Download Integrated Programme Sec 2 SBGE, LSS Biology Module Topic

Integrated Programme
Sec 2 SBGE, LSS Biology Module
Topic: Simple Genetics
Genetic Materials
A. Heredity
B. Chromosomes-Genes-DNA
C. RNA
D. Difference Between RNA and DNA
E..Protein Synthesis
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A. Heredity
 Some of our traits are largely determined by heredity
 Physical traits
o e.g. eye colour, ability to roll tongue, shape of earlobes
 mental traits
 e.g. intelligence and personality
 Traits are information determined by DNA
o Deoxyribonucleic acid
 make up our chromosomes
 Chromosomes normally occur in pairs in animal and plant cells
o Except in sex cells
o All animals and plants of the same species have the same number of
chromosomes
 A human cell contains 23 pairs (2n=46) of chromosomes
o Except in the sex cells (n=23)
B. Chromosomes-Genes-DNA
Chromosomes
 Chromatin is a nuclear material that contains the genetic code.
 Heterochromatin : condensed form of chromatin
 Euchromatin : Thread-like form
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Just before cell division, chromatin condenses further
into individual chromosomes
Dividing chromosomes appear as two chromatids
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Each chromosome contains 1 long DNA strand and
histones (proteins)
Genes
 A gene is the basic unit of heredity
 Genes occur in pairs and are located on chromosomes
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Each gene in a pair is inherited from a different parent
Dominant trait is one that prevents another trait from expressing itself
Recessive trait is expressed only if the dominant gene is not present
Phenotype is the observable traits of an individual
o the way in which a trait expresses itself physically
o a particular set of genetic instructions inherited form one’s parents
Two gene copies are called alellomorphs or alleles
Homologous chromosomes
o a pair of matching chromosomes
Alleles occupy the same locus (location) along the length of homologous
chromosomes]
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Genotype describes the alleles each cell has for a certain feature
o E.g. PP for purple flowers to represent the feature of flower colour
o Usually capitalised letters are used to represent the dominant trait
o Lower case letters are used to represent the recessive trait
o PP then represents alleles carrying two copies of dominant gene for colour
of flower
 Phenotype expressed as dominant trait
 Purple flowers
o carrying two copies of recessive gene
 Phenotype expressed as recessive trait
 Purple flowers
o Pp as shown in picture represents alleles carrying 1
copy of dominant gene and 1 copy of recessive
gene
 Phenotype expressed as dominant trait
 Purple flowers
Deoxyribonucleic Acid (DNA)
 The total length of DNA in human genome is 1.8
metres.
 The nucleus is only 6 micrometers in diameter.
How is the DNA packed in the nucleus?
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Structure of DNA
 DNA forms a double helix
 DNA is a polymer (polynucleotide)
 Monomer - nucleotides
o Nucleotides are found in the centre and linked together by phosphodiester
bond
o Each nucleotide consists of
 5-carbon sugar (deoxyribose)
 nitrogen containing base
 a phosphate group
o The sugar-phosphate units forms the backbone.
Phosphate group
Adapt from:users.rcn.com/.../ N/Nucleotides.html
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Nitrogen bases
 4 different types of nitrogen bases found in DNA
 adenine (A)
 guanine (G)
 cytosine (C)
 thymine (T)
 Adenine and guanine belong to a group of chemicals called
purines
 cytosine and thymine are pyrimidines.
 Each purine always bond with a pyrimidine.
 A always pair with a T
 C always pair with a G.
 The base pairs are held together by hydrogen bond
 2 H-bonds between A and T
 3 H-bonds between C and G
Function of DNA
 DNA carries the information needed to construct and operate an organism
o information is stored in genes
 Heredity is based on the transfer of genes between generations
How is the structure of DNA related to its function?
 Contains a sequence of bases that codes for a sequence of amino acids
 Cells use this genetic code to make proteins that build organisms
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Base sequence is read in blocks of three
o Known as a codon
o E.g. GAU
DNA strand is long
so can store a lot of information.
Complementary base pairing on DNA
allows for replication and transcription
Stable sugar-phosphate backbone
to give strength
Weak hydrogen bonds between the bases
allows the 2 strands of to separate
allowing for replication and transcription.
Differences Between Chromosome, Gene and DNA
Chromosome
Involve a series of DNA
strands coiled up into
single units
Occurs in pairs, have a
finite set in each cell
Large
Gene
Specific sites on the
chromosome that code
for a function
Usually seen as a set of 3
bases, numerous
Small
DNA
Refer to the complex
molecule that makes up
the chromosome
Occurs as a double helix
Microscopic
C.Ribonucleic Acid (RNA)
 Found in nucleus and cytoplasm
 Found in the mitochondria and chloroplasts too
 Involved in protein synthesis
 Amount of RNA is proportional to the rate of protein synthesis in the cell
 Polymer of nucleotides
 Single stranded
 Each nucleotide consist of
o 5 carbon ribose sugar
o Phosphate group
o Nitrogen containing base
 Adenine A, guanine G, cytosine C and uracil U
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 3 types of RNA
o Messenger RNA (mRNA)
o Transfer RNA (tRNA)
o Ribosomal RNA (rRNA)
Messenger RNA (mRNA)
 single strand of nucleotides
 mRNA are assembled in the nucleus using DNA as template
 carries genetic message from DNA to ribosome and provides a template for
protein
synthesis
 carries codons to bind to tRNA
Transfer RNA (tRNA)
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Found in the cytoplasm
Carries amino acids to ribosomes for assembly into new amino acid chain
Carries anticodon on one end and amino acid binding site on the other end
o tRNA binds to mRNA by means of anticodon
each tRNA specific for an amino acids
o 20 amino acids naturally synthesised by the body
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Ribosomal RNA (rRNA)
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Makes up part of the ribosome
Brings together all the chemicals and enzymes associated with protein
synthesis
Site of protein synthesis
rRNA
D. Differences Between DNA and RNA
Sugar
Bases
Strands
Size of
molecule
Life span
Site of action
DNA
Deoxyribose
C, G, A, T
Two
Enormous
Long term
Nucleus
mRNA
Ribose
C, G, A, U
One
Small compared to size of
DNA, varies with size of gene
Short term
Nucleus and cytoplasm
tRNA
Ribose
C, G, A, U
One
Small, constant
size
Short term
Cytoplasm
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E. Protein Synthesis
 2 stages
o Transcription
o Translation
Transcription
 Genetic code in DNA is transferred, or transcribed to mRNA
o mRNA then passes the genetic code to the ribosomes
o which act as a template for protein synthesis.
 DNA helix unwind
 RNA polymerase attaches to DNA
o mRNA molecules assembled by adding of nucleotides
 mRNA then detaches from DNA
o complete mRNA molecule leaves the nucleus.
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Translation
 Base sequence on the mRNA is translated into a polypeptide
 mRNA strand passes out of nucleus and attaches to ribosome
 tRNA binds to mRNA at the ribosome
o anticodon of tRNA that is complementary to codon of mRNA
 tRNA delivers amino acids one by one
o polypeptide chain grows
 Process continues until an mRNA stop codon is read
 A completed polypeptide is released
Process of transcription
Enzymes separate the two strands of DNA molecule and RNA polymerase
catalyses the assembly of mRNA, adding one nucleotide at a time until the gene
has been transcribed.
Sites of transcription and translation in the cell.
Transcription occurs in the nucleus and involves assembly of mRNA molecule.
Translation occurs on the ribosomes and involves assembly of a protein.
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