DNA to Genes to Chromosomes (Part 3)

Question 1

When does X-inactivation (Lyonization) occur?

Answer 1

during the early stages of development of female embryos.

Question 2

How do Barr bodies form?

Answer 2

One of the X-chromosome is inactivated and condensed to form the Barr body in females

Question 3

What are some characteristics of X-inactivation?

Answer 3

• X-inactivation is random (Paternal X chromosome active in approximately 50% of cells and maternal X chromosome is active in approximately 50% of cells)
• X-inactivation is fixed: The same X chromosome is inactivated in all the descendants of the cell

Question 4

How is X-inactivation regulated? What is the gene involved?

Answer 4

• X-inactivation is regulated by a region called X-inactivation center (Xic) that has the gene, XIST gene involved in inactivation

Question 5

How does transcriptional interference occur in X-inactivation?

Answer 5

XIST RNA coat one of the X-chromosomes, that results in transcriptional interference

Question 6

What effect does methylation have upon genes?

Answer 6

Silences them

Question 7

Methylation of cytosine bases (CG islands) close to the promoter, result in what?

Answer 7

transcriptional repression

Question 8

How is imprinting similar to X-inactivation?

Answer 8

the imprinted gene is methylated and transcriptionally silenced.

Question 9

What are homologous chromosomes?

Answer 9

Members of a pair of chromosomes (called as homologous chromosomes) carry matching genetic information; they have the same genes in the same sequence.

Question 10

What are alleles?

Answer 10

• At any specific locus, however, they may have either identical (homozygous) or slightly different (heterozygotes) forms of the same gene
• Alleles are different versions of a gene, have slight differences in the base sequence and amino acid sequence of the protein synthesized

Question 11

If the homologous chromosomes have the same allele of a gene, then the person is ____ for that allele

Answer 11

homozygous

Question 12

If the homologous chromosomes have different allelic forms of a gene, then the person is ______ for that allele

Answer 12

heterozygous

Question 13

What does the locus of a gene refer to?

Answer 13

Its location on a chromosome.

Question 14

Where is the Huntington’s gene locus?

Answer 14

Chromosome 4p

Question 15

Where is the beta globin gene cluster found?

Answer 15

Chromosome 11

Question 16

Where is the alpha globin gene found?

Answer 16

Chromosome 16

Question 17

Regarding MIM numbers, autosomal dominant disorders begin with what? What is an example?

Answer 17

• 1

- e.g. Marfan syndrome (MIM 154700)

Question 18

Regarding MIM number, autosomal dominant disorders begin with what? What’s an example?

Answer 18

• 2

- e.g. Cystic fibrosis (MIM 219700)

Question 19

Regarding MIM number, X-linked disorders begin with what? What’s an example?

Answer 19

• 3

- e.g.Duchenne muscular dystrophy (MIM 310200)

Question 20

Regarding MIM numbers, mitochondrial disorders begin with what? What is an example?

Answer 20

• 5

- Leber hereditary optic neuropathy (MIM 535000)