In a living cell seen underthe microscope, it is oftendifficult to see the nucleusWhen certain stains are applied, thenucleus takes up the stain morestrongly than the cytoplasm does,making the nucleus easily visibleStaining 2
By means of some special techniques ofillumination, the nucleus and other structuresmay be seen in the living cell.But in this presentation and in most textbooks, thenucleus is shown dark, as if it had been stainedIt is the differences in chemical compositionbetween nucleus and cytoplasm that make one takeup a particular stain more strongly than the other3
INHERITANCE• Is the transmission of genetic informationfrom generation to generation.
Terms you should know• CHROMOSOME: thread of DNA, made up of a string ofgenes.• GENE: a length of DNA that is the unit of heredity andcodes for a specific protein. A gene may be copied andpassed on to the next generation.• ALLELE: any of two or more alternative forms of a gene.• HAPLOID NUCLEUS: a nucleus containing a single setof unpaired chromosomes (e.g. sperm and egg)• DIPLOID NUCLEUS: a nucleus containing two sets ofchromosomes (e.g. in body cells)
In a cell which is not about to divide, thestructures in the nucleus are not distinctnuclearmembranecytoplasmCell 1 4
Just before cell division, thread-likestructures appear in the nucleusCell 2 5
These structures are called chromosomesThey get shorter and thicker and take upstains very stronglychromosomesCell 36
The shortening and thickening continues.Now the chromosomes are seen to bein pairsTo continue the sequence for cell division click hereCell 47
It may not be obvious from this illustration, but thechromosomes are always in pairs. They are distinguishedby their size.Chromosome pairs8
The chromosomes are always in pairs because oneof them is derived from the male parent and theother from the female parentDifferent species have different numbers andshapes of chromosomesMembers of the same species have identicalsets of chromosomes10
kangaroo (6 pairs) a sedge (21 pairs) hawkweed (4 pairs*)chicken (18 pairs) fruit fly (4 pairs) human (23 pairs)Chromosome numbers 11
Although chromosomes can be seen distinctlyonly at the time of cell division, they are presentand active all the timeThe chromosomes carry the DNADNA controls all the chemical reactions in the cellDNA also determines the species of organismand its individual characteristics (See thepresentation on ‘DNA’ for more detail)12
Chromosomes consist of DNA molecules supportedby a ‘scaffold’ of proteins. The diagram illustrates suchan arrangement but it is really more complex than thisChromosomes and DNAchromosomeDNAproteindouble helix13
The DNA in the chromosomes carries the genesConsequently, the genes are spaced out alongthe chromosomeThe genes consist of distinct stretches of the DNABy means of their DNA content, the genes controlthe activities of the cell, the type of cell it becomes,the species of the organism and the individualcharacteristics of that organism14
genes for eye colourgenes for hair coloursingle genechromosomeChromosomes and genesgenes for tallnessThe diagram illustrates the relationship between chromosomesand genes but it does not represent an actual chromosome.The genes for these characteristics are not necessarily on thesame chromosome and the number of genes shown is arbitrary15
ABCDEFGHIabcdefghIBecause the chromosomes are inpairs, the genes they carry are alsoin pairsThe individual genes of a pair, controlthe same characteristic, e.g. B and bcould control eye colour; G and g couldcontrol hair colourEach member of a pair of genes comesfrom either the male or the femaleparent just as the chromosomes doChromosomes and genes 16
Just before cell division, it can be seen that the chromosomeshave replicated; that is each chromosome has made a copy ofitself (including its DNA). These copies and the originals arenow called chromatidschromatidsChromatids 17
The chromatidstend to separate butare held together by aspecial region calledthe centromerechromatidsCentromere 18
CELL DIVISION:MITOSIS• Is a nuclear division giving rise togenetically identical cells in which thechromosome number is maintained by theexact duplication of chromosomes.
The next sequence of slides shows the way thechromosomes are distributed during cell divisionFor clarity, only two pairs of chromosomes arerepresentedThe chromosomes coloured blue are derived fromthe male parent; the chromosomes shown in redare from the female parent19
Two pairs of chromosomes.Each chromosome hasreplicated to form chromatidsCell division 120
The nuclear membranedisappearsFibres appear in thecytoplasm and form aspindleThe chromosomes moveto the ‘equator’ of thespindleCell division 221
The spindle fibresshorten and appearto pull the chromatidsapart by their centromeresCell division 3 22
The chromatids are nowchromosomes.The chromosomesmigrate to opposite endsof the cell as the spindlefibres shortenCell division 423
The cell begins to divideCell division 5 24
The nuclear membraneforms againThe cell constrictsThe chromosomesbecome less distinctCell division 625
Two cells formedEach cell now has a full setof chromosomes identical tothe parent cellThe chromosomes revert totheir elongated thin shapeand eventually cannot beseenCell division 7 26
Plant cells divide bybuilding a new cell wallAnimal cells divide bya constriction of theircytoplasmPlant and animal cells 27
This process of cell division, which produces cellscontaining identical sets and numbers ofchromosomes, is calledMITOSISMitosis 28Mitosis ensures that every cell of an organismcarries an identical set of genesCan you see a problem with this?
For example, what can a gene for brown eyes doin a stomach cell?The problem is that if every cell carries the sameset of genes, how do cells become specialised inshape and function to do different jobs?The answer is that the genes which are not relevantto the cell’s function are not activated.We say the gene for brown eyes is not expressedin a stomach cell29
The next slide shows a photomicrograph of onion root cells.In a root tip, a great many cells are dividing bymitosis, leading to rapid growth.The preparation is made by softening the root tip tissue,squashing it on a microscope slide and staining thechromosomes and nuclei.The ‘squash’ technique spreads the cells out.See if you can associate the various stages of cell divisionwith the stages 1-6 in the previous slide.Bear in mind that the previous slide is a purelydiagrammatic representation.31
GROWTH• Permanent increase in size and dry mass byan increase in cell number or cell size orgrowth.
DEVELOPMENT• Increase in complexity.
Question 1Staining techniques show up the nucleus because(a) The nucleus is enclosed in a nuclearmembrane(b) Nucleus and cytoplasm have differentchemical properties(c) The cytoplasm is less concentrated thanthe nucleoplasm(d) The nucleus is in the middle of the cell33
Question 2When are chromosomes present in the nucleus ?(a) Only just before cell division(b) Only during cell division(c) Only after cell division(d) All the time34
Question 3Chromosomes are in pairs because(a) They have replicated(b) Each is derived from either the maleor female parent(c) They are joined at the centromere(d) They have to be shared at cell division35
Question 4How many chromosomes are there in a human cell?(a) 100+(b) 92(c) 46(d) 2336
Question 5Which of these statements are correct ?DNA controls(a) The species of the organism(b) The function of the cell(c) Features of the organism such as size(d) Chemical reactions in the cell37
Question 6Which of these statements are correct?A chromosome contains(a) Protein(b) Cellulose(c) DNA(d) Genes38
Question 7The process by which a chromosome makes acopy of itself is called(a) Reproduction(b) Recombination(c) Relocation(d) Replication39
40What is the correct sequence of events in mitosis ?a b c d e f(c) c, b, a, e, d, f(a) c, d, a, b, e, f(b) b, c, a, d, e, f(d) c, b, d, a, e, fQuestion 8