27 Molecular genetics

Transcription

1 Class: Name: ( ) Date: 27 Molecular genetics 27.1 From DNA to proteins (Book 4, p. 27-3) A The genetic code (Book 4, p. 27-3) The base sequence of DNA determines the amino acid sequence in a polypeptide in a way called the (1). In a of DNA, (2) consecutive bases code for one amino acid. This genetic code is thus called a (3) ( 三聯體密碼 ). Since there are four bases in DNA, this would produce (4) different triplet codes. The genetic code is (5) ( 簡併的 ) (i.e. some codes code for the same amino acid). The genetic code has no gaps and is read in a (6) - manner. The genetic code is (7) (i.e. the same triplet code codes for the same amino acid in all organisms). triplet codes on DNA code for amino acids and signals Triplet codes on a DNA code for the same amino acid stop B How are proteins synthesized (Book 4, p. 27-4) The process by which genetic information contained in a gene is decoded ( 解碼 ) to make a protein is called (8) ( 基因表達 ). Genetic information is transferred from the nucleus to the cytoplasm and then decoded to make a protein DNA nucleus transcription through (9) ( 轉錄 ) and (10) ( 轉譯 ). polypeptide translation cytoplasm (Second Edition)

2 1 Transcription a The weak (11) that hold the two DNA s of a gene break. The two DNA s unwind. hydrogen bonds hydrogen bonds break and two DNA s unwind b c One of the DNA s acts as a template for the synthesis of (12) () ( 信使 RNA). This is called the (13) ( 模板鏈 ). The other DNA is called the (14) ( 編碼鏈 ). Free ribonucleotides are attracted to the (15) through complementary base pairing. (16) (RNA 聚合酶 ) catalyses the joining of the adjacent ribonucleotides to form (17). Every three bases on the that is complementary to the triplet code on the DNA is called a (18) ( 密碼子 ). template template triplet code free ribonucleotides with bases complementary to template codon coding coding - The base sequence of the produced is complementary to the (19), but the same as the (20) of DNA except having (21) instead of T. - After transcription, the leaves the nucleus and enters the cytoplasm. (Second Edition)

3 2 Translation a Translation occurs in the cytoplasm where ribosomes ( 核糖體 ) are located. The produced in transcription attaches to a (22). ribosome codon 1 (start) codon 2 codon 3 codon n (stop) b Meanwhile, free amino acids join to specific (23) (trna) ( 轉移 RNA) molecules in the cytoplasm. Each trna molecule has an (24) ( 反密碼子 ) that consists of three ribonucleotides. Each anticodon specifies a particular (25). amino acid trna anticodon c The trna molecule with an anticodon complementary to the first (start) codon on the binds to the. This carries a specific amino acid to the ribosome. trna d Another trna molecule carrying an amino acid binds to the next codon. The two amino acids are then joined by a (26) bond. start codon peptide bond joining two amino acids (Second Edition)

4 e The ribosome moves to the next codon and another amino acid is added to the dipeptide. The first trna is released. The process repeats until a (27) codon is met. Finally, a polypeptide is made. the first trna leaves to pick up another amino acid third trna binds to stop codon direction of translation The genetic code expressed in terms of codons Key: Ala = Alanine Arg = Arginine Asn = Asparagine Asp = Aspartic acid Cys = Cysteine Gln = Glutamine Glu = Glutamic acid Gly = Glycine His = Histidine Ile = Isoleucine Leu = Leucine Lys = Lysine Met = Methionine Phe = Phenylalanine Pro = Proline Ser = Serine Thr = Threonine Trp = Tryptophan Tyr = Tyrosine Val = Valine (Second Edition)

5 27.2 Mutations (Book 4, p ) A What are mutations? (Book 4, p ) Mutation is a sudden and permanent change in the DNA of an organism. Mutations can be classified as (1) ( 基因突變 ) and (2) ( 染色體突變 ). 1 Gene mutations Gene mutations are changes in the (3) sequence of the DNA in a gene. Different forms of gene mutations: Base sequence on coding Amino acid sequence* Normal ATG CAT GTA TTG Met His Val Leu (4) ( 缺失 ) ATG ATG TAT TG Met Met Tyr (5) ( 插入 ) ATG GCA TGT ATT G Met Ala Cys Ile (6) ( 取代 ) ATG TAT GTA TTG Met Tyr Val Leu (7) ( 倒位 ) ATG ACT GTA TTG Met Thr Val Leu * Refer to the table on p. 24 for the genetic code. Deleting or inserting a number of bases that is not a multiple of three will shift the (8) ( 閱讀框 ). The whole amino acid sequence after the point of mutation is altered. The resulting protein is usually (9) -. A substitution or an inversion of base(s) usually leads to the change of one (10) only. This may result in one different amino acid in the polypeptide. If that amino acid alters the conformation of the protein sufficiently, the mutation would have an obvious effect on the (11). Sickle-cell anaemia ( 鐮狀細胞性貧血 ) is caused by a (12) of a base in the gene that codes for a polypeptide chain in haemoglobin. - In the gene, base T is replaced by base (13) at one position. Thus the polypeptide made has one different amino acid (14) instead of glutamic acid. - When oxygen levels are low, the abnormal haemoglobin molecules form long fibres which cause the red blood cells to become sickle-shaped. These sickle-shaped cells can block blood vessels, causing damage in organs. (Second Edition)

6 2 Chromosome mutations Chromosome mutations are changes in the structure or number of chromosomes. i) Change in chromosome structure It can be brought about by: (15) (16) there is a loss of genes there is a gain of genes (17) (18) ( 易位 ) the order of genes is reversed some genes are exchanged ii) Change in chromosome number Down syndrome ( 唐氏綜合症 ) is a disorder in which an individual has an extra chromosome 21 in their body cells. - It occurs when homologous chromosomes or sister chromatids of chromosome 21 fail to (19) during gamete formation. They go to the same daughter cell. - If the abnormal gamete containing an extra chromosome 21 fuses with a normal gamete, a zygote with (20) chromosome 21 will be formed. People with Down syndrome are usually shorter than average. They have a distinctive round face with smaller ears. They also have some degree of mental retardation. (Second Edition)

7 Parent cell (2n) homologous chromosomes 21 First meiotic division Second meiotic division sister chromatids fail to separate and they move to the same daughter cell Gametes normal (n) chromosome 21 missing (n 1) with an extra chromosome 21 (n + 1) fertilization abnormal ovum (n + 1) normal sperm (n) zygote (2n + 1) Chromosome mutation that occurs during gamete formation and formation of a zygote with an extra chromosome 21 (only homologous chromosomes 21 are shown) B What are the causes of mutations? (Book 4, p ) Mutations can occur naturally and randomly. These mutations are called (21) ( 自發突變 ). Mutations can also be induced by agents called (22) ( 誘變劑 ). These mutations are called (23) ( 誘發突變 ). Nitrous acid ( 亞硝酸 ) Mutagen Possible source Food preservatives Chemical (24) Cigarette smoke Abestos ( 石棉 ) Construction materials Mustard gas ( 芥子氣 ) Chemical warfare (25) ( 紫外光 ) Sunlight Radiation (26) - Medical examination (27) ( 伽瑪射線 ) Radiotherapy, nuclear bombs (Second Edition)

8 Many chemical mutagens can change the (28) structure of DNA. High-energy radiations can ionize water or other molecules to form (29) ( 自由基 ) which are highly reactive and they can damage DNA molecules. C What are the effects of mutations? (Book 4, p ) Mutations that have an effect on organisms are usually harmful. This is because a mutation may lead to a change in the shape of a (30), making it fails to function properly. Most gene mutations have no effect on organisms because: - some mutations may produce a different code for the same amino acid (due to the (31) nature of the genetic code) - changes in an amino acid may not alter the shape of the protein - (32) exist in pairs in cells. Mutations in one allele may be masked by the normal allele. Mutations occurring in gametes or gamete-producing cells are (33). They provide a source of (34) in a population, which are essential for natural selection ( 自然選擇 ) to work and to bring about evolution. (Second Edition)

9 Answers Ch 27 Molecular genetics genetic code 2 three 3 triplet code degenerate 6 non-overlapping 7 universal 8 gene expression 9 transcription 10 translation 11 hydrogen bonds 12 messenger RNA 13 template 14 coding 15 template 16 RNA polymerase codon 19 template 20 coding 21 U 22 ribosome 23 transfer RNA 24 anticodon 25 amino acid 26 peptide 27 stop gene mutations 2 chromosome mutations 3 base 4 deletion 5 insertion 6 substitution 7 inversion 8 reading frame 9 non-functional 10 triplet code 11 phenotype 12 substitution 13 A 14 valine 15 Deletion 16 Duplication 17 Inversion 18 Translocation 19 separate 20 three 21 spontaneous mutations 22 mutagens 23 induced mutations 24 Tar 25 Ultraviolet light 26 X-rays 27 Gamma rays 28 chemical 29 free radicals 30 protein 31 degenerate 32 alleles 33 inheritable 34 variations (Second Edition)