F Genetic control
Ⅰ Syllabus Contents
1 Structure and replication of DNA
国际象棋游戏规则Structure: double helix
The basic unit is nucleotide. A nucleotide is made of a 5-carbon sugar(deoxyribose), a phophate and a nitrogen base. Four nitrogen bases in DNA are: A(adenine), C(cytosine), G(guanine) and T(thymine).
A = T(2 hydrogen bonds)
CG(3 hydrogen bonds)
Semi-conservative replication: the DNA double helix unzips, and each strand serves as a template for the formation of a new strand. The new molecules each consist of one old strand and one new strand.
Direction: from 5' to 3' end.
2 Role of DNA in protein synthesis
Ⅱ Samples of A-LEVEL biology examination for analyzing
1 describe the structure of RNA and DNA and explain the importance of base pairing and the different hydrogen bonding between bases;
the structure of DNA and RNA
DNA | RNA | |
Stand | Double strands | Single strand |
Unit | Nucleotide | Nucleotide |
Pentose sugar | Deoxyribose | Ribose |
Phosphate group | Yes | Yes |
Nitrogen-containing base | A,T,C,G | A,U,C,G |
Base pairing | A = T(2 hydrogen bonds) CG(3 hydrogen bonds) | A = U(2 hydrogen bonds) CG(3 hydrogen bonds) |
Complementary pairing
Base complementary pairing is very important for DNA replication and transcription. It ensures the accuracy of genetic information transmission. The number of four kinds of different bases and their sequence in a DNA molecule represent some genetic information. During DNA semi-conserve replication or transcription, one old DNA strand act as a template, four kinds of bases are pairing by A pair with T in DNA replication or A pair with U in transcription and C pair with G. There are little error occurred during base pairing.
Sample:
1.(9700_w11_qp_11,21) What makes the exact copying of DNA molecules possible?
A base pairing
B hydrogen bonding between nucleotides
C sugar-phosphate backbone
D the double helix shape
Key: A
Interpretion: Base complementary pairing is very important for DNA replication. It ensures the accuracy of genetic information transmission. The number of four kinds of different bases and their sequence in a DNA molecule represent some genetic information. During DNA semi-conserve replication, one old DNA strand act as a template, four kinds of bases are pairing by A pair with T in DNA replication and C pair with G. There are little error occurred during base pairing.
2 explain how DNA replicates semi-conservatively during interphase;
DNA molecules replicate during interphase. The hydrogen bonds between the bases break, allowing free nucleotides to fall into position opposite their complementary ones on each strand of the original DNA molecule. Adjacent nucleotides are then linked, through their phosphates and sugars, to form new strands. Two complete new molecules are thus formed from one old one, each new molecule containing one old str
and and one new.
Sample:
1.(9700_w11_qp_11,23) Bacteria were grown in a medium containing 15N. After several generations, all of the DNA contained 15N. Some of these bacteria were transferred to a medium containing the common isotope of nitrogen, 14N. The bacteria were allowed to divide once. The DNA of some of these bacteria was extracted and analysed. This DNA was all hybrid DNA containing equal amounts of 14N and 15N.
The remaining bacteria were left in the medium with 14N and allowed to divide one more time.
The DNA of some of these bacteria was extracted and analysed.
What is the composition of this DNA?
A 25 % hybrid DNA
B 50 % hybrid DNA
C 75 % hybrid DNA
D 100 % hybrid DNA
Key: B
Interpretion:
In the first DNA replication, one old DNA strand contains 15N act as template and replicates a new DNA st
rand contain 14N, so does the other old one. As a result, one DNA molecule contain 15N is divided into two DNA molecules contain both 15N and 14N, namely hybrid DNA.
In the second DNA replication, one old DNA strand contains 15N act as template and replicates a new DNA strand contain 14N, formed a hybrid DNA. However, the other old DNA strand contains 14N and replicates a new DNA strand also contain 14N, formed a non-hybrid DNA.
In other words, only 50% DNA produced in the second DNA replication are hybrid DNA.
3 state that a gene is a sequence of nucleotides as part of a DNA molecule, which codes for a polypeptide and state that a mutation is a change in the sequence that may result in an altered polypeptide;
The sequence of bases on a DNA molecules codes for the sequence of amino acid in a protein or polypeptide. Each amino acid is coded for by three bases. A length of DNA coding for one complete protein or polypeptide is a gene.
Mutation can be defined as an unpredictable change in the base sequence in a DNA molecule(gene mutat
ion) or in the structure or number of chromosomes (chromosome mutation). New alleles arise by gene mutation. Gene mutation include base substitution, deletions or additions.
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