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Cell & Nuclear Division
Cell Division consists of:
1
...
Process of separating nuclear DNA to daughter cells
b
...
Cytoplasmic Division
a
...
Unicellular Organisms: Cell division results in genetically identical daughter cells by
asexual reproduction
2
...
After an organism is fully grown, cell division continues to function in renewal and
repair, replacing cells that die from normal wear and tear or accidents
...
e
...
At the onset of mitosis, Centriole pairs duplicate and each pair moves to the opposite
poles of the cell, establishing the two poles of the cell
...
Cells are thus small in size
and low in ATP
...
Intensive cellular
gene expression and synthesis of appropriate
organelles and proteins

S Phase (2n, 2x): Each DNA molecule undergoes
DNA replication, resulting in the production of
Anaphase
two identical molecules of DNA
...
Further synthesis of appropriate
organelles and proteins occurs
...
On the cytoplasmic side of the
furrow is a contractile ring of microfilaments
...
A cell plate grows across the metaphase plate
...


Roles of Meiosis:




Result in the production four haploid daughter cells, each containing half the number of
chromosomes of the original parent cell
Production of gametes with haploid sets of chromosomes
Maintain the constancy of chromosome number from generation of generation by
preventing the doubling of chromosomal number with each generation

Meiosis Cell Cycle
Interphase

Metaphase
II

Anaphase II

Prophase I

Prophase II

Telophase II

Metaphase
I

Cytokinesis
I

Anaphase I

Telophase I

Meiosis I: known as reduction division where
chromosome number and ploidy level are
reduced by half

 Amount of DNA is the same as that of the
parent cell before replication
Cytokinesis
 Synapsis and crossing-over of homologous
II
chromosomes have occurred and their
subsequent segregation to different daughter cells

Interphase (2n, x -> 2n, 2x)





Precedes meiosis I and includes DNA replication
This process of replication is similar to the DNA replication preceding mitosis
The chromosomes only replicate once throughout meiosis
Pair of centrioles also replicate during interphase

Prophase I (2n, 2x)











Nucleolus disappears and nuclear membrane disintegrates
Spindle fibres begin to form and kinetochore spindle fibres attach to the centromeres of
chromosomes
Chromatin condenses and thickens until the chromosomes become distinct
Homologous chromosomes pair up and form a bivalent
Four chromatids in each bivalent are collectively known as a tetrad
The physical pairing is known as synapsis, where the homologues are bridged by a
synaptonemal complex
This process is precise and brings the genes on each chromosome into precise alignment
Chiasmata is formed between non-sister chromatids of homologous chromosomes at one
or more points
Crossing-over takes place where non-sister chromatids undergo exchange of genetic
material
...

Mendel’s Second Law of Independent Assortment is based on his work involving dihybrid crosses
(Dihybrid inheritance), stating segregation of one pair of alleles is independent of the
segregation of other pairs
...
dominance,
codominance,
incomplete
domicance

Inheritance of genes
DNA replication


During S phase of interphase, genes on chromosomes are replicated; via semi-conservative
DNA replication
In asexually
reproducing
organisms

• through mitosis, daughter chromosomes are separated into
daughter cells with the same number of chromosomes as their
parents

• In metaphase I, independent assortment of homologous
chromosomes occurs; resulting in new combination of paternal
and maternal chromosomes/ alleles
In sexually
reproducing • At the end of meiosis I, homologous chromosomes are then
separated into daughter cells
organsims
• At the end of meiosis II, daughter chromosomes are separated
into haploid gametes
• In random fertilisation, fusion of gametes forms new individuals
in the next generation and genes are passed on
• this restores the diploid number of chromosomes in the next
generation
Monohybrid Inheritance: the inheritance of a single character of contrasting traits
Incomplete Dominance: condition in which neither of the two alleles is completely dominant to
the other, so that the heterozygote has a phenotype which is intermediate
E
...
Colour of Snapdragon
Postulate that the allele CR allows for the production of a functional enzyme required for the
synthesis of red pigment and CW for a non-functional enzyme
...
Consequently, heterozygotes are pink
...
The heterozygote simultaneously expresses the phenotypes of both types of
homozygotes
E
...
coat colour of short horn cattle
Both alleles of a gene code for functional products
...
In the example, the heterozygote has a roan coat that consists of a mixture of red
and white hairs
...
g
...
g
...
Huntington Disease

Linkage (Mendel’s Law does not apply): when genes are situated on the same chromosome

Complete Linkage: assumption that 2 genes are located so closely together on the same
chromosome that they tend to be inherited together as one unit because no chiasma can be
formed in between them
...
Therefore, offspring produced show a majority of parental allele combinations
and hence phenotypes and a minority of recombinant allele combinants and hence phenotypes
Detection of Linkage





test cross with a double homozygous recessive individual can be performed to detect if
genes are unlinked, completely linked or incompletely linked
if genes are unlinked, 4 different phenotypes in the ratio of 1:1:1:1 are produced
if genes are completely linked, 2 phenotypes in the ratio of 1:1 are produced
if genes are incompletely linked, 4 phenotypes with a larger percentage of parental
phenotypes and smaller percentage of recombinant phenotypes are produced

Coupling: two dominant alleles are on one chromosome and two recessive ones are on the
homologous partner
Repulsion: dominant allele is linked with recessive on one chromosome
Chromosome Mapping






chance of crossing over occurring between two linked genes on the chromosome is
proportional to the distance between them
the further apart two linked genes are, the greater the statistical chance that crossing over
will separate them than if they were closer, and therefore the greater the proportion of
recombinants that will be formed
Cross-over value: recombination frequency
COV = (number of individuals showing recombination)/(total number of offspring) x 100%

Causes of Genotypic/Genetic Variation
Process
Meiosis &
Sexual
Reproduction

Mechanism
Crossing over, independent assortment and
separation of homologous chromosomes of
independent arrangement and separation of
chromatids and random fertilisation
Gene Mutation Deletion, insertion and substitution
Chromosomal Deletion, duplication, inversion and translocation;
mutation/
change in number of chromosomes due to nonaberration
disjunction

Effects
Reshuffles existing alleles
to form new genetic
combination; does not
result in new alleles
May result in new alleles
May or may not results
in new alleles

Effect of Environment on Phenotype


Degree of expression of genes may be influenced by the environment in which the
organism develops

Effect of Temperature: Coat colour in Himalayan rabbits





All Himalayan rabbits are homozygous for the Ch allele of the gene coding for a heatsensitive form of an enzyme, tyrosinase, which is needed for melanin production
Tyrosinase is active then the air temperature is below 33oC, thus there is growth of black
fur
The fur-producing cells will not produce the melanin pigment when exposed to higher
temperatures, thus hair of the rabbits appear light/white
Heat from environment prevents the development of black fur

Effect of soil acidity on Hydrangea Macrophylla

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

Hydrangea may have different flower colours, despite carrying the same alleles
The soil acidity in which the plants grow affects the plants’ ability to take up aluminium
In acidic soils (pH 5
...
5 or higher), aluminium is unavailable and flower colour is pink
purple
Sometimes a single plant has both blue and pink flowers due to varying soil condition
around the plant

Gene interactions: the idea that two or more genes influence one particular character




Various gene products function in a metabolic pathway that contributes to development of
one particular phenotype
Two independently assorting genes may interact to influence a single character
Biochemical basis of comb shape in chickens: different combinations of alleles from the
two genes result in different phenotypes of a single character, presumably due to the
interaction of their gene products, each of which contributes to the comb shape at the
biochemical or cellular level

Epistasis: when the expression of an allele of one gene suppresses/inhibits the expression of
alleles of a different gene at a different locus; suppressed gene is termed the hypostatic gene
*Dominant/recessive/duplicate recessive epistasis

Type of interaction
Non-epistatic
Recessive epistasis
Dominant epistasis
Duplicate recessive
epistasis
Duplicate interaction
Duplicate dominant
epistasis
Dominant and
recessive epistasis in
dihybrid cross

Phenotypic
Ratio
A_B_
9:3:3:1
9
9:3:4
9
12:3:1
9:7

Genotype
A_bb aaB_ aabb
3
3
1
3
4
12

3
7

9

9:6:1
15:1

9

6

13:3

13
(including
aabb)

1

Example
Comb shape in chickens
Coat colour in Labrador
retrievers/mice
Colour in summer squash
Flower colour in sweet
pea

1
1

15
3

Variation describes the recognisable differences in characteristics between organisms of the same
natural population or species
Discontinuous variation
Observable  Definite and clear cut; can be
phenotype
divided into discrete phenotypic
classes
 Intermediates are not observed
No
...
ABO blood group
factors acting on the different genotype
 Degree of expression allowed to genetic
potential hinges on environmental factors
eg
...
It
assumes that any differences are due to chance
Alternative Hypothesis: there is significant difference between the observed and expected results
...
The deviation is significant; hence reject the null
hypothesis in favour of alternative hypothesis
...
The deviation is not significant; hence do not
reject the null hypothesis

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