Gene Expression Flashcards
What are mutations?
Changes to the nucleotide sequence of DNA during DNA replication (interphase)
Ways mutagenic agents increase rate of mutations
- Base analogs / acting as a base (chemicals substitute for a normal nucleotide base)
- Altering bases (chemicals delete/alter base)
- Changing DNA structure (chemicals/radiation change DNA structure)
Types of mutations
- Substitution (swapping one base for another)
- Addition (extra base added)
- Deletion (base is removed)
- Inversion (sequence of bases is reversed)
- Duplication (one or more bases are repeated)
- Translocation (sequence of bases is moved from one part of the genome to another)
How may a mutation cause a change to a single AA?
- Changes one DNA triplet
- Changes the translation of one amino acid
- Changes the primary structure of the protein
- Changes the hydrogen / ionic bonding
- Changes the tertiary structure of the protein
How may a mutation cause a change to many AA?
- Frame shift changes the sequence of all following DNA triplets
- Changes the translation of all following amino acid
- Changes the primary structure of the protein
- Changes the hydrogen / ionic bonding
- Changes the tertiary structure of the protein
Which genes control cell division?
Tumour suppressor genes and proto-oncogenes. Mutations to these genes can cause cancer
Tumour suppressor genes
- Make proteins that slow down rate of mitosis or speed up rate of apoptosis
- Mutation to a tumour suppressor gene may lead to a non-functional protein
- Non-functional protein means cells divide uncontrollably
Proto-oncogenes
- Make proteins that increase rate of mitosis
- If proto-oncogene mutates it becomes an oncogene
- Oncogenes can be over expressed resulting in uncontrollable cell division
Types of tumours
- Benign (non-cancerous, grow slowly, harmless)
- Malignant (cancerous, grow quickly, destroy tissues, spread in blood)
Identifying tumour cells
- Larger, darker nucleus (sometimes multiple)
- Irregular shape
- Different antigens on surface
- Divide (mitosis) more frequently
- Don’t produce all proteins needed to function correctly
- Don’t respond to growth regulating processes
Role of abnormal methylation in tumour growth
- Hypermethylation of tumour suppressor genes means proteins that slow cells division are not transcribed, resulting in uncontrollable cell division
- Hypomethylation of proto-oncogenes causes them to act as oncogenes, so more proteins that speed up cell division are transcribed, resulting in uncontrollable cell division
Role of oestrogen in breast cancer
- Oestrogen-oestrogen receptor complex can activate transcription of oncogene
- Oestrogen can increase rate of cell division and thus DNA replication so mutations more likely
What are stem cells?
Unspecialised cells that can divide and differentiate into different types of specialised cells
Types of stem cells
- Totipotent (differentiate into ANY type of specialised cell, found in early mammalian embryos)
- Pluripotent (differentiate into MANY type of specialised cell, found in mammalian embryos)
- Multipotent (differentiate into FEW type of specialised cell, found in adult bone marrow)
- Unipotent differentiate into ONE type of specialised cell, found in adult bone marrow)
How do stem cells become specialised?
Conditions within cells control which genes are expressed (transcribed + translated into proteins)
Obtaining induced pluripotent stem cells (iPS cells)
- Use a modified virus as a vector
- Virus inserts transcription factor genes from pluripotent cells into the DNA of unipotent stem cells
- Transcription factors are expressed, making the unipotent stem cells pluripotent
Obtaining embryonic stem cells
- Embryos made in a lab by IVF
- Pluripotent stem cells are removed after a few days
- Embryo is destroyed
- Pluripotent stem cells can differentiate into most types of body cells (not placental cells)
Obtaining adult stem cells
- Removed from the bone marrow of adults in operation
- Multipotent stem cells can differentiate into some types of body cells
How are stem cells used in medicine?
- Bone marrow transplants (multipotent stem cells can differentiate into healthy blood cells)
- Growing new organs (iPS cells, genetically identical, avoiding rejection)
Ethical issues of stem cells
- Taken from IVF embryos which could develop into a foetus if implanted
- From fertilisation zygote has right to live
- Adult stem cells are not totipotent or pluripotent
What are transcription factors?
Proteins that control rate of protein synthesis by switching some genes on/off. These can be activators or repressors
How do transcription factors work?
- Transcription factors move from the cytoplasm into the nucleus
- Binds to the promoter region of DNA at the start of the gene
- Activators help RNA polymerase bind to DNA so the gene is transcribed
- Repressors prevent RNA polymerase binding to DNA so the gene is not transcribed
Role of oestrogen in controlling transcription factors
- Oestrogen (steroid hormone) enters cytoplasm through phospholipid bilayer as lipid soluble
- Oestrogen binds to a TF called an oestrogen receptor, forming an oestrogen-oestrogen receptor complex
- Complex binds to promoter region of DNA, initiating transcription
What is RNA interference (RNAi)?
Small double stranded RNA molecules stop mRNA from target genes being translated into proteins
siRNA
- siRNA combines with proteins to form a siRNA-protein complex
- siRNA unwinds becoming single stranded
- siRNA binds to mRNA by complimentary base pairing
- siRNA-protein complex breaks down the mRNA into pieces, preventing translation
- mRNA pieces are recycled
microRNA
- microRNA combines with proteins to form a microRNA-protein complex
- microRNA binds to mRNA by complimentary base pairing
- miRNA-protein complex prevents ribosome attaching, preventing translation
What is epigenetics?
Heritable changes in gene expression without changing the base sequence of DNA caused by changes in the environment that inhibit transcription
Methylation of DNA
- Methyl (-CH3) can attach to DNA at CpG sites in DNA
- Methylated CpG sites prevent transcription enzymes attaching, preventing transcription
- More methylation terminates transcription
Acetylation of histones
- Acetyl groups make histones space out (DNA is less tightly coiled)
- Allows transcriptional enzymes to attach so gene can be expressed
- Enzymes remove acetyl groups, preventing transcription
- Less acetylation of histones terminates transcription
