6.9.4 Eukaryotic Genomic Organization: Transposons and Amplified Genes

Question 1

gene families

Answer 1

• are thought to have arisen from the duplication of a parent gene

Question 2

Pseudogenes

Answer 2

• are copies of existing genes that lack regulatory sequences like promoters and are often found in gene families.

Question 3

Gene amplification

Answer 3

• takes place when many copies of a gene are synthesized in order to increase production of the gene
  product. Examples of gene amplification are found in salamander eggs and cancer cells
• Researchers have detected gene amplification in fertilized salamander eggs. Before fertilization, millions of copies of the rRNA genes are synthesized. These amplified genes exist as tiny DNA circles separate from the chromosome. After fertilization, the copies become active and act as a template for rRNA transcription. The newly developing embryo needs a multitude of rRNA in order to synthesize the vast amount of protein products it will require.
• Gene amplification may also occur in cancer cells of cancer patients who have undergone chemotherapy. The drugs kill many cells, but some are resistant. These cells have been found to contain amplified sequences of the genes responsible for drug resistance.

Question 4

Transposons

Answer 4

• are segments of DNA that are capable of moving within a genome
• Transposons make up over 50 percent of the corn genome. A general transposon contains a DNA sequence coding for the enzyme transposase that is flanked by inverted repeats. Transposase functions by making staggered cuts within a strand of DNA and allowing a copy of the entire transposon sequence to be inserted into the cut site.

Question 5

Retrotransposons

Answer 5

• produce reverse transcriptase, which can make a new copy of DNA from RNA and insert it into the genome.
• are transposons that move by means of
  an RNA intermediate that is formed from a transcript of the retrotransposon DNA. In order to be inserted into the genome, the RNA intermediate must be converted back to DNA. The enzyme reverse transcriptase, which is coded for in the retrotransposon, accomplishes this conversion. Another enzyme that is coded for in the retrotranspo son will insert the resulting DNA into the new site.

Question 6

Which of the following regarding transposons is not true?

Answer 6

• Transposons cannot be transcribed and translated to form a protein product

Question 7

Identify the retrotransposon and the reverse transcriptase in this model of retrotransposon movement.

Answer 7

• 1, 3

Question 8

Pseudogenes

Answer 8

• have sequences that are very similar to functional genes.
• lack necessary regulatory sequences.
• probably evolved from the same ancestor as the functional genes.
• all of the above

Question 9

Sequences of a gene that can move from one location to another are

Answer 9

• transposons

Question 10

Which of the following is not true about gene amplification?

Answer 10

• It may result in the loss of certain genes