Quiz 7 Flashcards
Cancer is caused by:
Mutations and epigenetic mechanisms that are regulated abnormally
Mutations usually occur in:
Somatic cells; many mutations needed; genes mutated or expressed aberrantly control many basic aspects such as DNA repair, cell cycle control, apoptosis, cellular differentiation, cell migration, and cell-cell contacts
Two fundamental properties of cancer cells:
Unregulated cell division (proliferation) and metastatic spread
Benign tumor
Cells divide abnormally but do not spread to other parts of the body
Malignant tumor
Cells divide abnormally and can spread to other parts of the body
Metastasis
When cancer cells spread to other parts of the body and form new tumors in the tissues where they arrive
Clonal origin
Arise from a common single cell that accumulated numerous specific mutations; however, cells in a tumor are not genetically identical because as cancer grows cells acquire new mutations and then divide–cancer consists of subpolulation of cells that share the specific mutations of the original cell
Driver mutations
Mutations that contribute to tumor progression
Passenger mutations
Do not contribute to cancer but might give the cancer cell an advantage if conditions change
Cancer stem cells
Cells in cancer that proliferate; similar to normal stem cells as they can self-renew–divides into two types of daughter cells (one a cancer stem cell and the other a cell type within the cancer)
In order for cancers to arise:
Several driver mutations are needed
Tumorigenesis
Development of a malignant tumor as driver mutations accumulate
Clonal expansion
Important feature of tumorigenesis; each driver mutations confers a growth and survival advantage to a cell which then divides more rapidly and creates a subpopulation
Cancer cells are characterized by:
Genetic instability (much higher rate of chromosomal alterations and mutations such as point mutations) and epigenetic abnormalities (epigenetic alterations are important for the early stages of tumorigenesis and epigenetic mechanisms have the same effect as mutations in tumorigenesis)
Mutator phenotype
High level of genomic instability in cancer cells
Cancer cells are hypomethylated:
Increased genetic expression of genes that normally not be used and increased genetic instability as noncoding proteins are not methylated and thus susceptible to translocations; genes in cancer cells can also be hypermethylated, silencing genes that would normally be expressed
Chromatin remodeling complexes
Mutations in proteins that are part of chromatin complexes can occur in cancers, leading to abnormal chromatin remodeling, leading to abnormal gene expression
Histone modifications
Writers, readers, and erasers can be mutated in cancers, causing abnormal histone modification patterns and/or responses
Epigenetic cancer therapy
Attempts to reprogram gene expression patterns that are characteristic for cancer cells, returning genes to normal and healthy pattern; focus on drug development to target genes silenced by epigenetic mechanisms
Genetic defects in cell cycle
Cell growth and differentiation are strictly regulated; in cancer cells, many genes that control these functions are mutated or abberantly expressed, causing abnormal cell division
Healthy cells that stop proliferating enter G0, and can exit to reenter cell cycle, but cancer cells are unable to enter G0 and thus divide continously (growth factor or hormone binds to receptor on plasma membrane and the message is transmitted through cytoplasm in a process call signal transduction; said pathways are often mutated in cancer cells)
Cell cycle is regulated by genes that:
Either suppress or promote cell division called cyclins and cyclin dependant kinsases, as well as proteins that regulate the cell cycle checkpoints
Cyclins and CDKs
A specific cyclin binds to a specific CDK, activating the CDK/cyclin complex which activates other proteins that are responsible for pushing the cell cycle to the next stage; a healthy cell regulates synthesis and destruction of different cyclins at different times during the cell cycle (because if a specific cyclin is not present it doesn’t progress to next stage of cycle)
Cell cycle checkpoints
G1/S, G2/M, and M, cells decide whether to proceed to next stage of cycle or halt progress if DNA replication or chromosome assembly is bad or DNA is mutated, a.k.a. cell cycle arrest
Apoptosis
If DNA damage is so severe that repair is impossible, the cell may initiate programmed cell death, during which proteases called caspases execute it
Steps of apoptosis
Nuclear DNA becomes fragmented, internal cellular structure breaks down, the cell dissolves into small spherical structures called apoptotic bodies, and the said apoptotic bodied are destroyed by immune cells
Example of apoptosis
Bcl2 prevents it and BAX initiates it; in a healthy cell Bcl2 inhibits BAX so no apoptosis occurs, and if cell is damaged Bcl2 is inhibited to BAX initiates apoptosis
Proto-oncogenes
Activated if a cell needs to divide; promotes cell division by acting as gas oedal
Tumor suppressor genes
Suppress cell division, so act as break pedal (also initiate apoptosis
Oncogones
In cancer cells, proto-oncogenes are altered so their activities aren’t controlled normally; this is called an oncogene, which is a mutated proto-oncogene that is mutated or abnormally expressed and contributes to the development of cancer (gain of function alteration)
ras Proto-oncogene
Ras genes encode signal transduction molecules that regulate cell growth and division; mutations that convert the ras proto-oncogene to an encogene freeze the Ras protein into its active conformation, constantly stimulating cells to divide
When tumor suppressor genes are mutated or inactivated:
Cells are unable to respond normally to cell cycle checkpoints and/or undergo apoptosis
p53 tumor suppressor
Transcription factor that repressed or stimulates transcription of more than 50 different genes; can arrest the cell cycle at several phases and activate apoptosis; cells lacking p53 are unable to arrest at the cell cycle checkpoints or activate apoptosis in response to DNA damage
Simplified model of metastasis
Cancer cells leave primary tumor and break out of tissue of origin, then migrate in blood or lymph vessels to other parts of the body, and finally leave the vessels and begin secondary tumors
Metastasis in detail
In primary tumor, some cancer stem cells acquire the ability to metastasize as they become able to digest components of the extracellular matrix and basal lamina that surrounds and senate the body’s tissues and normally inhibit migration of cells
Then, they enter the blood or lymph which is very perilous; if they survive and successfully invade and colonize a new tissue in the body, the cancer cells continue to mutate and undergo clonal expansions as in the primary tumor
Inherited cancers
Small fraction of cancers occur not from somatic cells but hereditary components; individual inherits mutated allele of a tumor suppressor gene from one parent; if only the wild-type allele in the cell is altered so that it does not work, then this cell now has no functioning alleles of the tumor suppressor, called a loss of heterozygosity
Viruses and cancer
Linked to 12% of human cancers; alone don’t cause them but contribute to tumorigenesis
Retroviruses
Contribute to cancer in animals as they are viruses within the RNA genome that are converted by enzyme in reverse transcriptase to DNA, which is then integrated into the host cells genome, making it a provirus
Three ways retroviruses contribute to cancer
Retrovirus integrates close to a cellular proto-oncogene thereby causing it to be expressed abnormally; retrovirus may incilde an oncogene which it brings to the cell it infects, called an acute transforming retrovirus; the retrovirus may have a normal viral gene that stimulates cell division or act as a gene-expression regulator for cancer-related cellular genes (no known acute transforming retroviruses in humans but some retrovrisues such as HIV and human T-cell leukemia virus)
Examples of human viruses that contribute to cancer
Some DNA viruses have genes that stimulate cells cycle; epstein-barr, hepatitis B and C, HPV
Carcinogen
Any substance or process that damages DNA has the potential to be carcinogenic if it causes mutations or epigenetic alterations (i.e. if it affects proto-oncogenes or tumor suppressor genes
Examples of natural carcinogens
Aflatoxin (in mold on corn and peanuts), natural pesticides and antibiotics, natural radiation such as UV light, and radon gas; alkylating agents in the gut, mistakes during DNA replication, byproducts of normal metabolism, chronic inflammation
Chemotherapy and radiotherapy
Target cells that are actively dividing are sensitive to these treatments because they have a compromised ability to repair; chemotherapy targets many different aspects of cell biology (antihormome drugs prevent cell division, mitosis, alkylating agents inhibit DNA replication and transcription, antimetabolites
Radiotherapy causes DNA damage that results in cell death but also damages healthy cells, though they can handle it better
Recombinant DNA technology
Manipulation of DNA to generate recombinant DNA (combination of two or more DNA molecules, usually from different biological sources and not found together in nature)
Restriction enzymes
Cuts DNA like a molecular pair of scissors
Cloning of DNA molecules
Example: put a human gene together with bacterial DNA (recombinant DNA) to make millions of clones (copies) of this recombinant DNA molecules
Recombinant DNA technology is used for:
Isolating and storing genes or pieces of DNA, analyzing genes or pieces of DNA, and commercial applications such as genetically modified foods and medicines
Basic steps of recombinant DNA technology
- Use cells to purify DNA that will be cloned
- Generate specific DNA fragments using restriction enzymes
- Join a DNA fragments with a cloning vector (aka vector), a DNA molecule that can carry the DNA fragments of interest, producing a recombinant DNA molecule
- Transfer the recombinant DNA molecule to a host cell (ex: bacterium)
- When the host cell divides, the recombinant DNA molecules also divides and is passed to the offspring, so it is easy to resource millions of recombinant DNA molecules in this fashion
- Recover (purify) the recombinant DNA from the bacteria
- Different possibilities: store DNA, analyze DNA, express DNA as protein and then investigate and sell it
Restriction enzyme
Cuts DNA like scissors by binding to a specific recognition sequence and cutting to produce cut DNA called restriction fragments
Palindromic
Most recognition sequences are palindromic and restriction enzymes during in offset manner; single stranded ends are called sticky ends and can anneal easily
Using restriction enzyme and DNA ligase to create recombinant DNA
Use the same restriction enzymes to cut two different pieces of DNA; DNA ligase then joins the restriction fragments to produce an intact recombinant DNA molecule
Cloning vector
It’s a DNA molecule that can carry cloned pieces of DNA; must be able to replicate DNA, have restriction enzyme recognition sequences that allow insertion of a DNA fragment, and have at least one selectable marker gene that enables us to identify host cells that have taken up the vector
Recombinant cloning vector
A cloning vector that carries a cloned piece of DNA
Plasmids
DNA molecule that replicates on its own in bacterial cells; not part of main chromosome, much smaller than the bacterial main chromosome, and circular
Multiple cloning site
Many cloning vectors contian a multiple cloning site, which is a genetically manipulated segment of DNA in the cloning vector that contains many recognition sequences for restriction enzymes, thus enabling us to choose which restriction enzyme we would like to use
Transformation
Process by which bacterium takes up a plasmid cloning vector; general method of transforming bacteria host cells with a plasmid into a host cell involves mixing millions of recombinant cloning vectors with millions of host cells, and a fraction of host cells will take up the recombinant cloning vector
Selectable marker gene
Encodes a protein that gives the host cell a distinct characteristic, enabling us to detect the host cells that have taken up the recombinant cloning vector (many cloning vectors have two)
Ex: gene that makes host cell resistant to antibiotic, expose all cells to antibiotics, only host cells with vector survive
Blue-white screening
Example of selectable marker genes; blue-white screening using LacZ, which becomes disrupted if vector takes up DNA fragment; if not taken up is not disrupted and produces protein that can cut the molecule X-gal, making it turn blue
If X-gal added, transformed cells that turn blue have non-recombinant cloning vector, while those that turn white have recombinant cloning vector
Limitation with bacterial plasmid cloning vectors
Can only take up relatively short DNA fragments (25,000 bp)
Solution: use other types of cloning vectors for big DNA fragments such as phage vectors (45,000), bacterial artificial chromosomes calledBACs (100,000 to 300,000), and yeast artificial chromosomes called YACs (100,000 to 1,000,000)
Expression vector
Cloning vector that is manipulated so that it can express a gene of interest to produce large quantities of encoded protein; has promoter so high levels of transcription
Bacterial cells as host cells
Easy and relatively cheap, but unable to do post-translational modifications
Solution: use eukaryotic host for eukaryotic genes that need post-translational modifications
Yeast cells as hosts
Widely used because they are eukaryotic, grown easily, can modify eukaryotic proteins, and are considered safe
Animal cells as hosts for cloning
Mammalian cells are excellent hosts for cloning mammalian genes because it’s a natural environment and allows for posttranslational modification
