Question

use the following diagram to answer question #22

1. when replication is finished, strand 3 and 4 will have the same base composition

as
a. strands 1 and 2.
b. strands 3 and 4.
c. strands 2 and 3.
d. strands 2 and 4.

1. transcription of part of a dna molecule with a nucleotide sequence of

aaacaactt results in a mrna molecule with the complementary sequence of
a. uuuagaaccb. uuugugaacc. tttgaagcc. d. cccacctcc.

1. if one strand of dna has the base sequence aag caa, the complementary strand

has which of the following sequences?
a. uucguub. aagcaac. utcgtud. ttcgtt

1. the genetic code consists of _ bases that stand for one amino acid.

a. 1b. 2c. 3d. 4

Explanation:

22

DNA replication follows the semi - conservative model. Parental strands (1 and 2) separate, and each serves as a template for new strands (3 and 4). Strand 3 is complementary to strand 1, and strand 4 is complementary to strand 2. But since strand 1 and strand 2 are complementary to each other (in a double - stranded DNA), strand 3 (complementary to 1) will have the same base composition as strand 2 (because 1 and 2 are complementary), and strand 4 (complementary to 2) will have the same base composition as strand 1. So, strands 3 and 4 will have the same base composition as strands 1 and 2.

23

In transcription, DNA (A - T, G - C) is transcribed to mRNA (U - A, C - G). For the DNA sequence AAACAAACTT:

• A in DNA pairs with U in mRNA, T pairs with A, C pairs with G.
• The DNA sequence AAACAAACTT:
• First three A's: UUU
• C: G (but wait, no, in mRNA transcription, for DNA base A, mRNA is U; for DNA base T, mRNA is A; for DNA base C, mRNA is G. The DNA sequence AAACAAACTT. Let's break it down: AAA (DNA) → UUU (mRNA), C (DNA) → G (no, wrong, wait DNA is AAACAAACTT. Let's do it correctly. DNA: A - A - A - C - A - A - A - C - T - T. mRNA: U - U - U - G - U - U - U - G - A - A. But wait, maybe a better way. The base - pairing rules: DNA to mRNA: A→U, T→A, C→G. The DNA sequence is AAACAAACTT. So, AAA (DNA) → UUU (mRNA), C (DNA) → G (no, wait DNA is AAACAAACTT. Let's write the complementary mRNA base by base:

DNA base	mRNA base
A	U
A	U
C	G
A	U
A	U
A	U
C	G
T	A
T	A	. So the mRNA sequence is UUUGUUUGAA. But looking at the options, maybe there is a typo. Wait, another approach: The general formula. DNA: 5' - AAACAAACTT - 3', mRNA is synthesized in 5' - 3' direction. The complementary mRNA sequence (using base - pairing rules: A - U, T - A, C - G) is 5' - UUUGUUUGAA - 3'. But looking at the options, option B: UUUGUUGAA. Maybe a formatting error in the problem. Assuming correct base - pairing, for DNA AAACAAACTT (assuming it's 5' - AAACAAACTT - 3'), mRNA is 3' - UUUGUUUGAA - 5' (but usually written 5' - UUUGUUUGAA - 3'). If we consider the triplet codons (though not necessary for this question), but among the options, option B (UUUGUUGAA) is the closest as per base - pairing (A→U, T→A, C→G).

24

In DNA, A pairs with T, and G pairs with C. For the DNA strand AAGCAA (assuming 5' - AAGCAA - 3'):

• A (first) pairs with T
• A (second) pairs with T
• G pairs with C
• C pairs with G
• A (fifth) pairs with T
• A (sixth) pairs with T. The complementary strand is TTCGTT (5' - TTCGTT - 3').

25

The genetic code is a triplet code. Three bases (a codon) code for one amino acid. For example, AUG (methionine), UUU (phenylalanine) etc.

Answer:

1. A. strands 1 and 2
2. B. UUUGUUGAA
3. D. TTCGTT
4. C. 3