⭐⭐⭐ MUTATION AND CANCER ⭐⭐⭐

⭐⭐⭐ MUTATION  AND CANCER ⭐⭐⭐

1) DEFINITION OF MUTATION :-

Mutation is a change in a DNA sequence.

2) FUNCTION OF DNA:-

  (A) Basic unit of DNA is a nucleotide.

Nucleotide consists of a sugar , a phosphate group and a base. 

  (B) Adenine, thymine, cytosine and guanine are the four nitrogenous bases found in DNA.

  (C) Thymine pairs with Adenine and cytosine pairs with guanine. Thus, the bonds between nitrogenous bases form the steps of ladder of DNA.

  (D) The pattern of arrangement of these nucleotides in DNA differs from person to person. Hence, each individual has unique DNA.

  (E) The sequences of nucleotides in DNA code for different proteins which form our body structures.

  (F)  Codon is a sequence of three nucleotides that corresponds with a specific amino acid or stop signal during protein synthesis.

  (G) Protein synthesis involves process of transcription i.e process by which the information in a strand of DNA is copied into messenger RNA (mRNA) and then translation i.e process of translating the sequence of a messenger RNA (mRNA) into a sequence of amino acids during protein synthesis. In this way , protein is formed.

3) WHAT WILL HAPPEN IF MUTATION OCCURS ?

  Due to change in structure of DNA during mutation, there can be changes in the structure of an encoded protein resulting in production of faulty protein or there may be decrease or complete loss in its expression.

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4)TYPES OF MUTATION:-

  (A)  Hereditary or germline mutation :-

     (I) This type of mutation is inherited from parents.

     (II) It is present in every cell of body.

     (III) Mutation is present in the germ cell i.e egg or sperm of parent. This is further transmitted to child after fertilization of egg and sperm.

     (IV) Egg and sperm are haploid. The zygote formed after fertilization is diploid and contains all of the genetic information (DNA) needed to become a baby. Half the DNA comes from the mother's egg and half from the father's sperm. Thus, mutation is transferred to child and every cell is affected.

  (B) Acquired or somatic mutations:-

     (I) It is not inherited but occurs at some time in a person's life.

     (II) These changes may be caused due to certain  factors like

 # radiation, 

 # UV rays, 

 # exposure to carcinogenic substances,   # cigarette smoke, 

 # certain dietary substances, etc 

        It can also occur if there is any error in the formation of DNA copies during cell division. In order for a cell to divide to make 2 new cells, it has to copy all of its DNA.

     (III) This type of mutation is present only in certain cells of body and not in each and every cell.

     (IV) Somatic mutations occur in non-reproductive cells and thus are not  passed into the offspring.

     (V) Acquired mutations are the most common cause of cancer.

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5) ROLE OF MUTATION IN CAUSATION OF CANCER :-

  (A) HEREDITARY MUTATION :-

     (I) About 5% to 10% of all cancers result directly from mutations inherited from a parent.    

     (II) Even if the hereditary mutation is present in the cells of whole body, hereditary cancers arise in a specific organ/ tissues only. For eg breast cancer.

Why this is so?

     (III) Reason no 1

        - Even if there are mutations in all cells ( eg. BRCA1 and BRCA2 ),

cancer is not a monogenic disorder, rather is a sum of different defects and environmental factors that contribute to its onset and progression.

       - For eg. mutations in BRCA1 and BRCA2 genes are associated to increased cancer risks in breast, ovary.

       - BRCA1 and BRCA2  have a important role in DNA repair. Mutations in those genes cause faulty DNA which may further result in accumulation of other mutations that may contribute to cancer development.

      - Why only Breast and Ovarian tissues are affected ? 

BRCA1 and E-ER (estrogen and estrogen receptor) signaling synergistically regulate the mammary epithelial cell proliferation and differentiation. Thus, as the hormone estrogen and it's function is associated with breast and ovary, mutation in BRCA affects these tissues.

       - This proves that cancer is not a monogenic disorder, rather is a sum of different defects that contribute to its onset and progression. 

      - Hence, even if same mutation is present in whole the body, only some tissues will have a permissive environment for the cancer to develop and grow.

      (IV) Reason no 2

         - Genetic material is the same in all the cells present in the whole body, but cells in different tissues possess unique structure and functions.

(Eg. Neuron and hepatocyte differs in structure as well as function) 

        - This is because the expressed genes are different in different types of cells. 

Different cells may use different genes.  Genes that the particular cell doesn't need are turned off and not used.

       - Thus, mutated gene may be turned on gene for some organ and turned off gene for other organ. This may lead to cancer in those specific tissues where the mutated gene is being constantly expressed (turned on gene ).

  

     (V) Reason no 3

       - We have 2 copies of genes – one from each parent. 

       - When someone inherits an abnormal copy of a gene, often there is no problem, because the other gene copy is still working. 

      - But if the other gene copy stops working because of an acquired mutation due to some environmental factor ( for eg, cigarette smoke acts as carcinogen affecting lungs ) , the gene’s function can be lost altogether. 

     - When the gene that stops working is a tumor suppressor gene, cells can grow out of control, which can lead to cancer.

  (B) ACQUIRED MUTATION :- 

     (I) Acquired mutations can be caused by things that we are exposed to in our environment, including cigarette smoke, radiation, hormones, diet , Tobacco, Ultraviolet (UV) radiation , Viruses ,etc.

These things can cause DNA damage and mutation. 

     (II) Acquired mutations are not found in every cell in the body. For eg - tobacco chewing can cause oral cancer or smoking can cause lung cancer.

The carcinogens affect those vulnerable tissues which they encounter.

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