7.1 DNA Structure & Replication Essential Idea: The structure of DNA is ideally suited to its function. DNA is a double helix, consisting of two anti-parallel chains of polynucleotides that are held together by hydrogen bonds between complementary bases on the different strands. This structure allows the double helix to be replicated, with one ‘old’ strand combining together with a new strand in semi- conservative replication. And DNA is transcribed into mRNA, which is then translated into a polypeptide. DNA’s function is to transmit genetic information from one generation to the next. It is a stable macromolecule that can be replicated with a high degree of fidelity in the enzyme assisted process of replication. These copies are passed on from one generation of cells to the next. By Darren Aherne
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7.1 DNA Structure & Replication
Essential Idea: The structure of DNA is ideally suited to its function.
DNA is a double helix, consisting of two anti-parallel chains of polynucleotides that are held together by hydrogen bonds between complementary bases on the different strands. This structure allows the double helix to be replicated, with one ‘old’ strand combining together with a new strand in semi-conservative replication. And DNA is transcribed into mRNA, which is then translated into a polypeptide.
DNA’s function is to transmit genetic information from one generation to the next. It is a stable macromolecule that can be replicated with a high degree of fidelity in the enzyme assisted process of replication. These copies are passed on from one generation of cells to the next.
By Darren Aherne
Understandings, Applications and Skills
Understandings, Applications and Skills
7.1 S1 Analysis of results of the Hershey and Chase experiment providing evidence that DNA is the genetic material.
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• Until the Hershey Chase experiment, it seemed that protein was the genetic material because it had great variety in structures
• Hershey & Chase took advantage of the fact that DNA contains phosphorus, but not sulfur, & protein contains sulfur, but not phosphorus. Viruses have protein coats surrounding DNA.
• They grew viruses in two environments- type 1 with radioactive phosphorus, and type 2 with radioactive sulfur.
• Rosalind Franklin worked at King’s College in London as a technician doing X-ray crystallography.
• She improved the resolution of the cameras used in order to obtain the most detailed images yet of X-ray diffraction of DNA.
• These detailed images allowed her to make very exact measurements related to the structure of DNA.
• Her work was shared with James Watson without her permission.
• Watson and Crick used her measurements to show that the phosphate groups were on the outside of the DNA double helix, and that the nitrogenous bases were more hydrophobic and thus on the inside.
• Watson & Crick published the structure of DNA first, without crediting Franklin. They were awarded the Nobel prize. Franklin died of ovarian cancer she developed as a result of her work.
7.1.U2 DNA structure suggested a mechanism for DNA replication.
Antiparallel Strands• The two strands have their 5’ and 3’
terminals at opposite ends
3’ – 5’ Linkages• 5’ end of a DNA strand- the deoxyribose
is linked to a phosphate• 3’ end of a DNA strand- a hydroxyl (-OH)
is attached to the 3’ carbon in a deoxyribose
• Nucleotides in a strand are linked by covalent phosphodiester bonds, linking the 3’ of one nucleotide to the phosphate attached to the 5’ on the adjacent nucleotide
7.1.U4 DNA replication is continuous on the leading strand and discontinuous on the lagging strand.7.1.U5 DNA replication is carried out by a complex system of enzymes.
Genes are coding regions of DNA—they code for a polypeptide.
• In humans, only about 1%-2% of DNA codes for a polypeptide. These coding regions are called exons.
• The remainder of the DNA is non-coding.
• The non-coding regions are typically made up of repetitive sequences of DNA.
From Biology Course Companion, Allott, A, Oxford University Press, 2014
Some Functions of Non-Coding Regions of DNA
Production of RNA Some regions on DNA function to produce tRNA and rRNA
Gene expression Non-coding regions can have an role in regulating the expression of genes by promoting or inhibiting.
Telomeres Telomeres are located on the ends of eukaryote chromosomes, they have a protective function because DNA cannot be replicated all the way to the ends, so telomeres prevent loss of important genes.
Locus: the location of a gene on a chromosome (plural: loci)
Variable Number Tandem Repeats (VNTR): are chromosomal regions in which a short DNA sequence(such as GC or AGCT) is repeated a variable number of times end-to-end at a single location (tandem repeat).
Schematic of a VNTR in 4 alleles. Each rectangle represents a repeated series of bases in DNA.
In this example, Locus A is a tandem repeat of the motif GC: there are four alleles, with two, three, four, or five repeats (A2, A3, A4, and A5, respectively). Locus B is a tandem repeat of the motif AGCT: there are only two alleles, with two or three repeats (B2 and B3, respectively).
The example shows a DNA fingerprint that includes both loci simultaneously. Individual #1 is heterozygous at Locus A (A2 / A5) and homozygous at Locus 2 (B2 / B2: note that this genotype gives a single-banded phenotype in the fingerprint). Individual #2 is heterozygous at both loci: (A4 / A3 and B3 / B2) respectively). The two individuals are distinguishable at either locus. Typical fingerprints include a dozen or more VNTR loci.`