SACE Stage 2 Biology Notes
CONTENTS
Topic 1: DNA & Proteins P2-12
Topic 2: Cells P13-29
Topic 3: Homeostasis P30-40
Topic 4: Evolution P41-51
, DNA and Proteins
DNA, RNA, Chromosomes and Genes
DNA
Deoxyribonucleic acid. Double helical structure (2 strands twisted in
ladder-like structure) comprised of nucleotide monomers. Nucleotide
composition:
• Phosphate group (negatively charged)
• Deoxyribose sugar (pentase sugar)
• 4 bases; adenine, thymine, guanine, cytosine (bases pair
together and weak hydrogen bonds are formed between
them, enabling DNA to be replicated)
DNA is found in all organisms and has the same function; stores and
transmits genetic information.
DNA in Prokaryotes
In prokaryotes, DNA is unbound (not bound to histones [histone = protein]) and found in a single,
circular chromosome in the nucleoid region of the cell as the cell has no membrane-bound
organelles. Rings of DNA separate to nuclear DNA are also found (called plasmids). DNA ONLY
contains exons (no introns)
Mitochondrial DNA (mtDNA) and DNA in Chloroplasts
Same as DNA in prokaryotes; single, circular, not bound to histones, no introns. DNA in these
organelles provides evidence that mitochondria and chloroplasts were once “free-living”
prokaryotes.
DNA in Eukaryotes
DNA found in nucleus (as cells have membrane-bound organelles) and bound to histones. DNA
found as several, linear chromosomes. Contains introns and exons.
Genetic Code
Universal genetic code (i.e. same code in all organisms) of A-T, C-G. This is evidence for evolution.
Complementary base pairing enables DNA to replicate.
RNA
Ribonucleic acid (RNA), usually single-stranded. Believed to have existed before DNA. Comprised
of nucleotide monomers. Nucleotide composition:
• Phosphate group
• Ribose sugar
• 4 bases: adenine, uracil, guanine and cytosine (A-U, G-C)
Types of RNA include:
• Ribosomal RNA (rRNA): RNA found in ribosomes
• Messenger RNA (mRNA): replicated genetic code from DNA so it can be taken to ribosomes
in cytoplasm for translation
• MicroRNA: believed to be important in gene expression after transcription by binding to
mRNA and stopping ribosome from attaching
• Transfer RNA (tRNA): translate mRNA codons into sequence of amino acids in translation
© Rosanna Barani 2020 2
, Chromosomes (CH)
Condensed DNA that is short and thick. Prior to chromosomes, DNA existed as chromatin, which is
uncoiled, stretched out and thin so DNA is not visible under a microscope. DNA forms chromosomes
to enable DNA to be transferred to new cells. Telomeres (segments of repeated DNA) present at
the end of chromosomes as protection from DNA damage.
Chromosomes have 2 states; non-replicated and replicated.
Non-Replicated Chromosomes
Exist prior to DNA replication. Chromosome without sister
chromatids
Replicated Chromosomes
Amount of DNA has doubled but number of chromosomes stays
the same. One chromosome comprised of sister chromatids
held by centromere.
Genes
Genes are segments of DNA that code for one protein or RNA
molecule.
• Position of a gene on chromosome = locus
• Total number of genes in organism = genome.
An allele is a unique form of the gene (dominant or recessive),
providing genotypic variation in members of the same species.
Several thousand genes in each chromosome. Contain both coding
(exons) and non-coding (introns) segments of DNA, but only exons
are translated into a protein.
DNA Replication (Semi-Conservative)
DNA is replicated prior to cell division so that it can be passed on to daughter cells, creating
genetically identical daughter cells (1 parent cell 2 genetically identical daughter cells). Is
termed “semi-conservative” as each DNA molecule produced has one old strand and one new
strand. DNA replication is made possible from complementary base pairing rules:
• Adenine – thymine
• Guanine - cytosine
Process
1. DNA helicase (an enzyme) unzips double-helix structure across weak hydrogen bonds, creating
exposed bases (replication fork)
2. Free nucleotides pair up with exposed bases according to complementary base pairing rules
3. DNA polymerase joins free nucleotides together to synthesise new sugar-phosphate backbone
4. 2 genetically identical DNA molecules are produced, each consisting of a ‘new’ strand and an
‘old’ strand
© Rosanna Barani 2020 3
, Protein Synthesis
Production of proteins at ribosomes (ribosomes either free-floating in cytoplasm or on rough
endoplasmic reticulum). As DNA cannot leave nucleus of the cell, the gene coding for the specific
protein must be replicated into a messenger RNA (mRNA) molecule so that it can reach the
ribosomes. The mRNA molecule is then transcribed by transfer RNA (tRNA) into a sequence of amino
acids (forming a polypeptide) which fold to create a protein.
Molecules Involved
o Messenger RNA (mRNA): replicated gene that carries DNA code out of nucleus and to
ribosomes
o Transfer RNA: places amino acids (aa) in order to create polypeptide based on mRNA codons.
The tRNA anticodon is complementary to mRNA codon. Amino acids are attached at the
acceptor stem
o Ribosomal RNA (rRNA): makes up ribosome, enabling it to function
o Sometimes, microRNA (miRNA) regulates gene expression after transcription by binding to mRNA
molecules, stopping ribosome from attaching and thus, stopping translation
Transcription (copying DNA into RNA – occurs in nucleus of eukaryotes, nucleoid region of prokaryotes)
1. DNA polymerase unzips gene along hydrogen bonds, creating exposed
bases
2. Free RNA nucleotides use exposed base as template
according to complementary base pairing rules (uracil is
used instead of thymine)
3. RNA polymerase joins free nucleotides together to form sugar
phosphate backbone of mRNA molecule containing introns and
exons
Translation (mRNA is transcribed into aa sequence – in ribosomes)
1. After splicing, mRNA molecule leaves nuclear pores/nuclear
envelope and travels in cytoplasm to ribosome
2. Ribosome attaches to mRNA
3. Complementary tRNA anticodons pair with mRNA codons,
bringing in amino acids (attached to their acceptor stem)
according to codon
4. Peptide bonds form between aa, creating polypeptide chain
5. tRNA break away and mRNA is broken down by enzymes so
RNA nucleotides can be reused
Splicing
DNA in eukaryotic cells contains both exons (coding segments)
and introns (non-coding segments). Though the majority of DNA
contains introns, only exons code for a protein. After transcription, pre-mRNA is formed and contains
both exons and introns. However, through splicing, introns are cut out by ligase enzymes whilst exons
are re-joined, creating mature mRNA which travels to the ribosomes.
© Rosanna Barani 2020 4
CONTENTS
Topic 1: DNA & Proteins P2-12
Topic 2: Cells P13-29
Topic 3: Homeostasis P30-40
Topic 4: Evolution P41-51
, DNA and Proteins
DNA, RNA, Chromosomes and Genes
DNA
Deoxyribonucleic acid. Double helical structure (2 strands twisted in
ladder-like structure) comprised of nucleotide monomers. Nucleotide
composition:
• Phosphate group (negatively charged)
• Deoxyribose sugar (pentase sugar)
• 4 bases; adenine, thymine, guanine, cytosine (bases pair
together and weak hydrogen bonds are formed between
them, enabling DNA to be replicated)
DNA is found in all organisms and has the same function; stores and
transmits genetic information.
DNA in Prokaryotes
In prokaryotes, DNA is unbound (not bound to histones [histone = protein]) and found in a single,
circular chromosome in the nucleoid region of the cell as the cell has no membrane-bound
organelles. Rings of DNA separate to nuclear DNA are also found (called plasmids). DNA ONLY
contains exons (no introns)
Mitochondrial DNA (mtDNA) and DNA in Chloroplasts
Same as DNA in prokaryotes; single, circular, not bound to histones, no introns. DNA in these
organelles provides evidence that mitochondria and chloroplasts were once “free-living”
prokaryotes.
DNA in Eukaryotes
DNA found in nucleus (as cells have membrane-bound organelles) and bound to histones. DNA
found as several, linear chromosomes. Contains introns and exons.
Genetic Code
Universal genetic code (i.e. same code in all organisms) of A-T, C-G. This is evidence for evolution.
Complementary base pairing enables DNA to replicate.
RNA
Ribonucleic acid (RNA), usually single-stranded. Believed to have existed before DNA. Comprised
of nucleotide monomers. Nucleotide composition:
• Phosphate group
• Ribose sugar
• 4 bases: adenine, uracil, guanine and cytosine (A-U, G-C)
Types of RNA include:
• Ribosomal RNA (rRNA): RNA found in ribosomes
• Messenger RNA (mRNA): replicated genetic code from DNA so it can be taken to ribosomes
in cytoplasm for translation
• MicroRNA: believed to be important in gene expression after transcription by binding to
mRNA and stopping ribosome from attaching
• Transfer RNA (tRNA): translate mRNA codons into sequence of amino acids in translation
© Rosanna Barani 2020 2
, Chromosomes (CH)
Condensed DNA that is short and thick. Prior to chromosomes, DNA existed as chromatin, which is
uncoiled, stretched out and thin so DNA is not visible under a microscope. DNA forms chromosomes
to enable DNA to be transferred to new cells. Telomeres (segments of repeated DNA) present at
the end of chromosomes as protection from DNA damage.
Chromosomes have 2 states; non-replicated and replicated.
Non-Replicated Chromosomes
Exist prior to DNA replication. Chromosome without sister
chromatids
Replicated Chromosomes
Amount of DNA has doubled but number of chromosomes stays
the same. One chromosome comprised of sister chromatids
held by centromere.
Genes
Genes are segments of DNA that code for one protein or RNA
molecule.
• Position of a gene on chromosome = locus
• Total number of genes in organism = genome.
An allele is a unique form of the gene (dominant or recessive),
providing genotypic variation in members of the same species.
Several thousand genes in each chromosome. Contain both coding
(exons) and non-coding (introns) segments of DNA, but only exons
are translated into a protein.
DNA Replication (Semi-Conservative)
DNA is replicated prior to cell division so that it can be passed on to daughter cells, creating
genetically identical daughter cells (1 parent cell 2 genetically identical daughter cells). Is
termed “semi-conservative” as each DNA molecule produced has one old strand and one new
strand. DNA replication is made possible from complementary base pairing rules:
• Adenine – thymine
• Guanine - cytosine
Process
1. DNA helicase (an enzyme) unzips double-helix structure across weak hydrogen bonds, creating
exposed bases (replication fork)
2. Free nucleotides pair up with exposed bases according to complementary base pairing rules
3. DNA polymerase joins free nucleotides together to synthesise new sugar-phosphate backbone
4. 2 genetically identical DNA molecules are produced, each consisting of a ‘new’ strand and an
‘old’ strand
© Rosanna Barani 2020 3
, Protein Synthesis
Production of proteins at ribosomes (ribosomes either free-floating in cytoplasm or on rough
endoplasmic reticulum). As DNA cannot leave nucleus of the cell, the gene coding for the specific
protein must be replicated into a messenger RNA (mRNA) molecule so that it can reach the
ribosomes. The mRNA molecule is then transcribed by transfer RNA (tRNA) into a sequence of amino
acids (forming a polypeptide) which fold to create a protein.
Molecules Involved
o Messenger RNA (mRNA): replicated gene that carries DNA code out of nucleus and to
ribosomes
o Transfer RNA: places amino acids (aa) in order to create polypeptide based on mRNA codons.
The tRNA anticodon is complementary to mRNA codon. Amino acids are attached at the
acceptor stem
o Ribosomal RNA (rRNA): makes up ribosome, enabling it to function
o Sometimes, microRNA (miRNA) regulates gene expression after transcription by binding to mRNA
molecules, stopping ribosome from attaching and thus, stopping translation
Transcription (copying DNA into RNA – occurs in nucleus of eukaryotes, nucleoid region of prokaryotes)
1. DNA polymerase unzips gene along hydrogen bonds, creating exposed
bases
2. Free RNA nucleotides use exposed base as template
according to complementary base pairing rules (uracil is
used instead of thymine)
3. RNA polymerase joins free nucleotides together to form sugar
phosphate backbone of mRNA molecule containing introns and
exons
Translation (mRNA is transcribed into aa sequence – in ribosomes)
1. After splicing, mRNA molecule leaves nuclear pores/nuclear
envelope and travels in cytoplasm to ribosome
2. Ribosome attaches to mRNA
3. Complementary tRNA anticodons pair with mRNA codons,
bringing in amino acids (attached to their acceptor stem)
according to codon
4. Peptide bonds form between aa, creating polypeptide chain
5. tRNA break away and mRNA is broken down by enzymes so
RNA nucleotides can be reused
Splicing
DNA in eukaryotic cells contains both exons (coding segments)
and introns (non-coding segments). Though the majority of DNA
contains introns, only exons code for a protein. After transcription, pre-mRNA is formed and contains
both exons and introns. However, through splicing, introns are cut out by ligase enzymes whilst exons
are re-joined, creating mature mRNA which travels to the ribosomes.
© Rosanna Barani 2020 4
