DNA Chromosomes and Inheritance
DNA and Chromosomes:
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Chromosomes:
o thread-like strands of genetic information
o made of deoxyribonucleic acid (DNA) and protein
o DNA is a set of instructions for the cell
o 46 chromosomes in each Diploid cell in the human body
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Genes:
o Many genes on each chromosome
o Each gene codes for one type of protein
o Part of the chromosome that carries heredity material

Nucleic acid:
o Long molecule found in nucleus of cells
o DNA – deoxyribonucleic acid
o RNA – ribonucleic acid
The Structure of DNA:
 Nucleotides are subunits which make up DNA
 Each nucleotide is made up of 3 chemical groups:
o A sugar molecule - Deoxyribose
o A base– four types:
 A = Adenine
 T = Thymine
 G = Guanine
 C = Cytosine
o A phosphate - P
Nucleotide:
Phosphate molecule
Deoxyribose Sugar
Base
Adenine, Cytosine, Guanine, Thymine
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Each nucleotide is joined to another by a backbone of sugar-phosphate groups to form a
chain of nucleotides.
DNA molecule = two chains of nucleotides joined by hydrogen bonds between bases.
A section of a DNA molecule

The two nucleotide chains spiral around one
another to form the DNA double helix structure.
Genetic code:
 Genetic code is the sequence of bases in the DNA
molecule
 Genetic code determines the order of amino acids in
a protein
 3 nucleotides (known as a triplet) code for each
amino acid in a polypeptide chain
o e.g. CAG codes for the amino acid valine
o There are 20 different types of amino acids
 Bases from one chain pair up with bases from the
other chain as follows:
o G always pairs with C
o T always pairs with A
 One strand in double-stranded DNA is complimentary
the other.
DNA Double Helix
Types of Amino Acids
Semi-conservative DNA replication
 DNA replication - the process by which DNA molecules in the nucleus produce an accurate
copy of themselves
 In semi-conservative DNA replication, the two resulting DNA copies each have one strand of
parental DNA and one newly constructed strand.
DNA replication takes place as follows:
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The nucleotides which attach at (c) are free floating
nucleotides found in the nucleus.
The enzyme DNA polymerase is involved in the
nucleotide attachment process
The enzyme ligase is involved in joining the pieces of
back-bone in (d)
Other Enzymes Involved:
Helicase – Separates the two strands
SSB – Prevents the strands from joining back together
Polymerase – Attaches nucleotides to the strand (extends DNA
strand)
Primase – Creates RNA primers (a primer is a short chain of
nucleotides which serves as a starting point for replication –
polymerase can only extend a chain and not initialise one)
Sliding Clamp – Holds polymerase on DNA
Ligase - Links short DNA chains
RNAse H – Removes RNA primers
Protein Synthesis:

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Protein Synthesis – the process by which cells make protein molecules from coded
instructions on the chromosome
Gene – A section of the DNA that gives instructions for making a polypeptide.
Polypeptide molecules are brought together to form longer and more complex protein
molecules.
Nucleus:

DNA stores code needed
for the production of
proteins
Information is stored in
the form of triplet code
DNA is the reference
library of the cell

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Cytoplasm:
Messenger RNA (mRNA)
acts as a carrier of triplet
codes from the nucleus
to the cytoplasm

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Amino acids are
assembled to produce
polypeptide chains.
Polypeptide chains are
joined to form proteins
Transcription – the process by which a section of the DNA code is copied into RNA
Translation – The process by which the mRNA is used to assemble amino acids in a particular
order in the cytoplasm.
A molecule of RNA produced from the DNA contains the code for the manufacture of one
polypeptide chain
Transcription:
Thymine in DNA is replaced by
Uracil in RNA
Inheritance:
Homologous Chromosomes:
 There are 23 pairs of homologous
chromosomes in the human cell
 Homologous chromosomes are
identical in length shape and
appearance
Chromosomes from the body cell of a man
Homologous chromosomes:
Chromosomes that are paired during meiosis. Such
chromosomes are alike with regard to size and also position
of the centromere. They also have the same genes, but not
necessarily the same alleles, at the same locus or location.
Chromosomes, DNA and Genes Overview:
 A pair of homologous chromosomes consist of two chromosomes each of which is a
molecule of DNA
 Each DNA molecule is a series of codes that carry instructions to make polypeptide
molecules
 Each code occupies a specific place on the chromosome called a locus
 Each locus on a chromosome is a gene
 A gene is the functional unit of the chromosome – codes for specific polypeptide molecule
Genetic Inheritance:
 Haploid Cells :
o Gamete cells
o 23 chromosomes
o One copy of each gene
o Represented as n
 Diploid Cells:
o Normal cells
o 46 chromosomes
o 2 copies of each gene
o Represented by 2n

One Diploid cell forms 2 haploid cells by meiosis:

23 chromosomes are inherited from the father and 23 from the mother:
Alleles:
 Different forms of the same gene
 Same locus, different chromosomes
 Haploid cells – one allele for each gene
 Diploid cells – two alleles for each gene
Genotype:
 Refers to the particular forms in which chromosomes exist at the loci on the chromosomes
 Genotype determines the characteristics of an organism
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Genotype is determined by the sequence of base pairs in the DNA strand.
Homozygous Genotype – Both alleles the same
Heterozygous Genotype – alleles are different
Dominant alleles show their characteristics whether genotype is homozygous or
heterozygous – written as capital letter (e.g. Hh and HH)
Recessive alleles only show their characteristics when the genotype is homozygous – written
as small letter (e.g. hh)
Phenotype:
 The name given to the actual appearance of the genotype in an organism
 Physical/ physiological characteristics
Monohybrid Crossing:
 Looks at the way specific characteristics are passed from the parent generation to the
offspring generation
Example:
Parental Phenotypes: Brown Eyes
x
Blue Eyes
Parental Genotypes:
x
bb
BB
Gametes:
B
B
Offspring genotypes:
b
Bb
Bb
Bb,
b
Bb
Bb
Bb,
Bb,
Bb,
Offspring phenotypes: All brown-eyed
Test Cross:
 Used to determine an unknown genotype:
Co-dominance:
 Both alleles equally dominant
 Both alleles are expressed in the phenotype
 E.g.
Genotype
CRCR
CWCW
CWCR


Phenotype
Red Flowers
White Flowers
Pink Flowers
Examples: human blood, hair colour
More complex ratios e.g. 1:2:1
Sickle-cell anaemia
 Painful and fatal disease
 Causes red blood cells to distort when blood is
deoxygenated
 A certain allele causes sickle cell anaemia when in
homozygous form
 Normal Allele = HBA
Sickle-cell Allele = HBS
 Phenotypes:
o HBAHBA = Normal Blood
o HBSHBS = Sickle-cell anaemia
o HBA HBS = Co-dominance + Resistant to Malaria
Multiple Alleles:
 In some cases a single characteristic is controlled genetically by one gene which has 3
alleles
 For the characteristic to appear in the phenotype only two of these alleles must be
present.
Inheritance in blood groups:
 4 human blood groups: Group A. Group B, Group AB, Group O
 Controlled by single gene with three alleles:
o A
o B
o O
 A and B are dominant to O
 A and B are co-dominant
Sex Inheritance:
 Sex is determined by sex chromosomes X and Y
 X is much longer than Y
 Female Genotype = XX
 Male Genotype = XY
Sex-Linkage:
 Examples of medical conditions caused by sex
linked genes:
o Haemophilia- blood clotting is defective
and sufferer may bleed uncontrollably
from the slightest cut
o Sex-linked colour blindness – inability to
distinguish red from green
 Both examples caused by defective gene on X chromosome
 Male (XY) suffers from the condition
Female (XX) usually does not, but carries a defective gene
 Y chromosome is small and does not contain many genes
o Appearance of recessive allele on a region of the X chromosome which does
not have a corresponding region on the Y chromosome will result in the
expression of the recessive trait in the phenotype.
o Therefore sex-linked conditions only occur in men