8.1 Alteration of DNA base sequences can alter protein
structure
● Addition: extra base added to DNA sequence, causes frame shift to the right
○ ATG CCT AAG → ATG CCT CAA G
● Deletion: base removed from DNA sequence, causes frame shift to the left
○ ATG CCT AAG → ATG CTA AG
● Substitution: base replaced by another, may have no effect as genetic code is
degenerate
○ ATG CCT AAG → ATG CCT TAG
● Inversion: section of bases detach from the DNA sequence, but when they reattach,
they are inverted
○ ATG CCT AAG → ATG TCC AAG
● Duplication: addition of the same base to the DNA sequence, causes frame shift to
the right
○ ATG CCT AAG → ATG CCT TAA G
● Translocation: section of bases on one chromosome detach and reattach onto a
different chromosome
8.2.1 Most of a cell’s DNA is not translated
Stem cells:
● Totipotent: stem cells able to divide and produce any type of body cell
○ Does this by translating only specific sections of DNA, meaning only certain
genes are expressed, so only certain proteins are produced, resulting in cell
specialisation.
○ Occurs only for a limited time in early mammalian embryos
● Pluripotent: stem cells able to divide indefinitely and differentiate into any type of
body cell
○ But not extra-embryonic tissues which are formed in early development that
don’t become part of the fetus, such as cells in the placenta.
○ Has uses in treating human disorders by replacing damaged cells, such as
by differentiating into β cells in the pancreas to treat type 1 diabetes.
○ Found in embryos
● Multipotent: stem cells able to divide into a limited range of related cell types within
a particular tissue
○ Differentiation is more limited as less genes are expressed or more genes are
permanently switched off
○ Found in mature mammals, such as bone marrow cells
● Unipotent: stem cells that can only differentiate into one cell type
○ Found in essentially any tissue in mature mammals
○ Such as unipotent stem cells that differentiate to form cardiomyocytes which
are heart muscle cells
● Induced pluripotent stem cells (iPS): adult somatic cells that have been genetically
modified to become pluripotent by using appropriate protein transcription factors
, ○ Switches genes back on, allowing those genes to be expressed and
transcribed
○ Therefore allows the cell to differentiate to many different body cell types.
8.2.2 Regulation of transcription and translation
Transcription:
● Transcription factors: proteins that bind to specific DNA sequences called promoter
regions upstream of a target gene, stimulating or inhibiting transcription of that
target gene.
○ Activators increase the rate of transcription by helping RNA polymerase bind
to the start of the target gene
○ Repressors decrease the rate of transcription by preventing RNA polymerase
from binding
● Oestrogen: lipid-soluble steroid hormone that can diffuse directly through the cell
membrane
○ Inside the cell, oestrogen binds to an oestrogen receptor which is a
transcription factor
○ Binding causes a conformational change in the tertiary structure of the
receptor, exposing its DNA binding site
○ Forms an oestrogen-oestrogen receptor complex which is the active form of
the transcription factor
○ The complex moves to the nucleus where it binds to a promoter region at the
start of the target gene
○ Acts as an activator, helping RNA polymerase bind to the start of that target
gene, increasing the rate of transcription.
● Epigenetics: heritable changes in gene expression that occur without altering the
base sequence of DNA. Caused by environmental factors that inhibit transcription
by:
○ Increased DNA methylation, where methyl groups are added to DNA,
causing DNA to bind more tightly to histones, which stops transcription
factors and RNA polymerase from binding
○ Decreased histone acetylation, causing DNA to bind more tightly to histones,
preventing transcription factors from accessing the DNA
Translation:
● RNA interference (RNAi): process where small RNA molecules produced from
double-stranded RNA, prevent translation of mRNA transcribed from target genes.
Occurs in eukaryotes and some prokaryotes.
● Small interfering RNA (siRNA):
○ Transcribed mRNA leaves the nucleus and enters the cytoplasm
○ Double-stranded siRNA associates with proteins and unwinds itself
○ A single siRNA strand of siRNA then binds to the target mRNA by
complementary base pairing
○ The associated proteins on the siRNA cut the mRNA into fragments
○ Preventing translation of the mRNA into a polypeptide
structure
● Addition: extra base added to DNA sequence, causes frame shift to the right
○ ATG CCT AAG → ATG CCT CAA G
● Deletion: base removed from DNA sequence, causes frame shift to the left
○ ATG CCT AAG → ATG CTA AG
● Substitution: base replaced by another, may have no effect as genetic code is
degenerate
○ ATG CCT AAG → ATG CCT TAG
● Inversion: section of bases detach from the DNA sequence, but when they reattach,
they are inverted
○ ATG CCT AAG → ATG TCC AAG
● Duplication: addition of the same base to the DNA sequence, causes frame shift to
the right
○ ATG CCT AAG → ATG CCT TAA G
● Translocation: section of bases on one chromosome detach and reattach onto a
different chromosome
8.2.1 Most of a cell’s DNA is not translated
Stem cells:
● Totipotent: stem cells able to divide and produce any type of body cell
○ Does this by translating only specific sections of DNA, meaning only certain
genes are expressed, so only certain proteins are produced, resulting in cell
specialisation.
○ Occurs only for a limited time in early mammalian embryos
● Pluripotent: stem cells able to divide indefinitely and differentiate into any type of
body cell
○ But not extra-embryonic tissues which are formed in early development that
don’t become part of the fetus, such as cells in the placenta.
○ Has uses in treating human disorders by replacing damaged cells, such as
by differentiating into β cells in the pancreas to treat type 1 diabetes.
○ Found in embryos
● Multipotent: stem cells able to divide into a limited range of related cell types within
a particular tissue
○ Differentiation is more limited as less genes are expressed or more genes are
permanently switched off
○ Found in mature mammals, such as bone marrow cells
● Unipotent: stem cells that can only differentiate into one cell type
○ Found in essentially any tissue in mature mammals
○ Such as unipotent stem cells that differentiate to form cardiomyocytes which
are heart muscle cells
● Induced pluripotent stem cells (iPS): adult somatic cells that have been genetically
modified to become pluripotent by using appropriate protein transcription factors
, ○ Switches genes back on, allowing those genes to be expressed and
transcribed
○ Therefore allows the cell to differentiate to many different body cell types.
8.2.2 Regulation of transcription and translation
Transcription:
● Transcription factors: proteins that bind to specific DNA sequences called promoter
regions upstream of a target gene, stimulating or inhibiting transcription of that
target gene.
○ Activators increase the rate of transcription by helping RNA polymerase bind
to the start of the target gene
○ Repressors decrease the rate of transcription by preventing RNA polymerase
from binding
● Oestrogen: lipid-soluble steroid hormone that can diffuse directly through the cell
membrane
○ Inside the cell, oestrogen binds to an oestrogen receptor which is a
transcription factor
○ Binding causes a conformational change in the tertiary structure of the
receptor, exposing its DNA binding site
○ Forms an oestrogen-oestrogen receptor complex which is the active form of
the transcription factor
○ The complex moves to the nucleus where it binds to a promoter region at the
start of the target gene
○ Acts as an activator, helping RNA polymerase bind to the start of that target
gene, increasing the rate of transcription.
● Epigenetics: heritable changes in gene expression that occur without altering the
base sequence of DNA. Caused by environmental factors that inhibit transcription
by:
○ Increased DNA methylation, where methyl groups are added to DNA,
causing DNA to bind more tightly to histones, which stops transcription
factors and RNA polymerase from binding
○ Decreased histone acetylation, causing DNA to bind more tightly to histones,
preventing transcription factors from accessing the DNA
Translation:
● RNA interference (RNAi): process where small RNA molecules produced from
double-stranded RNA, prevent translation of mRNA transcribed from target genes.
Occurs in eukaryotes and some prokaryotes.
● Small interfering RNA (siRNA):
○ Transcribed mRNA leaves the nucleus and enters the cytoplasm
○ Double-stranded siRNA associates with proteins and unwinds itself
○ A single siRNA strand of siRNA then binds to the target mRNA by
complementary base pairing
○ The associated proteins on the siRNA cut the mRNA into fragments
○ Preventing translation of the mRNA into a polypeptide
