Topic 8
Alteration of the sequence of bases in DNA can alter the structure of proteins
● Frame shift: a mutation changes the nature of all base triplets downstream from the
mutation
● Gene mutations occur spontaneously
● Mutation rate increased by mutagenic agents
● Mutation:
○ Inversion
○ Duplication
○ Translocation
○ Substitution
○ Deletion
○ Addition
Most of a cell’s DNA is not translated
● Totipotent cells can divide and produce any type of body cell
○ During development, totipotent cells translate only part of their DNA, resulting in
cell specialisation
○ Totipotent cells occur only for a limited time in early mammalian embryos
● Pluripotent cells are found in embryos – pluripotent cells can specialise into any cell in
the body but lose the ability to become the cells that make up the placenta
○ (Totipotent cells can divide into the cells that make up the placenta in mammals)
● Multipotent and unipotent cells are found in mature mammals and can divide to form a
limited number of cell types
○ Multipotent stem cells can differentiate into a few different types of cell, e.g
multipotent stem cells in bone marrow can form both red and white blood cells
○ Unipotent stem cells can only differentiate into one type of cell, eg epidermal skin
cells and cardiomyocytes (heart muscle cells that make up a lot of tissue in the
heart)
● Pluripotent stem cells can divide in unlimited numbers and can be used in treating
human disorders (aka embryonic stem cells)
● Induced pluripotent stem cells (iPS cells) can be produced from adult somatic (body)
cells using appropriate protein transcription factors
○ The adult somatic cells are made to express a series of transcription factors that
are normally associated with pluripotent stem cells, and these transcription
factors cause the cells to express genes that are associated with pluripotency.
■ Can do this eg by infecting the cells with a specially-modified virus that
has the genes coding for the transcription factors within its DNA → when
the virus infects the cell these genes are passed into the adult cell’s DNA
so the cell is able to produce the transcription factors
, Regulation of transcription and translation
● In eukaryotes, transcription of target genes can be stimulated/inhibited when specific
transcription factors move from the cytoplasm into the nucleus
○ In the nucleus they bind to specific DNA sites near the start of their target genes.
○ Some transcription factors – activators – stimulate or increase the rate of
transcription (eg help RNA polymerase bind to start of target gene)
○ Some transcription factors – repressors – inhibit or decrease the rate of
transcription (eg bind to start of target gene preventing RNA polymerase binding)
● Role of oestrogen in initiating transcription:
○ Oestrogen is a steroid hormone
○ In the cytoplasm, oestrogen binds to a transcription factor called an oestrogen
receptor → forms an oestrogen-oestrogen receptor complex
○ The complex moves from the cytoplasm into the nucleus where it binds to
specific DNA sites near the start of the target gene
○ Complex acts as an activator of transcription
○ (In some cells the complex acts as a repressor of transcription → depends on the
type of cell and the target gene)
● Epigenetic control of gene expression in eukaryotes:
○ Epigenetic control works through the attachment or removal of chemical groups
(epigenetic marks) to/from DNA or histones
■ Increased methylation of DNA switches a gene off
● Methyl group attaches to DNA coding for a gene at a CpG site
● Increased methylation changes the DNA structure so that the
transcriptional machinery can’t interact with the gene and it is not
expressed
■ Decreased acetylation of histones switches a gene off
● Histones are proteins that DNA wraps around to form chromatin
(which makes up chromosomes) → chromatin can be highly
condensed or less condensed. When highly condensed, not
transcriptionally active
● When histones are acetylated, the chromatin is less condensed so
transcriptionally active because transcriptional machinery can
access the DNA
● So decreased acetylation of histones causes chromatin to become
more condensed so transcriptionally inactive
● Histone deacetylase enzymes are responsible for removing the
acetyl groups
○ Epigenetic marks don’t alter the base sequence; they alter how easy it is for
enzymes and other proteins to interact with and transcribe the DNA
○ Epigenetic changes to gene expression can occur in response to environmental
factors, eg pollution
○ Epigenetic changes can be inherited by offspring → most epigenetic marks on
the DNA base sequence that offspring inherit are removed between generations,
but some escape the removal process and are passed on.
Alteration of the sequence of bases in DNA can alter the structure of proteins
● Frame shift: a mutation changes the nature of all base triplets downstream from the
mutation
● Gene mutations occur spontaneously
● Mutation rate increased by mutagenic agents
● Mutation:
○ Inversion
○ Duplication
○ Translocation
○ Substitution
○ Deletion
○ Addition
Most of a cell’s DNA is not translated
● Totipotent cells can divide and produce any type of body cell
○ During development, totipotent cells translate only part of their DNA, resulting in
cell specialisation
○ Totipotent cells occur only for a limited time in early mammalian embryos
● Pluripotent cells are found in embryos – pluripotent cells can specialise into any cell in
the body but lose the ability to become the cells that make up the placenta
○ (Totipotent cells can divide into the cells that make up the placenta in mammals)
● Multipotent and unipotent cells are found in mature mammals and can divide to form a
limited number of cell types
○ Multipotent stem cells can differentiate into a few different types of cell, e.g
multipotent stem cells in bone marrow can form both red and white blood cells
○ Unipotent stem cells can only differentiate into one type of cell, eg epidermal skin
cells and cardiomyocytes (heart muscle cells that make up a lot of tissue in the
heart)
● Pluripotent stem cells can divide in unlimited numbers and can be used in treating
human disorders (aka embryonic stem cells)
● Induced pluripotent stem cells (iPS cells) can be produced from adult somatic (body)
cells using appropriate protein transcription factors
○ The adult somatic cells are made to express a series of transcription factors that
are normally associated with pluripotent stem cells, and these transcription
factors cause the cells to express genes that are associated with pluripotency.
■ Can do this eg by infecting the cells with a specially-modified virus that
has the genes coding for the transcription factors within its DNA → when
the virus infects the cell these genes are passed into the adult cell’s DNA
so the cell is able to produce the transcription factors
, Regulation of transcription and translation
● In eukaryotes, transcription of target genes can be stimulated/inhibited when specific
transcription factors move from the cytoplasm into the nucleus
○ In the nucleus they bind to specific DNA sites near the start of their target genes.
○ Some transcription factors – activators – stimulate or increase the rate of
transcription (eg help RNA polymerase bind to start of target gene)
○ Some transcription factors – repressors – inhibit or decrease the rate of
transcription (eg bind to start of target gene preventing RNA polymerase binding)
● Role of oestrogen in initiating transcription:
○ Oestrogen is a steroid hormone
○ In the cytoplasm, oestrogen binds to a transcription factor called an oestrogen
receptor → forms an oestrogen-oestrogen receptor complex
○ The complex moves from the cytoplasm into the nucleus where it binds to
specific DNA sites near the start of the target gene
○ Complex acts as an activator of transcription
○ (In some cells the complex acts as a repressor of transcription → depends on the
type of cell and the target gene)
● Epigenetic control of gene expression in eukaryotes:
○ Epigenetic control works through the attachment or removal of chemical groups
(epigenetic marks) to/from DNA or histones
■ Increased methylation of DNA switches a gene off
● Methyl group attaches to DNA coding for a gene at a CpG site
● Increased methylation changes the DNA structure so that the
transcriptional machinery can’t interact with the gene and it is not
expressed
■ Decreased acetylation of histones switches a gene off
● Histones are proteins that DNA wraps around to form chromatin
(which makes up chromosomes) → chromatin can be highly
condensed or less condensed. When highly condensed, not
transcriptionally active
● When histones are acetylated, the chromatin is less condensed so
transcriptionally active because transcriptional machinery can
access the DNA
● So decreased acetylation of histones causes chromatin to become
more condensed so transcriptionally inactive
● Histone deacetylase enzymes are responsible for removing the
acetyl groups
○ Epigenetic marks don’t alter the base sequence; they alter how easy it is for
enzymes and other proteins to interact with and transcribe the DNA
○ Epigenetic changes to gene expression can occur in response to environmental
factors, eg pollution
○ Epigenetic changes can be inherited by offspring → most epigenetic marks on
the DNA base sequence that offspring inherit are removed between generations,
but some escape the removal process and are passed on.
