Download Genetics

Genetics
Learning objectives
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To describe the transmission of genetic
information.
To define mitosis and meiosis.
To differentiate mitosis and meiosis.
To define dominant and recessive genes.
To define alleles.
To define genotype and phenotype.
To differentiate between sex chromosomes.
To define sex-linked disease.
Genetics
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Gene = Hereditary
unit, a sequence of
DNA
Genome = Entire set
of genes
Gamete = Mature
sex cell
Chromatin  not
dividing cell
Chromosome 
dividing cell
A chromosome
DNA
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Deoxyribonucleic acid
The material for genetic
information
Confined to the nucleus
Structure= sugar +
phosphate group
+organic nitrogenous
base ( A, G, C, T )
Genetic Code
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The base sequence
in DNA
It determines the
amino acid
sequences in protein
3 consecutive bases
(triple code) in the
DNA Codon
Genetic
codes
Protein synthesis
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The sequence of bases
in DNA genetic codes
~ genotypes
Genotypes determine
the phenotype through
production of enzyme.
One- gene- one enzyme
hypothesis
RNA
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Single-stranded
molecule
differ from DNA
( ribose instead of
deoxyribose ; U
instead of T)
mRNA, tRNA, rRNA
Protein synthesis
I. Synthesis of amino acids
~ in mitochondria & chloroplast
~ non-essential & essential amino acids
2. Transcription
~ DNA strand base sequence of mRNA
~ mRNA directs the protein synthesis
3. Translation
~ mRNA sequence of amino acid
Transcription
Chromosome
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Composed of DNA, protein & RNA
Invisible in non- dividing cells chromatin
shorten & intensifies in dividing cells
23 pairs in human
Member of ear pair of chromosomes =
homologue
Homologous pair = one from father, one from
mother
Diploid (2N) vs Haploid (1N)
Chromosomes in cell division
DNA replicate
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2 identical chains of DNA
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The DNA are surrounded by
protein coat, 2 identical
strands lyes side by side
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the strands are attached by
centromere
Cell cycle
The sequence of events occurring
between the formation of cell and its
division into daughter cell.
 Interphase ~ period of synthesis &
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growth, replication of DNA
 Nuclear division ~ separation of
chromatids
 Cytoplasm division ~ division of
cytoplasm
Mitosis
The division of nucleus into daughter
nuclei containing identical sets of
chromosomes to the parent cell.
 cell numbers
 Growth, replacement, repairs cells,
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asexual reproduction
 Interphase + Prophase + Metaphase+
Anaphase + Telophase
Prophase
Early metaphase
Metaphase
Anaphase
Telophase
Cytokinesis
Cytokinesis
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The division of
cytoplasm
1. Cell organelles become
evenly distributed
2. Invagination of cell
membrane
3. Continuous furrow
around the equator
4. Complete separation into
2 cells
Significance of mitosis I
Genetic stability
~ same number of chromosomes of
parent & daughter cells
~ same hereditary information in parent
& daughter cells
~ no variation in genetic information
 Growth
~  number of cells
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Significance of mitosis II
Cell replacement
~ mitosis produces new cells
 Asexual reproduction and regeneration
~ regeneration of missing parts e.g. tail
in lizard
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Meiotic cell division
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Also call reduction division.
Takes place in the reproductive tissue.
Single duplication of chromosomes
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2 cycles of nuclear division &
cytoplasmic division
A single diploid cell gives rise to 4
haploid cells.
The necessity for chromosome
reduction
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In normal somatic cells, chromosomes are
homologous & diploid(2N).
Gamete contains only one member of each
homologous pair and is haploid(1N) due to
meiosis.
In sexual reproduction, the zygote after
fertilization (sperm[1N] + ovum[1N]) is 2N.
So, meiosis will prevent the nuclear materials
from doubling in amount in each new
generation.
Process of meiosis
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First meiotic division:
~ interphase
~ prophase I
~ metaphase I
~ anaphase I
~ telophase
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Secind meiotic
division
~ interphase II
~ prophase II
~ metaphase II
~ anaphase II
~ telophase II
Crossing over
Significance of meiosis
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Conserve the number of chromosomes
in the cells of successive generation.
Random orientation of chromosomes
Crossing over of genetic material
Comparison between meiosis
& mitosis
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Continuous, regular process
One duplication of genetic material
Involves separation of chromosomes &
other cell organelles.
Similar mechanism of cell division
Involves increase in cell number.
Comparison between meiosis
& mitosis
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A single division of
chromosomes & nucleus.
The number of
chromomses remains
the same(2N).
No crossing over.
Daughter cells are
identical to parent cells
2 daughter cells are
formed.
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A single division of
chromosomes but a
double division of
nucleus.
The number of
chromosomes is halved,
2N to 1N.
Crossing over present.
Daughter cells are
genetically different
from parental ones.
4 daughter cells are
formed.
Cytokinesis
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Normally follows telophase and leads
into interphase.
The cell membrane invaginate and
eventually join up
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complete separation of the two cells
Inheritance
Terms to know
 Haploid
 Diploid
 Allele
 Homozygous
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Heterozygous
Genotype
Phenotype
Dominant
Recessive
Father of Genetics:
Gregor Mendel
Back cross
TT (homozygous dominant) x TT (homozygous dominant)
TT (all are dominant)
Back cross
Tt (heterozygous dominant) x Tt (heterozygous dominant)
Dominant (TT, Tt) & Recessive (tt) in ration 3:1
Test cross
TT (homozygous dominant) x tt (homozygous recessive)
Tt (all are dominant)
Test cross
Tt (heterozygous dominant) x tt (homozygous recessive)
Tt or tt (ratio is 1:1)
Sex chromosomes
Each body cell:
22 pairs autosomes + 1 pair sex chromosome
 Sex chromosomes: X & Y
 Except sex chromosomes, all homologous
pairs of autosomes are identical
 X- chromosome carries many genes while Y
chromosome carries fewer genes.
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Sex chromosomes
Female:
~ genotype: XX
(homogametic sex)
 Male:
~ genotype: XY
(heterogametic sex)
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Sex-linkage
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Characters controlled by the genes
which situated on the sex chromosomes,
especially X, are called sex-linked
characters.
For example
~ red-green colour blindness
~ Haemophilia
Genetic diagram: color blindness
Genetic diagram: color blindness
Carrier
Genetic diagram: Hemophilia
Multiple alleles
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Genes exists in more than 2 allelis forms in
the same locus of given pair of homologous
chromosomes.
Each allele produces a distinctive phenotype.
For example: ANO blood group system
~ the human blood groups are controlled by
three alleles IA, IB, I
Crossing over
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During pairing up, the homologous
chromosomes break and re-join with
non-sister chromatid of its homologous
member  exchange genetic segments.
Mutation
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Sudden & stable inherited change of
the genetic material(DNA).
Leads to differences among individuals
Provides raw materials for the species
of the organisms.
Enhances natural selection.
Takes place at any stage in the
development of all organisms.