8-1 DNA and the structure of proteins

Question 1

What are mutations?

Answer 1

• Mutations are changes in the sequence of nucleotides in DNA molecules.

Question 2

What are the types of mutations?

Answer 2

• Insertion/Deletion mutations
  o Where one or more nucleotide pairs are inserted or deleted from the sequence.
  o This type of mutation alters the sequence of nucleotides after the insertion/deletion point known as a frameshift.
• Duplication
  o One or more bases are repeated and therefore produces a frameshift.
• Inversion
  o A group of bases become separated from the DNA sequence and then re-join at the same position but in the reverse order.
  o This therefore affects the amino acid that is produced.
• Translocation
  o A group of bases become separated from the DNA sequence on one chromosome and are inserted into the DNA sequence on another chromosome.
  o This can often lead to significant effects on the phenotype.

Question 3

What are the causes of mutations?

Answer 3

• Chemical mutagens
• Ionising radiation
• Spontaneous errors in DNA replication

Question 4

What are stem cells?

Answer 4

• Stem cells are undifferentiated cells which can keep dividing giving rise to other cell types.

Question 5

What are the types of stem cells?

Answer 5

• Pluripotent cells
  o Able to give rise to many types of specialised cells apart from embryonic cells.
• Totipotent cells
  o Give rise to all types of specialised cells including embryonic cells.
  o Found in the embryo
  o Sometimes called embryonic stem cells.
• Multipotent
  o Can differentiate to multiple cell types but more limited.
  o Cells in the bone marrow and umbilical cord.
• Unipotent
  o Only into one type of cell.

Question 6

What are induced pluripotent stem cells?

Answer 6

• Pluripotent stem cells created from unipotent stem cells.

Question 7

What role does oestrogen play in transcription?

Answer 7

• The hormone oestrogen can alter transcription through altering molecules called transcription factors.

Question 8

What are transcription factors?

Answer 8

• These are molecules that bind to a specific site on DNA to begin the process of transcription.

Question 9

Describe the action of oestrogen in controlling transcription?

Answer 9

1. The lipid soluble nature of oestrogen means that it can freely diffuse across the cell membrane where it binds to a receptor molecule on a transcription factor.
2. The binding alters the shape of the DNA binding site on the transcription factor and makes it able to bind to the DNA.
3. The transcription factor therefore enters the nucleus via the nuclear pore where it binds to DNA, this stimulates the transcription of the gene that makes up the DNA.

Question 10

What is small interfering RNA?

Answer 10

• siRNA is used for short-term switching off of genes.

Question 11

Describe how siRNA prevents translation?

Answer 11

• siRNA binds to a complementary sequence of mRNA.
• As mRNA is usually single stranded the cell detects the double stranded form on mRNA and views it as abnormal.
• Therefore, the mRNA is broken down by enzymes
• Thus, preventing translation.

Question 12

What is Epigenetics?

Answer 12

• Epigenetics involves heritable changes in gene function, without changes to the base sequences of DNA.
• It shows that environmental factors can make changes to the function of genes which can be inherited.

Question 13

What is DNA methylation?

Answer 13

• DNA methylation is a process by which methyl groups are added to DNA.
• Methylation modifies the function of the DNA, typically acting to suppress gene transcription.
• DNA methylation alters the expression of genes in cells as they divide and become specialised.
• The change is permanent and prevents the cell from converting back into a stem cell or a different cell type.
• The methylation is through the addition of a CH3 chemical group to cytosine bases, which both prevents binding of transcriptional factors to DNA and stimulates decreased acetylation of histones.

Question 14

What is DNA acetylation?

Answer 14

• DNA acetylation also changes DNA structure.
• Histones are positively charged proteins closely associated with DNA, which is negatively charged.
• Decreased acetylation of histones increases their positive charge, so they bind to DNA more tightly.
• When this happens, transcriptional factors can no longer access the DNA, so the gene is switched off.