Question

1. the image to the right represents a growing polypeptide chain. what does directionality refer to in terms of polypeptides? label the image to the right accordingly.

use the chart below to help answer questions 9 - 12
polar amino acids: serine, threonine, cysteine, asparagine, glutamine, tyrosine, lysine, arginine, histidine, aspartate, glutamate
nonpolar amino acids: glycine, alanine, valine, leucine, methionine, isoleucine, phenylalanine, tyrosine, tryptophan

1. if an amino - acid substitution occurred in a polypeptide chain, predict which option would have the most significant impact on the structure and function of the final protein? why? a. serine is replaced by glutamate b. alanine is replaced by lysine c. glycine is replaced by valine

sickle cell anemia is an inherited disorder that affects the shape of red blood cells (see figures 1 and 2). individuals who have sickle cell anemia have blood that is chronically low in oxygen, and they suffer from frequent infections and chronic pain. sickle cell anemia is caused by a single nucleotide substitution in the hemoglobin - beta gene found on chromosome 11. this mutation causes one adenine nucleotide on the hemoglobin - beta gene to be switched to the nucleotide thymine. this, in turn, causes a change in a single amino acid in the resulting protein that helps form red blood cells. normal red blood cells should have glutamic acid (glutamate) as the sixth amino acid position in their polypeptide chain, but the mutation switches this glutamic acid to valine. use this information and the chart above to answer questions 10 - 12.

1. what codes for proteins?
2. in the case of sickle cell anemia, what happens to the gene? to the protein?
3. propose an explanation for how a single amino - acid substitution could cause such a dramatic difference in the final shape of red blood cells.

Explanation:

1. Alanine is a non - polar amino acid and lysine is a polar amino acid. A change from non - polar to polar can significantly affect protein structure and function as it can change the protein's solubility, its interactions with other molecules, and its overall folding pattern. In contrast, in option a, both serine and glutamate are polar, and in option c, both glycine and valine are non - polar, so the impact is less significant.
2. DNA (deoxyribonucleic acid) codes for proteins. DNA contains the genetic information which is transcribed into mRNA (messenger ribonucleic acid) and then translated into proteins.
3. In sickle cell anemia, a single nucleotide substitution occurs in the hemoglobin - Beta gene on chromosome 11 (an adenine is switched to thymine). This causes a change in a single amino acid in the resulting protein, where glutamic acid at the sixth amino acid position in the polypeptide chain of normal hemoglobin is replaced by valine.
4. A single amino acid substitution can cause a dramatic difference in the final shape of red blood cells because the amino acid sequence determines the protein's three - dimensional structure. The change from a polar (glutamic acid) to a non - polar (valine) amino acid in hemoglobin leads to abnormal protein - protein interactions. The non - polar valine can cause the hemoglobin molecules to aggregate together in low oxygen conditions, forming long, rigid fibers that distort the red blood cells into a sickle shape.

Answer:

1. b. Alanine is replaced by lysine
2. DNA
3. Gene: Single nucleotide substitution (adenine to thymine). Protein: Glutamic acid at sixth position replaced by valine.
4. Amino acid change affects protein - protein interactions and folding, causing aggregation and shape change of red blood cells.