Unit 1 – Cell Biology
DNA
Structure of DNA
• Proteins are large complex molecules
• They are made of repeating units called amino acids
• There are around 20 different types of amino acids
• Amino acids are joined together by peptide bonds to form polypeptide
chains
• E.g.
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A chain then folds to form a completed protein
Sometimes a number of polypeptide chains combine to form a
completed protein
The sequence of amino acids in a protein determines its structure and
function.
Proteins can be classified into two groups
• Fibrous
• Globular
Fibrous protein
• Used to construct connective tissues, muscle fibre and tendons
• E.g. collagen
Globular Protein
Form important components of hormones, antibodies and enzymes
Double Helix
• DNA carries the complete genetic information of an organism in the
form of a code
• The sequence of amino acids in a protein is determined by this code
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DNA is a type of nucleic acid called deoxyribonucleic acid
DNA consists of a double helix – 2 strands twisted into a coil
Each strand is made of repeating units called nucleotides
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P = phosphate
S = deoxyribose sugar
B = base
______ = strong chemical bond
DNA nucleotides contain deoxyribose sugar
• A = adenine
• C = cytosine
• T = thymine
• G = guanine
This means that there are four different nucleotides in DNA
Forming a DNA Strand
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The sugar of one nucleotide joins with the
phosphate of the next nucleotide
• This is repeated to form a chain
• The chain has
– A sugar phosphate backbone
– A sequence of bases that makes
Joining two strands
together
the genetic
code
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Two strands join together to form a molecule of DNA
– Adenine pairs with thymine
– Cytosine pairs with guanine
This is known as complementary base pairing
Weak hydrogen (H) bonds between the bases of one strand and the
bases of the other hold the strands together
The two DNA strands twist to form a double helix
DNA is organised into chromosomes
Genes
The DNA which makes up a chromosome is millions of base pairs long.
• A section of DNA which is thousands of base pairs long is called a
gene
• Each gene carries the code for one protein
DNA
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Replication
DNA replication occurs before nuclear division
DNA replication produces exact copies of the parent DNA molecule
This ensures that a complete set of information is passed onto new
cells and the next generation
• DNA replication will not occur unless all of the following are available
• Parent DNA to act as a template
• A supply of new nucleotides
• Enzymes to control the process
• ATP to provide energy
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Parent DNA molecule untwists
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Parent DNA molecule unzips when the hydrogen bonds break causing
the two strands to separate
Free nucleotides present in the cell pair with nucleotides on the
unzipped strands according to their complementary base pairing.
The bases are joined to each other by hydrogen bonds
• Sugar phosphate bonds join the nucleotides together to form a new
DNA strand
• Two identical molecules of DNA are formed
Each daughter molecule consists of
– One original strand
– One newly synthesised strand
RNA
• RNA is a type of nucleic acid called ribonucleic acid
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RNA is different from DNA in that it
– Is single stranded
– Has a sugar called ribose
– Has uracil instead of thymine
There are two types of RNA
– Messenger RNA (mRNA)
– Transfer RNA (tRNA)
mRNA
• Is formed in the nucleus of a cell
• Rewrites the base sequence of a section of DNA in a process called
transcription
• Carries the code for building a protein from the nucleus to the
cytoplasm i.e. acts as a messenger
tRNA
• Is found in the cytoplasm
• Picks up specific amino acids from the cytoplasm and brings them into
position on the surface of a ribosome where they can be joined
together in a specific order
• This is called translation
• The sequence of bases in a DNA molecule makes up the genetic code.
This determines which amino acids are joined together to make a
protein.
• The code is read as a series of triplets (groups of three bases)
Triplets
• DNA - a triplet of bases is known as a codon
• mRNA - a triplet of bases is known as a codon
• tRNA - a triplet of bases is known as an
anticodon
Main stages of protein synthesis
• The main stages are transcription and translation
Transcription
• Transcription takes place in the nucleus
• Part of a DNA strand unwinds and unzips
• mRNA synthesised along one DNA strand
• mRNA peels off and moves out of the nucleus to cytoplasm
Translation
• Translation takes place on the ribosomes in the cytoplasm
• mRNA attaches to ribosome
• tRNA molecules bring amino acids to ribosome
• Anti – codons and codons match up
• Peptide bonds form between amino acids
Result of Protein Synthesis
• A polypeptide is formed
• tRNA is reused
• mRNA is reused and broken down into free nucleotides again
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Ribosomes are the site of protein synthesis
Protein passes into the channels of the rough ER for transportation
Rough ER membranes pinch off to form a vesicle carrying protein
molecules
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Vesicles fuse with the golgi releasing protein into channels for
packaging
Proteins are coated with sugars containing glycoproteins
Vesicle pinches off golgi apparatus
Vesicle fuses with cell membrane and glycoproteins are secreted e.g.
mucus
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Cells and organelles
• Growing cells require large amounts of protein for growth. These cells
contain large numbers of ER and Ribosomes e.g. cells present in root
and shoot tips
• Secreting cells must package the protein before it is secreted. These
cells contain large numbers of golgi apparatus e.g. pancreatic cells,
salivary gland cells