6.1 What Is Cancer?

    Tumor: Unregulated cell division that form a mass of cells with no function

    Benign tumor: doesnt affect surrounding tissues

    Malignant tumor: invades surrounding tissues; cancerous

    Metastasis: cells break away from a malignant tumor and start a new cancer at another location

 

    Metastatic cells can travel throughout the body via the circulatory system or the lymphatic system.

   Once in either system the cancer cells can travel anywhere in the body

    Cancer cells differ from normal cells:

   Divide when they shouldnt

   Invade surrounding tissues

   Move to other locations in the body

 

    Risk factors: increase a persons risk of developing a disease (Table 6.1)

   Tobacco use: tobacco contains many carcinogens

   Alcohol consumption

   High-fat, low-fiber diet

   Lack of exercise

   Obesity

   Increasing age which weakens the immune system

   Cells that divide frequently such as ovarian cells

 

6.2 Passing Genes and Chromosomes to Daughter Cells

    Asexual reproduction:

   Only one parent

   Offspring are genetically identical to parent

    Sexual reproduction

   Gametes are combined from two parents

   Offspring are genetically different from one another and from the parents

 

    Before dividing, cells must copy their DNA

    Gene: section of DNA that has the instructions for making all proteins

    One molecule of DNA is wrapped around proteins to form a chromosome containing hundreds of genes.

    Different species have different numbers of chromosomes (we have 46).

 

    Chromosomes are uncondensed before cell division

    Duplicated chromosomes, held together at the centromere, are called sister chromatids

    They are duplicated through DNA replication

 

    DNA molecule is a double stranded structure similar to a twisted ladder.

    The sides of the ladder are composed of a sugar-phosphate backbone.

    Nucleotides are connected to each other by hydrogen bonding to form the rungs of the ladder.

   Adenine (A) pairs with thymine (T)

   Cytosine (C) pairs with Guanine (G)

 

    DNA molecule is split up the middle of the helix

    Nucleotides are added to each side via hydrogen bonding

    Result is two identical daughter molecules, each with one parental strand and one new strand (semiconservative replication)

 

    DNA polymerase: the enzyme that replicates DNA

    It moves along
the length of the unwound DNA
and helps form
the new strands

 

6.3 The Cell Cycle and Mitosis

    Cell cycle has three steps:

   Interphase: the DNA replicates

   Mitosis: the copied chromosomes are moved into daughter nuclei

  Mitosis occurs in somatic or body cells.

   Cytokinesis: the cell is split into 2 daughter cells

 

6.3 The Cell Cycle and Mitosis - Interphase

    Interphase has three phases:

   G1: cell grows, organelles duplicate

   S: DNA replicates

   G2: cell makes proteins needed to complete
mitosis

    Most of the cell cycle is spent in interphase

 

6.3 The Cell Cycle and Mitosis - Mitosis

    Mitosis produces genetically-identical daughter nuclei

    Mitosis is followed by cytokinesis which splits the two nuclei into two daughter cells

    Four stages:

   Prophase

   Metaphase

   Anaphase

   Telophase

 

6.3 The Cell Cycle and Mitosis - Mitosis

    Prophase:

   Chromosomes condense

   Nuclear envelope disappears

   Microtubules pull the chromosomes around during cell division

  Animal cells: microtubules attached to centrioles at the poles of the cell

 

6.3 The Cell Cycle and Mitosis - Mitosis

    Metaphase:

   Chromosomes are aligned across the middle of the cell by microtubules

    Anaphase:

   centromeres split,

   sister chromatids are pulled apart toward opposite poles

    Telophase:

   Nuclear envelopes reform around chromosomes

   Chromosomes revert to uncondensed form

 

6.3 The Cell Cycle and Mitosis - Cytokinesis

    Cytokinesis is the stage in which two daughter cells are formed from the original one

    After cytokinesis, cells reenter interphase.

    Animals:

   Proteins pinch the original cell into two new cells

    Cytokinesis in Plants:

   Starts with vesicles forming the cell plate.

   This results in a new cell wall being formed between the cells forming daughter cells.

  The cell wall is made from cellulose

 

6.4 Cell Cycle Control and Mutation

    Cell division is a tightly controlled process

    Normal cells halt at checkpoints

    Proteins survey the condition of the cell

    Cell must pass the survey to proceed with cell division

    3 checkpoints: G1,G2, and metaphase

    Growth factors stimulate cells to divide

    Growth factors bind to receptors to trigger a response from a cell

    Mutation: a change in the sequence of DNA

   Changes to DNA can change the structure and function of the protein coded by the DNA

   Mutations may be inherited or caused by carcinogens

 

6.4 Cell Cycle Control and Mutation

    Proto-oncogenes: genes that code for the cell cycle control proteins

    When proto-oncogenes mutate, they become oncogenes

   Their proteins no longer properly regulate cell division

   They usually overstimulate cell division

 

    Tumor suppressor genes: genes for proteins that stop cell division if conditions are not favorable

   When mutated, can allow cells to override checkpoints

 

    Depending on the number of mutations and whether the tumor suppressor protein is functional will determine whether it is a benign or malignant tumor that is formed.

 

6.4 Cancer Development Requires Many Mutations

    Progression from benign tumor to cancer requires many mutations.

    Angiogenesis: tumor gets its own blood supply

    Loss of contact inhibition: cells will now pile up on each other

    Loss of anchorage dependence: enables a cancer cell to move to another location

    Immortalized: cells no longer have a fixed number of cell divisions due to an enzyme called telomerase

 

6.4 Cell Cycle Control and Mutation – Multiple Hit Model

    Multiple hit model: process of cancer development requires multiple mutations

    Some mutations may be inherited (familial risk)

    Most are probably acquired during a persons lifetime

 

6.5 Cancer Detection and Treatment

    Early detection increases odds of survival

    There are different detection methods for different cancers

    Some cancers produce increased amount of a characteristic protein

    Biopsy: surgical removal of cells or fluid for analysis

   Needle biopsy: removal is made using a needle

   Laparascope: surgical instrument with a light, camera, and small scalpel

 

6.5 Cancer Detection and Treatment - Treatment Methods

    Chemotherapy: drugs that selectively kill dividing cells

   Combination of different drugs used (cocktail)

   Interrupt cell division in different ways

   Helps prevent resistance to the drugs from arising

   Normal dividing cells are also killed (hair follicles, bone marrow, stomach lining)

 

    Radiation therapy: use of high-energy particles to destroy cancer cells

   Damages their DNA so they cant continue to divide or grow

   Usually used on cancers close to the surface

   Typically performed after surgical removal of tumor

    If a person remains cancer free after treatment for 5 years they are in remission and after 10 years they are cured.

 

6.6 Meiosis

    Specialized form of cell division in gonads to produce gametes

    Reduces number of chromosomes in each
cell by one-half

   Chromosomes come
in homologous pairs

   Gamete gets one of
each pair

 

    Chromosomes can be visualized using a karyotype.

   Human somatic cells have 22 pairs of autosomes and one pair of sex chromosomes

 

    Gametes are haploid or have 1 set of 23 chromosomes

    When the egg and sperm nuclei fuse it forms a zygote which is diploid

    Somatic or body cells are also diploid

 

    Starts with interphase – DNA is duplicated

    Meiosis takes place in two stages:

    Meiosis I

   Separating out the homologous pairs into 2 separate cells

    Meiosis II

   Separating out the sister chromatids in each cell to produce 4 haploid cells.

 

    Crossing over: exchange of equivalent portions of chromosomes between members of a homologous pair

   Results in new types of gametes being formed

   Linked genes typically cross over together

    Random alignment: the way in which different pairs of chromosomes align and get separated during meiosis I is random

   Results in different types of games being formed

 

6.6 Meiosis - Mistakes in Meiosis

    Nondisjunction: failure of homologues to separate normally during meiosis

    Results in a gamete having one too many chromosomes (trisomy) or one too few chromosomes (monosomy)

    Most embryos that result from such gametes will die before birth

    Several chromosome abnormalities are known in humans (Table 6.2)

 

    For cancer mutations to be passed on to offspring, they must take place in cells that give rise to gametes.

    Mutations caused by environmental exposures are not passed on unless the mutation occurs in the gametes.

   Mutations in somatic cells (e.g., skin cancer from UV ray exposure) are not heritable.