucleotides diffuse to regions of single-stranded DNA and pair with its bases along both strands
these are ribonucleotides, not deoxyribonucleotides, and they are pomerized into short pieces of Primer RNA Pic
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replication fork
as DNA uncoils and seperates, it has a forked appearance
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Monohybrid Crosses
Sexual reproduction between two individuals is called cross #
the meiotic divisions that precede a cross reduce the number of sets the chromosomes per cell from the diploid number to the haploid number
each sex cell- that is , each sperm cell and egg cell- contains one complete set of genes.
each sperm cell contains all the genes necessary to construct a new plant; the same is true of each egg cell
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within a population, mutation produce new alleles and genotypes of individuals within the population differ. of the plants that grow in an area and that can interact sexually,
many may have the same allele of a particular gene but other individuals any have other alleles, other versions of the gene
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test crossing
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if traits has Complete Dominance, it is difficult to know what genotype of any particular plant is unless the plant shows the recessive trait.
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all gametes produced by homozygous recessive parents carry the recessive allele, which is unable to mask the homologous allele in resulting F1 zygote
after the actual genotypes pf plants are known, those that are homozygous dominant can be gathered and planted in special areas,
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kept free of all natural pollinators, and allowed to breed among only themselves
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Multiple alleles
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a protein of average size consist of about 300 amino acids, so the coding portion of its mRNA must have about 300 codons, each containing three nucleotide.
the gene is therefore at least 900 sites exist at which point mutations can occur, and of course, any mutation may involve several nucleotide
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when genes are polymorphic, hahving multiple alleles, numbers such as X1,X2,X3,X4 ans so on are used rather than capital and lowercase letters
certain mutations still result in the production of a protein with the normal sequence, but most lead to altered protein structure
within population of plants, as many types of gametes can be produced as there are different types of alleles
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Concepts
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the chemical basis of genetic inheritance is the gene,
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if all plants of a species had exactly identical nucleotide sequences in their DNA, then all those plants would be identical physically
alleles
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Mutation can occur in any gene in any individual, and thus, gradually a population of separate plants comes to have a variety of alleles
as a result of mutation, the population of individuals has varying genotypes and phenotypes
not identical, as is apparent from considering humans
Genotypes
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and the expression of those alleles in the individual size, shape, or metabolism is its Phenotype pic
Selective advantageous
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in asexual reproduction, each offspring is identical to its parents and siblings
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even though this seems like the most safe mechanism for producing offspring to carry the parents genes into future generation,
the progeny are never more fit than the parent and no progeny is adapted to any environment other than the parents environment
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Mutation
Causes of mutations
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as DNA unwinds ahead of the DNA Polymerase,
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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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Effects of mutation
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if it occurs in spacers DNA between two genes, it may have no effect whatsoever.
point mutation and small insertion and deletion in introns appear to be unimportant generally- they change portions of hnRNA that will not be incorporated into mRNA #
Within exons, a small mutation may not be important if it only changes a codon into another codon that specifies the same or a similar amino acid.
changing codons to ones that specify very different amino acids may not matter if they are located in a part that is not critical to the proteins' functioning
a mutation in an exon may cause the gene to code for proteins whose active sit is disrupted and the proteins cannot function
Statistically, Mutation are almost always harmful
enzymes tend to be approximately 300 to 400 amnion acids long and hundreds of trillions of proteins could possibly exist
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any mutation that changes the structure of proteins, rRNA, or tRNA is more likely to produce a less useful than more useful forms
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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when a leaf falls off in the fall, the mutated gene decomposes along with the rest of the leaf.
the same is true of any mutation in roots, wood or bark
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the mutation may occur in a leaf cell nucleus, but in gene that is inactive in leaves, such as a gene that affects root hair growth or bark information
DNA Repair Processes
because most mutations are deleterious and occur frequently enough to be significant problems, it is selectively advantageous for organisms to have DNA repair mechanisms that recognize and remove mutations
certain mechanisms recognize base mismatches, loops or other problems; other enzymes minimize the number of errors that occur in the first place
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mutations occur in the genes that code for repair enzymes, resulting in serious problems
Mutations
a Mutation is any change, however large or small, in DNA
the smallest mutation, affecting the least amount of DNA
Point Mutation
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Deletion
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Insertion
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Inversion
Under some conditions a piece of DNA becomes tangled and breaks and during repair, it puts in backwards
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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
because no preferential sites for crossing-over seem to exist, the farther apart two genes are the greater the possibility that crossing over will occur between them
most chromosomes are so long that crossing over occurs several tomes within each chromosome during prophase 1
if the two genes are close together on a chromosome, however, crossing over may occur and the two alleles may move together during meiosis 1,
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Gene on Separate chromosomes: independent Assortment #
if the twp genes are on separate chromosomes, the alleles of one gene move independently of alleles of other gene, Independent assortment
the chromosomes have duplicated during S phase and each has two copies of each allele; all chromosomes align on the metaphase plate during the metaphase 1.
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there are two Y alleles, two y alleles , two S alleles and two s alleles at the metaphase plate
During anaphase 1, homologous chromosomes separate from each other and both Y alleles move to one pole because they are on the two chromatids of one chromosome
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both y alleles, located on the two chromatids of the homologous chromosomes
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During meiosis 2 , the two chromoatids of each chromosomes separate from each other resulting in four types of haploid cells in equal numbers; SY, sY, Sy, sy
after the possible types of gametes are known, the punnet square can be set up #