Transcribing the genetic code from DNA to mRNA

The genetic code is held in the order of bases along the DNA molecule. Sections of DNA called cistrons(commonly referred to as genes)contain the information needed to make a particular polypeptide. However, DNA does not carry out polypeptide synthesis directly. When the DNA in а cistron is activated, the information is transferred to a molecule of ribonucleic acid (RNA) called messenger RNA (mRNA),which acts as a template for the synthesis of the polypeptide.

The central dogma of biology

The relationship between DNA, mRNA, and polypeptides in a eukaryotic I cell is often called the central dogmaof biology.

§ mRNA is made on a DNA template in the nucleus, in a process called 1 transcription.

§ The mRNA then moves into the cytoplasm, where it combines with ribosomes to direct protein synthesis by a process called translation.

§ When the information in a cistron is used to make a functional polypeptide chain by transcription and translation, gene expressionis said to have taken place.

MRNA is made from the DNA template

mRNA is a large polynucleotide polymer, chemically similar to DNA but differing in that:

§ mRNA consists of only one chain of nucleotides, not two

§ mRNA contains the sugar ribose instead of deoxyribose

§ mRNA contains the base uracil instead of thymine.

During transcription, DNA acts as a template for making mRNA by complementary base pairing. Thus a particular short sequence of DNA may be transcribed as follows:

DNA base sequence: TAGGCTTGATCG

mRNA base sequence: AUCCGAACUAGC

The triplet code: frame-shift experiments

Twenty amino acids make all the proteins in living organisms.

§ If a code consisted of one base for one amino acid, only four combinations would be provided (there are four bases).

§ If two bases coded for one amino acid there would be 16 (4²) possible combinations.

§ A three-base (triplet)code provides 64 (4³) possible combinations, more than enough for all 20 amino acids.

Francis Crick and his co-workers confirmed that the genetic code is a triplet code.Using enzymes, they added or deleted nucleotide bases in the DNA of a

virus that infects bacteria. They found that when one or two bases were added or deleted, the viruses were unable to infect the bacteria. But when three bases were added or deleted, the virus was able to infect the bacteria. They concluded that adding or removing one or two bases caused a frame shiftwhich inactivated the gene. However, adding or removing three bases only partially affected the gene. Thus the sequence of bases shown above would contain the following sequences of DNA base triplets and mRNA codons:

DNA base triplet sequence: TAG GCT TGA TCG

mRNA codon sequence: AUC CGA ACU AGC

If one base (for example, guanine) is added to the DNA the frame shifts and the sequence of triplets and codons is changed:

DNA base triplet sequence: GTA GGT TTG АТС G

mRNA codon sequence: CAU CCG AAC UAG С

The results of the frame-shift experiments also showed that the code is non-overlapping:

§ Each triplet in DNA specifies one amino acid.

§ Each base is part of only one triplet, and is therefore involved in specifying only one amino acid.

A non-overlapping code requires a longer sequence of bases than an overlapping code (see box): however, replacing one base for another has a small or no effect.

Cracking the genetic code

To crack the genetic code, scientists had to work out which of the 64 codons determined each amino acid. To do this, they made mRNA molecules with a known sequence of bases. This mRNA was added to a cell-free system that contained isolated ribosomes, radioactively labelled amino acids, and all the enzymes needed for polypeptide synthesis. The polypeptides that were synthesised were then analysed to determine their amino acid sequence.

The first synthetic mRNA molecule made was a chain of uracil bases and was called poly-U. The polypeptide chain synthesised from it contained only phenylalanine. It was therefore concluded that the codon UUU codes for phenylalanine.

The complete genetic code was confusionfinally deciphered in 1966.

Quick check:

1. What is the “central dogma” of biology?

2. What is the name given to the result of adding one or two nucleotide bases to a DNA sequence?

3. Describe the relationship between DNA, messenger RNA, and proteins.

4. Explain how frame-shift experiments support the triplet code hypothesis.

5. Discuss the main features of the genetic code.

6. Divide the text into an introduction, principal part and conclusion.

7. Express the main idea of each part.

Suggested Answers and Solutions

Unit I. Text 1.1

p. 4. Ex. A:1H; 2A; 3F; 4G; 5D; 6C; 7E; 8B.

p. 10. Ex. II:respond-response; transform-transformation;

move-movement; develop-development;

respire-respiration; create-creation; define-definition.

p. 11. Ex. VI: 1)1-5; 2-6; 3-4; 2) 1-4; 2-6; 3-5;

3) 1-3; 2-4; 4) 1-4; 2-5; 3-6.

p. 12. Ex. VIII:1H; 2F; 3G; 4A; 5B; 6C; 7D; 8E.

Text 1.2

p. 19. Ex. II: 1) 1-4; 2-6: 3-5; 2) 1-2; 3-5; 4-6; 3) 1-4; 2-3;

4) 1-3; 2-4; 5) 1-3; 2-4.

pp.19. Ex. III:1. employment; 2. inheritance; 3. modification;

4. observation; 5. measurement; 6. prediction; 7. understanding;

8. discovery; 9. knowledge; 10. contradiction.

p. 20. Ex. V:1E; 2G; 3A; 4F; 5B; 6D; 7H; 8C.

P. 23. Ex. XII: Model answer

Dear Mrs Jones,

I am writing this letter to let you know about the areas of biology I am mostly interested in specializing in. These are molecular biology and genetics. I have chosen these areas because I have always been interested in the cell, this tiny living organisms, and the ways it works. I would like to know everything how its different systems interact. Also, DNA is another great mystery to me. I would like to learn how all this genetic information is stored and passed on from one generation to the next.

I realize that there are no hard lines between these two areas of study, that is, molecular biology and genetics, so I would like to know how my choice now will affect my career prospects later. What I hope to do when I graduate is work with doctors and chemists and do research in order to find cures for different diseases.

I would very much appreciate it if you could meet me during your office hours in order to discuss my options. Would Monday 17^th October at 10 am be suitable for you?

Thank you very much for your time. I am looking forward to hearing from you.

Yours sincerely,

Carly Browny.

Unit II. Text 2.1

P. 26. Quick check

1. Living things are made of cells and these cells have certain things in common.

2. a) glycogen granules; b) chloroplast, cell wall, vacuole membrane (tonoplast), vacuole; c) cell surface membrane, mitochondria, cytoplasm, and nucleus.

pp. 26. Ex. II:exist– existence – existent; store – store – store;

form – form – formal; divide – division – divisible;

act – act – active; suit – suitability – suitable;

differ – difference – different.

p. 27. Ex. IV:1L; 2 F; 3A; 4I; 5J; 6B; 7E; 8C; 9D; 10G; 11H; 12K.

p. 28. Ex.VII: 1) 1-3;2-4; 2) 1-5; 2-3; 4-6;

3) 1-3;2-5; 4-6; 4) 1-2; 3-5; 4-6.

p. 29. Ex. IX:1D; 2H; 3B; 4F; 5C; 6I; 7E; 8K, 9G, 10A, 11J, 12L.

Text 2.2

P. 33. Quick check

1. a) interphase, prophase, metaphase, anaphase, and telophase.

b) interphase.

2. mitosis involves one division and the formation of two daughter cells from each parent cell whereas meiosis involves two divisions and the formation of four daughter cells from each parent cell.

p. 33. Ex. II:replace – replacement – replaceable

continue – continuity - continuous

condition – condition - conditional

fuse – fuse - fusible

mutate – mutation - mutable

double – double – double

pp. 34. Ex. IV:1L; 2K; 3H; 4J; 5B; 6I; 7E; 8A; 9C; 10F; 11G; 12D.

p. 35. Ex. VII: 1) 1-4; 2-3; 2) 1-2; 3-4-5; 3) 1-3; 2-4-5; 4) 1-3-5;2-4.

pp. 36. Ex. IX:1F; 2I; 3D; 4A; 5B; 6G; 7E; 8C, 9H.

Text 2.3

Pp. 39. Quick check

1. a) a magnification in a light microscope is varied by changing the power of the glass lenses.

b) magnification of an electron microscope is varied by changing the strength of the electromagnets.

2. Electron microscopes use beams of electrons which have a shorter wavelength than light, giving electron microscopes a higher resolving power than light microscopes.

3. 0.2 micrometres.

p. 40. Ex. II:magnify – magnification - magnified

multiply – multiplication – multiple

reflect – reflection – reflective

absorb – absorbability – absorbable

prevent – prevention – preventative

p. 40. Ex. IV:1E; 2H; 3K; 4A; 5B; 6C; 7J; 8D; 9F; 10G;11I.

p. 41. Ex.VII: 1) 1-3; 2-4; 2) 1-4; 2-3; 3) 1-6; 2-4; 3-5;

4) 1-3; 2-4;

p. 42. Ex. IX:1G; 2E; 3A; 4H; 5C; 6I; 7F; 8D, 9B.

Unit III. Text 3.1

p. 44. Ex.A.:1C; 2D; 3E; 4F; 5G; 6A; 7B; 8I;9J; 10H

P. 47. Quick check

1. a nucleoside contains a base and pentose sugar whereas a nucleotide consists of a base, sugar, and phosphate;

2. condensation

3. TTAGGC.

pp. 47-48. Ex.II.:discover – discovery; project – project;

describe – description; receive – receiver;

remove – remove/remover;

condensate – condensation; react – reaction.

Text 3.2

p. 51. Ex. A. 1C; 2L; 3E; 4A; 5B; 6D; 7I; 8G; 9K; 10H; 11F; 12J.

P. 64. Quick check

1. a bead-like structure consisting of DNA and histones

onchromosomes;

2. control the distribution of chromosomes during cell division;

3. telomeres seal the ends of chromosomes; shoelace tips seal the

ends of shoelaces.

Unit IV. Text 4.1

p. 59. Ex. A: 1C; 2A; 3I; 4G; 5H; 6J; 7K; 8E; 9L; 10D; 11B; 12F.

P. 65. Quick check

1. an agent that causes a mutation (e.g. X-rays)

2. the genotype refers to the genetic make-up of an organism (i.e.the alleles it has) whereas the phenotype refers to the visible or otherwise measurable characteristics of an organism resulting from an interaction between the genotype and environment;

3. heterozygous

4. continuous variation

Text 4.2

p. 67. Ex. A: 1I; 2J; 3A; 4F; 5D; 6H; 7B; 8C; 9E; 10G.

P. 72. Quick check

1. non-separation of one or more homologous chromosomes during meiosis;

2. a) amniocentesis can be carried out at about 15-16 weeks of pregnancy whereas chorionic villus sampling can be carried out between weeks 8 and 12;

b) amniocentesis carries a lower risk than chorionic villus sampling;

3. chorionic gonadotrophin.

p. 72. Ex. II.increase – increase – increased;

prevent– prevention – preventative/preventive;

risk – risk – risky;

inherit – inheritance – inheritable/inherited;

perform – performance – performing/performable;

decide – decision – decisive;

analyse – analysis – analytic.

Unit V. Text 5.1

P. 79. Quick check

1. nerve fibre carrying nerve impulses away from the cell body

2. a) nerve impulses; b) hormones.

p. 79. Ex. III. 1c; 2f; 3b; 4g; 5d; 6a;

pp. 79-80. Ex. IV. 1. Favourable stimulus; 2.nervous system;

3. to process information; 4. motor neurones; 5. target cell; 6. water balance; 7. to respond appropriately; 8. considerable distances; 9. insulating material; 10. external environment.

P. 80. Ex. V.

1. Ductless (endocrine) glands secrete their hormones directly into the bloodstream.

2. The endocrine system consists of a number of glands that secrete hormones.

3. Responses to stimuli usually involve the coordinated actions of different parts of the body.

4. The CNS acts as an integration centre and processes information from many sources.

5. Each living organism has its own specific type of sensitivity.

6. Hormones regulate such processes as heart rate, metabolism, gastric secretion etc.

7. Sense organs and effectors occur in different parts of the body.

8. The nervous system of mammals is more complicated than one of single-celled organisms.

9. Neurones convey information in the form of nerve impulses.

10. Blood glucose concentration is regulated by the endocrine system.

p. 80-81. Ex. VI. 1. range from; 2. consists of; 3. occur at; around; 4. acts as; 5. such as; 6. major; 7. apart from; 8. characteristic features; 9. detect; appropriately; 10. involve.

Text 5.2

P. 86. Quick check

The resting potential is determined by an unequal distribution of charged ions inside and outside a neurone, making the inside negative relative to the outside.

p. 86-87. Ex. III. 1C; 2F; 3D; 4E; 5A; 6B.

P. 87. Ex. IV.

1. resting potential; 2. escape movements; 3. external environment;

4. sodium ions; 5. potential difference; 6. electrical charge;

7. light intensity; 8. to respond quickly; 9. nerve fibres; 10. to reach a peak.

p. 88. Ex. VII.The inside / interior (of the neurone); the size of the potential; to respond quickly; resting condition; potential difference; to reach the threshold level; to obey the law; unequal distribution; to measure; complete reversal of charge.

p. 88. Ex. IX. During the resting potential, the voltage-gated sodium and potassium ion channels are closed. When a stimulus is applied, sodium ion channels open rapidly, sodium ions move in, and the inside becomes more positive. If the stimulus reaches the threshold level, an action potential occurs. When the action potential reaches its peak, the sodium ion channels close slowly and potassium ion channels open slowly. Sodium ions stop moving into the cell but potassium ions diffuse more rapidly out. These changes cause the potential difference to drop. When the membrane returns to its resting potential, potassium ion channels close, but because they do this slowly, the potential dips below the resting level. Finally, when the potassium ion channels are closed, the membrane returns to its resting condition.

Unit VI. Text 6.1

P. 92. Quick check

1. The change over successive generations of the genetic composition (allele frequency of a population) that may result in the formation of new species from pre-existing species.

2. Neo-Darwinism incorporates new scientific evidence, particularly from genetics and molecular biology.

pp. 92. Ex. II:exist – existence – existent;

suggest- suggestion – suggestible;

reproduce – reproduction – reproductive;

develop – development – developmental;

inherit – inheritance – inheritable;

inhabit- inhabitant – inhabitable

evolve – evolution – evolutionary;

select – selection – selective.

p. 93. Ex. IV:1H; 2K; 3N; 4L; 5O; 6E; 7C; 8F; 9M; 10B; 11D;

12A; 13G; 14I; 15J.

p. 94. Ex.VII: 1)1-5;2-6;3-4; 2) 1-5; 2-4; 3-6;

3) 1-3; 2-4; 4) 1-4; 2-5; 3-6.

p. 95. Ex. IX:1E; 2H; 3A; 4B; 5G; 6C; 7D; 8F.

Text 6.2

P. 99. Quick check

1. Ability to pass on alleles to subsequent generations; the fittest individual in a population is the one that produces the largest number of offspring.

2. a. Disruptive selection; b. intermediates would be at a selective disadvantage because they would be easily seen against either a green or brown background.

p. 99. Ex. II:argue – argument – argumentative

occur – occurrence – occurring

increase – increase – increasing

compete – completion – competitive

expose – exposition - expository

distribute – distribution – distributive

describe – description - descriptive

pp. 101. Ex. IV:1H; 2D; 3G; 4M; 5A; 6K; 7B; 8N;

9E; 10O; 11C; 12F; 13J; 4I; 15L.

p. 101. Ex.VII: 1) 1-4; 2-5; 3-6; 2) 1-3; 2-4;

3) 1-4; 2-5; 3-6; 4) 1-4; 2-6; 3-5.

pp. 102. Ex. IX:1D; 2F; 3B; 4A; 5G; 6H; 7C; 8E.

Text 6.3

P. 106. Quick check

1. Directional selection.

2. a. Inbreeding reduces genetic diversity;

b. outbreeding increases genetic diversity.

3. Harmful recessive alleles may be less likely to be present in the homozygous condition and some allele combinations may interact positively.p

p. 107. Ex. II:suit – suitability - suitable

resist – resistance - resistant

interact – interaction - interactive

value – valuation , value - valuable

cultivate – cultivation – cultivable

desire – desire – desirable

p. 107. Ex. IV:1D; 2H; 3N; 4J; 5A; 6L; 7M; 8B; 9C;

10E; 11F; 12O; 13G; 14I;15K.

p. 108-109. Ex.VII: 1)1-3-4; 2-5-6; 2) 1-4; 2-3; 3) 1-4; 2-5; 3-6;

4) 1-3; 2-4.

p. 109. Ex. IX:1E; 2D; 3B; 4A; 5C; 6H; 7F; 8G.

Text 6.4

P. 114. Quick check

1. a. Associated with a shorter and flatter nose which has allowed the evolution of stereoscopic vision;

b. gives a powerful grip;

c. results in reduced number of offspring associated with increased parental care;

d. allows increased mobility of forearm.

p. 115. Ex. II:reflect – reflection – reflective

adapt – adaptation – adaptive

grasp – grasp - grasping

depend – dependence – dependent

support - - support – supportive

extinguish – extinction – extinct

separate – separation – separate

p. 115. Ex. IV:1K; 2H; 3F; 4J; 5A; 6I; 7B; 8N; 9L;

10C; 11O; 12G; 13E; 14M;15D.

p. 117. Ex.VII:1) 1-4; 2-3; 2) 1-4-7; 2-5; 3-6;

3) 1-3; 2-4-5; 4) 1-4; 2-3.

p. 117. Ex. IX:1E; 2C; 3F; 4B; 5H; 6D; 7A; 8G.

Unit 7. Text 7.1

P. 121. Quick check

1. a. Carbon dioxide; b. oxygen

2. a. Light-dependent stage occurs in the grana of chloroplasts;

b. the light-independent stage occurs in the stomata of chloroplasts.

p. 121. Ex. II:react – reaction – reactive

accumulate – accumulation – accumulative

produce – product productive

require – requirement – requisite

connect – connection – connective

pp. 122. Ex. IV:1G; 2D; 3A; 4M; 5I; 6N; 7B; 8L; 9E;

10O; 11C; 12J; 13F; 14H;15K.

pp. 123. Ex.VII: 1) 1-4; 2-3; 2) 1-3; 2-4; 3) 1-6; 2-5; 3-4;4) 1-4; 2-3.

p. 124. Ex. IX:1F; 2A; 3E; 4B; 5H; 6G; 7D; 8C.

Text 7.2

P. 128. Quick check

1. Water availability affects many activities in addition to photosynthesis. Water deprivation may kill a plant, but the cause of death may not be connected to photosynthesis.

2. The compensation point for a shade plant is at a lower light intensity than that of a sun plant.

3. Light intensity.

p. 128. Ex. II:measure – measure - measurable

subject – subject - subjective

interact – interaction – interactive

recognise – recognition – recognizable

concentrate – concentration – concentric

pp. 128-129. Ex. IV:1C; 2G; 3H; 4L; 5A; 6I; 7M; 8D;

9K; 10E; 11N; 12B; 13J; 14O;15F.

p. 130. Ex.VII: 1) 1-3; 2-6; 4-5; 2) 1-5; 2-3; 4-6;3) 1-3; 2-4; 4) 1-4; 2-3

p. 130-131. Ex. IX:1G; 2H; 3E; 4A; 5C; 6B; 7F; 8D.

Text 7.3

Pp. 135-136. Quick check

1. Two from wheat, soya beans, and rice.

2. Sugar cane is called a C 4 plant because it fixes carbon dioxide as a four-carbon compound.

3. At night.

4. aC4; b CAM; c C3

p. 136. Ex. II:adapt – adaptation – adaptable

fix – fixation – fixed

conserve – conservation - conservative

separate – separation – separate

dry – drought - dry

pp. 136-137. Ex. IV:1N; 2F; 3L; 4A; 5J; 6M; 7B; 8O; 9C;

10G; 11D; 12H; 13I; 14K;15E.

p. 138. Ex.VII: 1) 1-3; 2-4; 5-6; 2) 1-2; 3-6; 4-5;

3) 1-4; 2-3; 4) 1-4; 2-3.

pp. 138-139. Ex. IX:1E; 2D; 3A; 4F; 5H; 6B; 7G; 8B.

Unit VIII. Text 8.1

P. 143. Quick check

1. a) cuticle; b) the upper surface is more exposed to sunlight, making it hotter than the lower surface.

2. Photosynthesis.

3. Collenchyma consists of living cells with the corners of each cell reinforced by extra cellulose whereas mature sclerenchyma consists of dead cells impregnated with a thick layer of lignin.

p. 144. Ex. III:connect – connection – connective

require – requirement - required

support – support – supportive

protect – protection – protective

adapt – adaptation/adaptability – adaptable

expose – exposition – exposed

reproduce – reproduction – reproductive

p. 144. Ex. IV: 1) 1-4; 2-5; 4-6; 2) 1-4; 2-6; 3-5; 3) 1-3; 2-4; 4) 1-4; 2-3.

p. 145. Ex. VI: 1D; 3G; 3E; 4F; 5B; 6A; 7C.

pp. 145-146. Ex. VIII: 1B; 2F; 3G; 4C; 5E; 6H; 7D; 8I; 9A; 10J.

Text 8.2

P.150. Quick check

1. All stems have nodes and internodes.

2. Support, transport, tissue production, storage of food and water.

3. Parenchyma cells help support stems by becoming fully turgid and pressing against other cells.

p. 150. Ex. II: diperse – dispersal - dispersed

attach – attachment – attached

locate – location – located

identify – identification - identified

store – storage - stored

maintain – maintenance - maintained

penetrate – penetration - penetrated

strengthen – strength - strong

grow – growth - grown

add – addition – additional

p. 151. Ex. VI: 1)1-4; 2-5; 3-6; 2)1-3; 2-4; 3)1-6; 2-4; 3-5;

4)1-3; 2-4; 5)1-3; 2-4; 5-6.

p. 152. Ex. VIII: 1E; 2A; 3G; 4H; 5F; 6D; 7C; 8I; 9B.

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