2007 Mutation and Cell division M.T.Yeung P.1Mutation

(http://www.tokyo-med.ac.jp/genet/index-e.htm)- a sudden and relatively permanent, inheritable change in the amount or structure of genetic material (e.g.

DNA in the chromosome) of an organism(resulting in a change in the phenotype of an organism)

- sudden change in somatic cells (called somatic mutation) will not be inherited to individuals of the next generation, but can be transmitted to its daughter cells inside the body via mitosis.

- however, if mutation is occurred during gamete formation (i.e. germinal mutation) it might be inherited to offspring and form new varieties.

provide raw material (variation) for natural selection in evolution.

- The mutated individuals are called mutantsshow new phenotype that HAVE NEVER BE FOUND IN THEIR ANCESTORSthe change in character is due to mutation

Types of MutationA. Gene Mutation (Point Mutation):

- alternation in the sequence of nucleotides at a single locus on a chromosome (i.e. in a gene ) by:

1. Insertion (addition) or extra nucleotide(s) into a gene2. Deletion of original nucleotide(s) from a gene3. Duplication of original nucleotide(s) in a gene4. Inversion of nucleotide sequence in a gene5. Substitution of original nucleotide(s) by nucleotide(s) with other base(s)

--> change the order of amino acids making up a protein--> result may be silent / a mutated phenotype / a mutated lethal expression

- e.g.1 Sickle-cell anaemia (an inherited disease)- RBC : normal biconcave discs ---> sickle-shaped

- Distorted shape of RBC is due to the presence of abnormal haemoglobin S (HbS) which aggregates at low oxygen concentration.

inefficient carrying o f oxygen and clotting of small blood vessels

chronic haemolytic anaemia

Symptoms of the victims include extreme O2 shortage: weakness, becoming thin, and sometimes may lead to kidney and heart failure

Normal Hb β chains: val-his-leu-thr-pro-glu-glu---etc. (GAG in mRNA)

point mutation (substitution)Sickle-cell Hb β chains: val-his-leu-thr-pro-val-glu---etc.

(GUG in mRNA)

- the mutant gene is codominant with the wild type gene:

If heterozygous: mild anaemia (~30% -40% Hb S in blood), rarely die but more resistance to malaria since the parasitic protozoan – Plasmodium cannot easily invade the sickle-RBC (common in Africa and Middle East where malaria is prevalent)

If homozygous : severe anaemia and frequently early death (i.e. lethal)

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- e.g 2: Cystic fibrosis囊性纖維變性- common in Europe- caused by a mutated recessive gene

e.g.3: Huntington’s disease (亨廷頓氏症)- caused by a mutated dominant gene involuntary muscular movement and progressive mental deterioration

B. Chromosome Mutation- visible changes in gross structure or number of chromosome under microscope

2007 Mutation and Cell division M.T.Yeung P.31. Change in the number of chromosomes

a. Changes involve the entire set of chromosomes (genome number) / euploidy整倍體.i. Haploidy

= the organism has only 1 genome (N)(each kind of chromosome is represented once in a nucleus)

-- rare in animal adults, but common in plants

-- usually smaller, weaker and less vigorous,

-- highly sterile since chromosomes have no regular pairing partners during meiosis.

-- develop from unfertilized egg

ii Polyploidy = a condition where an organism with more than two sets of chromosomes or genomes.

(>2N, i.e. each chromosome represented by > 2 homologues) -- rare in animals, but common in plants

(one third of the angiosperms are polyploids e.g. roses, tulips and chrysanthemums) and they often have some advantage e.g. forming larger fruits etc.

-- associated with advantageous characteristics: greater size, hardiness, resistance to disease.

 

-- If genome number is even (N= 4, 6...etc.; e.g. tetraploid)--> fully fertile

If genome number is odd (N= 3, 5...etc.; e.g. triploid)--> sterile( since chromosomes have no regular pairing partners during meiosis)e.g. 3n water melons are seedless.

Phenotypes of polyploids:

Increased DNA content tends to increase size of cell; often translate more protein than diploids. Thus often larger and more vigorous than diploids; flowers often more deeply coloured. Development of complex organs often disrupted (and polyploidy is lethal in humans for this reason). Allopolyploids combine features of both parent species and so are often more phenotypically novel新穎的 than autopolyploids. Thus allopolyploids are more likely than autopolyploids to be strongly favoured or disfavoured by selection.

2007 Mutation and Cell division M.T.Yeung P.4

-- Autopolyploidy同源多倍性(occurs within the same species)http://www.biols.susx.ac.uk/ugteach/cws/evol1/polyploid.htmThis may be due to:

1.errors in meiosis producing unreduced diploid gametes (all the chromosomes at metaphase end up in the same nucleus) which then fertilize

2. errors in mitosis that cause a somatic doubling of the chromosome number so that body cells produced that are tetraploid. This can lead to polyploid offspring if those body cells or their cell descendents:

- contribute to asexual or vegetative reproduction- produce gametes (Thanks to the 'segregation of the germ-line' into gonads,

few cells in animals can do this. But many cells in plants can do so)

3. multiple fertilisation (called polyspermy in animals); for example triploidy in humans (the cause of about 15% of spontaneous abortions) is mainly due to two sperm fertilizing an egg rather than a haploid and diploid gamete fusing (although latter also occurs).

Mitosis and meiosis can be interfered by chemicals (colchicines 秋水仙素: blocks spindle formation and thus prevent anaphase movement) or environmental conditions (starvation, cold)

2007 Mutation and Cell division M.T.Yeung P.5

iii Allopolyploidy 異源多倍性 (Addition of genome of different species)-- Cross between 2 distinct taxonomic groups ---> hybrid

-- hybrid from 2 diploid parents of different species (case A below) genomes so different that each kind of chromosome lacks a homologue to act as its pairing partner at meiosis ---> sterile

-- Fertile allopolyploidy offspring may be resulted when

--- Diploid gametes are obtained from tetraploid parents (case B above) or

--- meiosis occurs after non-disjunction of chromosomes in mitosis of sterile hybrid parent cells.

--- fusion between unreduced gametes from sterile hybrid parent cells

“Endoreduplication” of polyploidy zygote develop and produce further polyploidy gametes.

e.g. : Common bread wheat is an example of fertile allopolyploid. (http://www.ansi.okstate.edu/course/3423/buchanan/corr/lesson/lesson4.htm)

Chromosome with two chromatids

Many crops are polyploids. (http://www.biols.susx.ac.uk/ugteach/cws/evol1/polyploid.htm)

Wild Einkorn Wheat Triticum boeoticum is a diploid.

Emmer Wheat (T. turgidum) is an allotetraploid (4 x 7 chromosomes) between Einkorn (2N gamete from endoreduplication) and a wild-grass (2N gamete from endoreduplication).

Bread Wheats (e.g. Triticum aestivum --- the most common wheat species today) are allohexaploids (6 x 7 chromosomes, in this case from three different species) between T. tauschii (2N gamete from endoreduplication) and T. turgidum (4N gamete from endoreduplication)

2007 Mutation and Cell division M.T.Yeung P.6

Reference ( 參考 ) : 染色體數目異常是由於染色體行態異常或複製異常，其原因有四方面:

(1) 不分離(nondisjunction）: 指在細胞分裂的中期和後期，某一對同源染色體或姐妹染色單體同時進入一個子細胞核為不分離；

(2) 染色體遺失(chromosome loss): 在細胞分裂的中後期，如果一個染色體未能進入下一個子細胞核，使子細胞缺少一個染色體；

(3) 染色體橋(chromosome bridge)的影響：染色體畸變中出現的雙著絲粒染色體在細胞分裂後期如不能被拉斷，就會在兩核之間形成染色體橋，它使細胞不能分裂，出現四倍體。

(4）核內再複製(endoreduplication):四倍體的細胞核進入下一個分裂週期，恢復正常複製與分離，出現四條染色單體排列的現象，稱為核內再複製。

(http://antioxy.fmmu.edu.cn/chapter6.htm)

2007 Mutation and Cell division M.T.Yeung P.7

b. Changes in the chromosome number in a genome (異倍體/非整倍體 Aneuploids)- involve only parts of a set of chromosomes - produced by non-disjunction of a pair of chromosome

(failure of the members of a pair of chromosome to segregate) --> establish progeny with chromosome one more or one less than normal (2n+1 trisomy三體性, 2n-1 monosomy 單體性)

- chance increased by radiation and other chemicals

- e.g. Down's Syndrome (Mongolism)(http://www.ansi.okstate.edu/course/3423/buchanan/corr/lesson/lesson4.htm)

-- one extra chromosomes in genome(i.e. 47 chromosomes), located at the 21st pair (Trisomy 21)

--> abnormalities of face, eyelids, tongue and other part of body; some speech

difficulties, greatly retarded physically and mentally, short life expectancy

This disorder is not likely to be passed from generation to generation since the individuals seldom live long enough to reproduce.

-- very common : 0.15% of all births, but the exact reason is unknown.(a relatively high incidence when mother’s age>35; Frequency highest when mother's age is > 40)

Down's syndrome—(唐氏症，蒙古症)：又稱「Trisomy 21」「mongolism」(蒙古症，現已少用) 「Down syndrome」；一種遺傳性的病症，屬於最常見的一種出生基因缺陷(genetic birth defect)，一種人體DNA(染色體)失常(chromosomal abnormality)，造成原因主要可分為兩個種類，常見的一種是在DNA的G組中出現一個多餘的21號DNA(染色體)，因此稱為Trisomy 21，全部的DNA數目成為47個而不是通常的46個，發生率隨女性生產年紀而增高；另一種造成原因約佔5%，多餘21號DNA是加在另一個DNA之上，此方式稱為「translocation」，嬰兒DNA總數目仍是46個，但附加(compound)一個DNA，這種情況有時不會產生症狀，父母親可能都是這種無症狀的「帶原者」(carrier)，此種情況與母親年紀無關，發生的機率約是10%如果母親是帶原者，2.5%如果父親是帶原者；唐氏症造或頭腦低智和身體功能缺陷，容易有先天性心臟疾病，發育較慢，平均的智商約是50至60，成年後患者易得患呼吸道感染、肺炎和肺病；唐氏症發生機率與女性生產年紀形成正比的關係，母親生產年紀愈大發生機率愈高，例如女性生產年紀是30、36、42、48歲，其發生機率分別是每900、300、60、12人中有一人；

2007 Mutation and Cell division M.T.Yeung P.8

2. Change in Chromosome Structure- during meiosis a chromosome may break into one or more segments at chiasmata and

rejoin with corresponding portion of chromatid on its homologous partner ---> change in the gross structure of the chromosome

a. Changes in the number of gene within a chromosomehttp://www.tokyo-med.ac.jp/genet/cai-e.htm

i. Deficiency (Deletion) - loss of small segment(s) of chromosome (i.e. loss of 1 or more genes)---> usually lethal, especially if involve considerable part

ii. Duplication (Addition) - addition of 1 or more genes

---> may results in abnormal body character if organism survives

b. Changes in the Arrangement of Gene Locii. Translocation

- a fragment of chromosome becomes attached to another non-homologous chromosome ---> phenotypically normal unless gene interaction that affect phenotype expression

ii. Inversion- the reversal of the order of a block of genes in a given chromosome- usually requires 2 breaks in a chromosome followed by re-insertion of the segment in the reversed direction.

2007 Mutation and Cell division M.T.Yeung P.9Types of Mutation According to Their Origin

1. Spontaneous Mutation- without a known cause, It may be due to the effect of cosmic ray from the outer space but also

probably caused by physical and chemical agents.- usually infrequent and very slow in nature

e.g. in man: ~ 1 mutation / 100,000 gametes- over a period of time, these mutations accumulate in population.

2. Induced Mutation- mutation rate can be speeded up by applying mutagenic agents (mutagens)- caused by the mutagenic agents (mutagens) artificially.

a. Physical mutagens- UV radiation, heat and ionizing radiations電離輻射 (e.g. -, -, -, x-ray... etc.)

b. Chemical mutagens- chemical substances which increase the mutability of genes

e.g. nitrous acid, mustard gas, formaldehyde, colchicines, base analogs- base analogs are chemically similar to the nucleic acid bases ( incorporated into DNA

molecule by mistake)

Significance of Mutation- most mutations confers disadvantages. few are useful- exerts very slight effect on the characteristics of an organism- mutant gene is usually recessive.- It is a kind of discontinuous variation and hence the basis for evolution.

- Variation and Evolution of species

Beneficial mutation increases the number of alleles within a population (very rare in fact) more inheritable variations can be acted upon by natural selection, variations provide a population with new features and abilities to adapt to the constantly changing environment. Through natural selection, more and more favorable characters would accumulate, shaping the population to become more and more adaptable to the environment. Mutation and variations are thus the basis of the evolution within the population.

Harmful mutation (majority)-- Inheritable diseases or lethal effect

cause serious problems for organisms. For example, in humans, some mutant genes produce inheritable diseases like haemophilia, colour blindness, albinism, etc.. Each person may have several hidden recessive alleles which, when becoming homozygous, will have a lethal or deleterious effect.

-- antibiotic resistance of bacteriaantibiotic resistant strains of many bacterial species, has already destroyed the usefulness of several drugs.

-- cancerit is believed that cancer can be caused by multiple mutation in several genes in cells. Hence the cells will keep dividing without control.

2007 Mutation and Cell division M.T.Yeung P.10Cell Division

Cell Theory was extended by Rudolph Virchow in 1855:" every cell is from a cell"

i.e. all new cells are derived from other cellso all cells in all organisms have been formed from successive divisions of some original ancestral cells.

A. Chromosomes in cell division

- responsible for the transmission of hereditary information from generation to generation

- may staining with certain basic dyes

- Chromatin -- fibers containing proteins and DNA-- extremely long and thin-- dispersed throughout the nucleus and individual threads cannot be seen in non-dividing eukaryotic cells

- Chromosome-- made up of chromatin-- involved in controlling of all materials in the cell-- during cell division:

- highly condense and become shorten, so individual chromosomes can be seen under microscope- control of materials synthesis in the cell ceases

- Homologous ChromosomesThey are the similar chromosomes in diploid cells:-- contain the same number and linear sequence of genes-- with similar morphology under microscope (same length and some centromere position, except the sex

chromosomes)-- one derived from the male parent and the other from the female parent-- pair up to from a bivalent during meiosis

- Chromatid-- each of the two identical double helix chains of DNA from replication 'surrounded' by protein 'coat' at

onset of cell division lying side by side-- attached to each other by a non-staining region 'centromere' where for attachment of microtubules of

spindle fibres.

- Number of chromosomes in various species-- each individual of a given species contains a characteristic number of chromosomes in most nuclei of

the body-- it is not the number of chromosomes that differentiate the various species, but rather the information

specified by genes-- most species of animals and plants have chromosome number between 10 and 50, number above and

below this are not common-- normal human being having exactly 46 (23 pairs) chromosomes

2007 Mutation and Cell division M.T.Yeung P.11B. The Cell Cycle- is the period from the beginning of one mitotic cell division to the beginning of the next and is

customarily represented in diagrams as a circle.

- the length of time for a complete cell cycle is termed the generation time

- can be used to describe the activities of actively growing and dividing cells (some cells are not capable of dividing once they reach a certain size, e.g. red blood cells, nerve, skeletal muscle)

- 3 main stages:i. Interphase / Resting phase

- Chromosomes are not visible but only chromatins can be seen after staining.First growth phase (G1) : cell organelles and biochemicals and protein synthesis

(cell growing and developing)Synthesis phase (S) : DNA replicationSecond growth phase (G2) : condensation of chromatin, centriole duplication (in animals),

increase energy storage

ii. Mitosis (Division of nucleus)1. Prophase:

- Chromosomes shorten and thicken gradually and become visible- centrioles (not in plants) move to two poles- aster (not in plants) appear- spindle fibres (they are microtubules) form spindle- nucleolus disappears- nuclear envelope disintegrates

2. Metaphase: - chromosomes line up at equator - spindle fibres attaché to centromere- chromatids are slightly apart

3. Anaphase:- centromere split and chromatids are totally separated by shortening of spindle fibres (partial

depolymerization of microtubules into tubulin molecules)- chromatids of each chromosome move towards two poles

4. Telophase:- new nuclear envelope forms- chromatids loosen to form chromatin- spindle fibres breakdown (depolymerization)- new nucleolus form

iii. Cytokinesis (Cytoplasmic Division)--> equal splitting of cytoplasm into two new daughter cells that each contain a newly formed

nucleus

- animal cells : constriction of the center of the parent cell from the outside inwards.

- plant cells : fusion of Golgi vesicles (containing constitute of middle lamella and primary cell wall)

forming cell plate across the equator from the center outwards until reaching the edges of the cell

membrane of vesicles becoming new cell membranes and cell plate becomes the middle lamella

cellulose is laid down on both sides of the plate to form the new cell wall

2007 Mutation and Cell division M.T.Yeung P.12C. Mitosis and Meiosis

- various phases in mitosis (see above)

- various phases in meiosis-- Interphase (DNA replication)

-- Prophase I --- chromosome shorten and visible, synapsis of homologous pair to form a bivalent (each bivalent

consist of two pair of sister chromatids, and can be called a tetrad = 2 x 2 strands ) --- chiasma(ta) (allow crossing over and exchange of genetic materials between the members of a

pair of homologous chromosomes) to occur) may occur at various points--- nucleolus disappears and nuclear envelope disintegrates--- centriole (not in plants) move towards two poles--- spindle fibers forms

-- Metaphase I--- bivalents line up randomly in respect to their order and orientation at equator--- homologous chromosome apart slightly and limited by chiasmata

-- Anaphase I--- members of homologous chromosomes separated and move towards two poles

-- Telophase I (in some cells this stage does not occur)--- nuclear envelope forms--- spindle disappear--- chromtids loosen

-- Meiosis II (Prophase II, Metaphase II, Anaphase II, Telophase II)--- similar to mitosis

- differences between mitosis and meiosis Prophase:1. synapsis in meiosis I but not in mitosis2. chiasma may occur in meiosis I but not in mitosis

Metaphase:3. bivalent (homologous pair) line up at equator in meiosis I while single line of individual chromosme

at equator in mitosis

4. separation of homologous chromosomes limited by chiasma in meiosis I while separation of chromatids of a chromosome limited by centromere in mitosis

Anaphase:5. separated homologous members move towards two poles in meiosis I while separated sister

chromatids move towards two poles in mitosis

Telophase:6. telophase I may not occur during meiosis I in some cells while telophase is the last essential step in

mitosis.

Karyokinesis (nuclear division)7. two nuclear division in meiosis but one nuclear division in mitosis

Daughter cells8. four daughter cells in meiosis but two daughter cells in mitosis9. for a diploid parent cell, daughter cells resulted from meiosis are haploid cells (half of the parent)

while daughter cells resulted from mitosis are diploid cells (same as the parent).

2007 Mutation and Cell division M.T.Yeung P.13

Summary of differences between various stages of mitosis and meiosis:Stage Characteristics Mitosis Meiosis

I IIProphase Pairing of homologous chromosomes

(synapsis forming bivalent)✘ ✔ ✘

Chiasmata formation (crossing over) ✘ ✔ ✘

Metaphase Lining of homologous pairs of chromosomes at the equator of spindle

✘ ✔ ✘

Lining of pairs of sister chromatids (individual chromosomes) at equator of spindle

✔ ✘ ✔

Anaphase Separation of homologous chromosomes ✘ ✔ ✘

Separation of sister chromatids (splitting of centromere)

✔ ✘ ✔

Telophase Number of chromosomes in daughter cells compared with that in parent cell

Same Half

Amount of DNA in daughter cells compared with that in non-dividing parent cell

Same Same Half

Presence of homologous pairs of chromosomes in a daughter cell

Both members of each pair

One member of each pair (with two sister chromatids connected at centromere)

One member of each pair(with one of the separated sister chromatids)

*telophase I may not occur during meiosis I in some cells while telophase is the last essential step in mitosis.

2007 Mutation and Cell division M.T.Yeung P.14

- significance of mitosis and meiosis respectivelyMitosis Meiosis

genetic stability- daughter cells are genetically identical to

their parent cell results in genetic stability within

population of cells derived from parental cells

gametes for sexual reproduction and increase genetic variation

- segregation of a pair of alleles and independent assortment of unlinked genes allows random combination of genetic materials in daughter nuclei

- crossing over of non-sister chromatids in bivalent allow recombination of genes in a linkage group

- allow fertilization (without double of DNA) of haploid gametes from different parents and preservation of a constant number of chromosomes from generation to generation.

growth, repair and replacement of tissues or body parts- the number of cells within an organism

increases and replaces by mitosis

asexual reproduction and regeneration of lost body parts