Question

The antisense (noncoding) strand of DNA is 5' ATG GAT AAA GIT TTA AAC AGA GAG GAA TCT 3 ' What is the a) sense strand? b) mRNA transcribed? c) polypeptide chain that is translated?

Short answer

a) The sense strand of DNA is 3' TAC CTA TTT CTA AAT TTG TCT CTC CTT AGA 5' b) The mRNA transcribed is 5' AUG GAU AAA GAU UUA AAC AGA GAG GAA UCU 3' c) The polypeptide chain is Met - Asp - Lys - Asp - Leu - Asn - Arg - Glu - Glu - Ser

Step-by-step solution

Obtain the antisense strand of DNA

We are given the antisense strand of DNA: 5' ATG GAT AAA GAT TTA AAC AGA GAG GAA TCT 3' Step 2: Find the sense strand of DNA

Determine the sense strand of DNA

To find the sense strand of DNA, we follow the complementary base-pairing rule: A pairs with T, and C pairs with G. So, the sense strand will be: 3' TAC CTA TTT CTA AAT TTG TCT CTC CTT AGA 5' Answer (a): The sense strand of DNA is 3' TAC CTA TTT CTA AAT TTG TCT CTC CTT AGA 5' Step 3: Transcribe the mRNA from the sense strand of DNA

Transcribe the mRNA

During transcription, the sense strand of DNA is used as a template to synthesize the mRNA molecule. Here, adenine (A) in the DNA pairs with uracil (U) in the mRNA, whereas all other bases pair with their complementary counterparts. So, the mRNA sequence transcribed from the sense strand is: 5' AUG GAU AAA GAU UUA AAC AGA GAG GAA UCU 3' Answer (b): The mRNA transcribed is 5' AUG GAU AAA GAU UUA AAC AGA GAG GAA UCU 3' Step 4: Translate the mRNA into a polypeptide chain

Translate the mRNA to a polypeptide chain

Each set of 3 nucleotides (codon) in the mRNA codes for a specific amino acid. Using the genetic code table, we can determine the amino acids that correspond to each codon in the mRNA sequence: AUG - Methionine (Met) GAU - Aspartic Acid (Asp) AAA - Lysine (Lys) GAU - Aspartic Acid (Asp) UUA - Leucine (Leu) AAC - Asparagine (Asn) AGA - Arginine (Arg) GAG - Glutamic Acid (Glu) GAA - Glutamic Acid (Glu) UCU - Serine (Ser) Combining the amino acids, we get the polypeptide chain: Met - Asp - Lys - Asp - Leu - Asn - Arg - Glu - Glu - Ser Answer (c): The polypeptide chain is Met - Asp - Lys - Asp - Leu - Asn - Arg - Glu - Glu - Ser

Key concepts

Antisense Strand

In DNA, the antisense strand is one of the two strands that make up the double helix. It is also known as the template or non-coding strand. This strand provides the template for the production of messenger RNA (mRNA) during the process of transcription.
The antisense strand is referred to as noncoding because its sequence is complementary to the mRNA. Remember, DNA is made up of nucleotide base pairs. In complementary base pairing, adenine (A) pairs with thymine (T) and cytosine (C) pairs with guanine (G).

• The example given in the exercise starts with an antisense strand: 5' ATG GAT AAA GAT TTA AAC AGA GAG GAA TCT 3'.
• This antisense strand will serve as a template for creating the sense strand and, eventually, mRNA.

Understanding the antisense strand is crucial as it carries the information necessary for translating genetic instructions contained within DNA into proteins.

Sense Strand

The sense strand, also known as the coding strand, complements the antisense strand. It carries the genetic code directly and matches the sequence of the mRNA (with the exception that thymine in DNA is replaced by uracil in RNA).
In the given exercise, the sense strand can be determined by applying the complementary base-pairing rules to the antisense strand. This means pairing adenine with thymine, and cytosine with guanine. The opposite is done if you start from the 5' end of the sense strand.

• Knowing the example: the sense strand derived from the antisense strand will be 3' TAC CTA TTT CTA AAT TTG TCT CTC CTT AGA 5'.
• This strand resembles the mRNA sequence, making it critical for gene expression because it effectively "codes" for the protein.

Thus, understanding the role of the sense strand helps clarify how genetic information translates into functional proteins.

mRNA Transcription

mRNA transcription is the process of creating a messenger RNA (mRNA) molecule from a DNA template. This involves copying the genetic information held in the DNA into mRNA. This is a crucial step in the central dogma of molecular biology: DNA → RNA → Protein.
During transcription, the enzyme RNA polymerase reads the antisense strand of the DNA to synthesize the mRNA. As the name suggests, messenger RNA acts as a messenger conveying genetic information from DNA to the ribosome, where proteins are synthesized.

• Applying transcription rules to the exercise example, the mRNA sequence transcribed will be 5' AUG GAU AAA GAU UUA AAC AGA GAG GAA UCU 3'.
• This transcribed mRNA corresponds to the coding information originally present within the DNA but in RNA language.

Understanding mRNA transcription is vital as it bridges the gap between DNA and protein creation, enabling cells to produce proteins essential for life processes.

Polypeptide Chain

The polypeptide chain is a sequence of amino acids linked together by peptide bonds, forming a protein. Proteins perform a myriad of functions within biological organisms and are critical for various structural, functional, and regulatory roles.
The process of translating an mRNA sequence into a polypeptide involves tRNA molecules bringing amino acids to the ribosome, where they are added to the growing chain. Each 3-nucleotide triplet, known as a codon, on the mRNA corresponds to a specific amino acid in the genetic code.

• For the mRNA given, translating its sequence results in the polypeptide: Met - Asp - Lys - Asp - Leu - Asn - Arg - Glu - Glu - Ser.
• This translation from nucleic acid language to amino acid language is what creates functional proteins.

Grasping the structure and function of polypeptide chains helps in understanding how genetic information is expressed as cellular functions.

Genetic Code

The genetic code refers to the set of rules used by living cells to translate information encoded within genetic material (DNA or mRNA sequences) into proteins. It defines how codons specify which amino acids will be added during protein synthesis.
This code is universal and redundant:

• There are 64 codons, and most amino acids are encoded by more than one codon.
• AUG, for example, is the start codon coding for Methionine, signaling the start of protein synthesis.

In our exercise example, each codon from the mRNA is matched to an amino acid using the genetic code:
• AUG starts the sequence with Methionine (Met).
• Other codons like GAU correspond to Aspartic Acid (Asp), ensuring the proper sequence for the resulting protein.
The genetic code is crucial for interpreting the information within genetic sequences and for understanding the mechanisms of heredity and variation.