Genetics concepts and terminologies

1. Overview of Basic GeneticConcepts and Terminology
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Mohammad Atif
Field Application Specialist
H.A. Shah Group of Companies

2. The basicstructure of a gene
It is well established that human characteristics areinherited from
parents to offspring in discreteunites calledgenes.
Vast amount of info regarding the precisemolecular mechanismsof
genetic transmission from parentto offspring.
A geneis the most fundamental unit of heredity that controls the
transmission and expressionof oneor more traits.
The chemical structure of ageneis deoxyribonucleic acid (DNA).
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3. The basicstructure of a gene
A genecan beviewed asatwo long strands of DNA which are normally
bound to eachother lengthwise by hydrogen bondsandare twisted
around eachother asadouble helix.
The subunits arecalled nucleotides which contain the nitrogenous
bases
There arefour different nitrogenousbases,called adenine (A),
guanine (G), cytosine (C), and thymine (T).
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4. Chromosomes
Wecan think of DNA asasequenceof the four letters A, G,C, and T.
An important feature of DNA is that A is always paired with T, and G
is always paired with C.
Genesarefound on chromosomesin the nucleus ofcells.
Chromosomesarevery long strands of DNA.
Every specieshasits own characteristic number of different
chromosomes.
Humanshave23 pairs of chromosomes,22autosomesand 1pair of
sex chromosomes.
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5. Chromosomes
The 22 autosomal chromosomesarenumbered in order of decreasing
length from 1 to22.
For every pair of chromosomes,oneis inherited from the mother of
an individual and oneis inherited from the father of an individual.
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6. Chromosomes
Chromosomesthat areof the samepair andcarry the sameset of
genesand arecalled homologous.(e.g. both chromosome21)
Mitosis is cell division that yields two identical diploid cells, both of
which have two pairs of each chromosome.
Meiosis is aspecial type of cell division that happensin reproductive
tissue yielding haploid cells (which haveoneof eachchromosome)
called gametes. In females,the gametesarethe eggcells andin males
the gametesarethe spermcells.
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7. Chromosomes
The centromere is aregion of the chromosomethat is the
attachment site for the spindle fiber that movesthe chromosome
during celldivision.
The centromere definestwo armsof the chromosome,the short arm p
andthe long armq.
Whentreated with special stains, eacharm appearsto bedivided into a
number of bands,which arenumberedfrom the centromere.
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8. Chromosomes
The approximate location of ageneis often specified by the
chromosomenumber (i.e. 1,2,...,22,X,Y), the arm (p,q), andthe
band (1,2,3,...).
Example: 12q24.2
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9. More Genetic Terminology
More than 99 percent of loci of the DNA sequenceson the 23
chromosomepairs areidentical in all humans
A genetic marker is astrand of DNA that is polymorphic: hassome
variation in the human population.
A geneticmarkercanhavetwo or moredifferent states andwedefine an
allele to bethe state at amarker.
Single Nucleotide Polymorphism (SNP) hastwo allelic types: highly
abundant (1 per 1000 base-pairs)
Short TandemRepeats(microsattelites) GTAGTAGTAGTAGTA...
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10. More Genetic Terminology
For achromosomepair, the two alleles at asingle genetic markerare
called an individual’s genotype.
Homozygous genotypeshavealleles that areidentical.
Genotypesthat have two different alleles aresaid to be
Heterozygous .
A haplotype is asequenceof allelesallong a chromosome.
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11. The Human Genome
The entire DNA characteristics of aspeciesis called its genome. The
human genomehasabout 3.2billion basepairs per haploid.
Approximately 2%of the human genomeis coding and98%of the
human genomeisnon-coding.
A geneis asequenceof DNA that is transcribed into mRNA
(messengerRNA), which, in turn, is translated into protein.
For RNA, uracil (U) is substituted for thymine in DNA.
In arecent build of the human genome,annotation data areavailable
for approximately 32,000 geneswith around 18,000 confirmedgenes.
Genesvary enormouslyin length from lessthan athousand basepairs to
over amillion basepairs(megabases).
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12. Coding Sequences
I II III
Exons
Introns
II IIIImRNA
Protein
Splicing
Transcription
Genesdo not form acontinuous sequencebut consists of several
coding segmentscalled exonsthat areseparatedbynon-coding
segmentscalled introns.
Non-coding regions andintrons aresometimescalled ”junk” DNA.
This term can bemisleading becausenon-coding regions mayindeed
have afunction.
Somenon-coding regions areknown to beinvolved in the regulation
of nearbycoding sequences.
Exons
Promoter
I II III DNA
Translation
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13. Recombination
A chromosomeinherited byan offspring from aparent is actually a
mosaicof the parent’s two chromosomes.
Genetic Recombination −→ genetic material is exchanged between
achromosome of paternal origin and the corresponding chromosome of
maternal origin.
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14. Genetic and Physical Maps
Crossoversarethe points of exchange
Recombination fraction between two loci on achromosomeis the
probability that the two loci endup on regions of different origin −→
occurs when the two loci areseparatedby an odd numberof crossovers
Genetic Maps −→ give the order anddistances (recombination
fraction) between genesor geneticmarkers.
Physical Maps −→ setsof orderedmarkersandthe physical distance
(basepairs) between them
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15. Short Questions
It is known that about 22 percent of the double-stranded DNA of an
organismconsists of thymine. Canthe other basepercentagesbe
determined? If so,what are they?
If T is 22% then A must also be 22% due to complementary base
pairing. This then accounts for 44% of the composition. C and G must
then account for 56%, and since they must also be equal, each
accounts for 28%.
A certain DNA virus hasabaseratio of (A+G)/(C+T)=0.85. Isthis
single- or double-stranded DNA? Explain.
It must be single-stranded. Otherwise, the ratio would be 1.
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