Alevelbiology讲义6Geneticcontrol.docx

1、Alevelbiology讲义6GeneticcontrolF Genetic control Syllabus Contents1 Structure and replication of DNAStructure: double helixThe 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(gu

2、anine) 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 o

3、f DNA in protein synthesis Samples of A-LEVEL biology examination for analyzing1 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 RNADNA RNAStandDouble strandsSingle strandUnitNucleotideNucleot

4、idePentose sugarDeoxyriboseRibosePhosphate groupYes YesNitrogen-containing baseA,T,C,GA,U,C,GBase pairingA = T(2 hydrogen bonds)CG(3 hydrogen bonds)A = U(2 hydrogen bonds)CG(3 hydrogen bonds)Complementary pairingBase complementary pairing is very important for DNA replication and transcription. It e

5、nsures 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 b

6、y 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 pairingB hydrogen bonding between nucleotidesC sugar-phosphate backbo

7、neD the double helix shapeKey: AInterpretion: 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

8、 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 duri

9、ng 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 m

10、olecules are thus formed from one old one, each new molecule containing one old strand 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

11、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

12、 DNA of some of these bacteria was extracted and analysed.What is the composition of this DNA?A 25 % hybrid DNAB 50 % hybrid DNAC 75 % hybrid DNAD 100 % hybrid DNAKey: BInterpretion:In the first DNA replication, one old DNA strand contains 15N act as template and replicates a new DNA strand contain

13、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. Ho

14、wever, 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 pol

15、ypeptide 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 pro

16、tein or polypeptide is a gene.Mutation can be defined as an unpredictable change in the base sequence in a DNA molecule(gene mutation) or in the structure or number of chromosomes (chromosome mutation). New alleles arise by gene mutation. Gene mutation include base substitution, deletions or additio

17、ns.Base additions or deletions usually have a very significant effect on the structure, and therefore the function, of the polypeptide that the allele codes for. Base additions or deletions always have large effects, because they alter every set of three bases that follows them in the DNA molecule.

18、They are said to cause frame shifts in the code. Often, the effects are so large that the protein that is made is totally useless. Or they may introduce a stop triplet part way through a gene, so that a complete protein is never made at all.Base substitutions, on the other hand, often have no effect

19、 at all because many amino acids have more than one triplet code, so even if one base is changed the same amino acid is still coded for. However, base substitutions can have very large effects. If, for example, the base sequence ATG mutated to ATT, this has produced a stop triplet, so the synthesis

20、of the protein would stop at this point.Sample:1.(9700_w11_qp_11,22) Which molecule has its synthesis directly controlled by DNA?A amylaseB cholesterolC glycogenD phospholipidKey: AInterpretion: amylase is protein and protein synthesis is controlled by DNA.4 describe the way in which the nucleotide

21、sequence codes for the amino acid sequence in a polypeptide with reference to the nucleotide sequence for HbA (normal) and HbS (sickle cell) alleles of the gene for the -haemoglobin polypeptide;The haemoglobin molecule, which is made up of two chains and two chains, is nearly spherical. The gene whi

22、ch codes for the amino acid sequence in the chains is not the same in everyone. In most people, the chains begin with the amino acid sequence:Val-His-Leu-Thr-Pro-Glu-Glu-LysBut in some people, the base sequence CTT is replaced by CAT, and the amino acid sequence becomes:Val-His-Leu-Thr-Pro-Val-Glu-L

23、ysThis small difference in the amino acid sequence makes little difference to the haemoglobin molecule when it is combined with oxygen. But when it is not combined with oxygen, the unusual chains make the haemoglobin molecule much less soluble. The molecules tend to stick to each other, forming long

24、 fibres inside the red blood cells. The red cells are pulled out of shape, into a half-moon or sickle shape. Sickle cell anaemia is a severe hereditary disease and may be fatal.HbA (normal) and HbS (sickle cell) (left is HbA, right is HbS)Sample:1.(9700_s10_qp_11,23) What is the minimum number of ba

25、se substitutions required to change the nucleotide sequence of the HbA (normal) allele to the HbS (sickle cell) allele?A 1 B 2 C 3 D 4Key: AInterpretion: the base sequence CTT is replaced by CAT, and the amino acid Glu becomes Val.5 describe how the information on DNA is used during transcription an

26、d translation to construct polypeptides, including the role of messenger RNA (mRNA), transfer RNA (tRNA) and the ribosomes;Transcription:During protein synthesis, a complementary copy of the base sequence on a gene is made, by building a molecule of mRNA against one DNA strand. The mRNA then moves t

27、o a ribosome in the cytoplasm.Translation:tRNA molecules with complementary triplets of bases temporarily pair with the base triplets on mRNA, bring appropriate amino acids. As two amino acids are held side by side, a peptide bond forms between them. The ribosome moves along the mRNA molecule, so th

28、at appropriate amino acids are gradually linked together, following the sequence laid down by the base sequence on the mRNA.Types and roles of RNAmRNA: copy the genetic information stored in the DNA.rRNA: components of ribosomes.Role of ribosomes: site of protein or polypeptide synthesis.tRNA: trans

29、fer the amino acids to the ribosome in the translation.Sample:1.(9700_s11_qp_11,22) The following events occur during transcription.1 Bonds break between complementary bases.2 Bonds form between complementary bases.3 Sugar-phosphate bonds form.4 Free nucleotides pair with complementary nucleotides.B

30、efore the mRNA leaves the nucleus, which events will have occurred twice?A 1 and 2 only B 1, 3 and 4 only C 2, 3 and 4 only D 1, 2, 3 and 4Key: AInterpretion: the first of bonds break between complementary bases occurs when DNA double-strand unwind. The second of bonds break between complementary ba

31、ses occurs when mRNA separates from one DNA strand. The first of bonds form between complementary bases occurs when mRNA is produced in transcription. The second of bonds form between complementary bases occurs when DNA recovers to wind after mRNA separates from one DNA strand. Practices1.(9700_w11_

32、qp_11,20) In a genetic engineering experiment a piece of double-stranded DNA containing 6000 nucleotides coding for a specific polypeptide is transcribed and translated.What is the total number of amino acids in this polypeptide?A 500 B 1000 C 2000 D 30002.(9700_s11_qp_11,21) The mechanism of action of four drugs that inhibit DNA replication is stated below. Aphidicholine inhibits DNA polymerase.