Hemoglobin is a critical oxygen-carrying protein. The protein functions as a tetramer with two α-globin subunits and two β-globin subunits. Individuals homozygous for a specific single point mutation in the region that encodes the sixth amino acid in β-globin develop sickle-cell anemia, a disease that causes red blood cells to form a sickle shape. The sickled shape leads to problems with blood flow due to the accumulation of the misshaped hemoglobin. The DNA sequence encoding the fifth, sixth, and seventh amino acids of the normal and mutant β-globin is given.

Normal DNA sequence: 5′-CCTGAGGAG-3′

Mutant DNA sequence: 5′-CCTGTGGAG-3′

A. The point mutation causes an amino acid replacement of (1)___________ by (2)___________ for the sixth amino acid of β-globin.

1)

2)

GLUTAMIC ACID

VALINE

DNA

1) Glutamic acid

2) Valine

1) The point mutation causes an amino acid replacement of glutamic acid by valine for the sixth amino acid of β-globin.

To understand why this mutation causes the amino acid replacement, we need to look at the DNA coding sequence and the corresponding amino acids. Each successive three nucleotides in the DNA sequence (known as a codon) codes for a specific amino acid in the protein.

The normal DNA sequence given is 5′-CCTGAGGAG-3′. Let's break it down into codons:
Codon 1: CCT
Codon 2: GAG
Codon 3: GAG

Each codon codes for a specific amino acid. Using the genetic code, we can determine the amino acids encoded by each codon:
Codon 1: CCT codes for proline (Pro)
Codon 2: GAG codes for glutamic acid (Glu)
Codon 3: GAG codes for glutamic acid (Glu)

Now let's look at the mutant DNA sequence: 5′-CCTGTGGAG-3′. Similarly, let's break it down into codons:
Codon 1: CCT
Codon 2: GTG
Codon 3: GAG

Using the genetic code, we can determine the amino acids encoded by each codon:
Codon 1: CCT codes for proline (Pro)
Codon 2: GTG codes for valine (Val)
Codon 3: GAG codes for glutamic acid (Glu)

Comparing the amino acids encoded by the sixth codon in the normal and mutant sequences, we can see that there is a substitution of glutamic acid (Glu) by valine (Val) caused by the point mutation.