DNA and Protein Synthesis

Grade 9 · Science · Study Guide

Explore how DNA encodes genetic information and how cells use that information to build proteins through transcription and translation.

DNA, or deoxyribonucleic acid, is the molecule that carries genetic instructions for the development, functioning, growth, and reproduction of all known organisms. It is organized into structures called chromosomes found in the nucleus of eukaryotic cells.

The structure of DNA is a double helix — two complementary strands wound around each other. Each strand is made of nucleotides, which consist of a sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C). Base pairing rules are strict: A pairs with T, and G pairs with C.

Protein synthesis occurs in two main stages. During transcription, the enzyme RNA polymerase reads a DNA template strand and synthesizes a complementary strand of messenger RNA (mRNA). In mRNA, uracil (U) replaces thymine. The mRNA then travels from the nucleus to the ribosomes in the cytoplasm.

During translation, ribosomes read the mRNA sequence in groups of three bases called codons. Each codon specifies a particular amino acid. Transfer RNA (tRNA) molecules carry amino acids to the ribosome, matching their anticodon to the mRNA codon. The ribosome links amino acids together to form a polypeptide chain, which folds into a functional protein.

Key terms

DNA (Deoxyribonucleic Acid)
The double-stranded molecule that carries genetic information using four nitrogenous bases (A, T, G, C).
Transcription
The process by which RNA polymerase copies a DNA template strand into a complementary mRNA strand.
Translation
The process by which ribosomes read mRNA codons and assemble a chain of amino acids into a protein.
Codon
A sequence of three mRNA bases that codes for a specific amino acid or signals start/stop of protein synthesis.
tRNA (Transfer RNA)
RNA molecules that carry specific amino acids to the ribosome and match anticodons to mRNA codons during translation.
Complementary Base Pairing
The rule that adenine pairs with thymine (in DNA) or uracil (in RNA), and guanine always pairs with cytosine.

Flashcards

Double Helix
The twisted-ladder shape of DNA, with two complementary strands wound around each other.
Nucleotide
The building block of DNA/RNA; consists of a sugar, phosphate group, and nitrogenous base.
mRNA
Messenger RNA; the single-stranded copy of a gene that carries instructions from the nucleus to ribosomes.
Ribosome
The cellular structure (made of rRNA and proteins) where translation occurs and proteins are assembled.
Anticodon
A three-base sequence on tRNA that is complementary to a specific mRNA codon.
Polypeptide
A chain of amino acids linked by peptide bonds; folds into a functional protein.
Gene
A specific sequence of DNA bases that encodes the instructions for building one protein or functional RNA molecule.
RNA Polymerase
The enzyme that unwinds the DNA double helix and synthesizes a complementary mRNA strand during transcription.

Questions

  1. Which enzyme is responsible for synthesizing mRNA during transcription?

    1. DNA polymerase
    2. RNA polymerase
    3. Ribosome
    4. Ligase
    Show answer

    Answer: RNA polymerase

    RNA polymerase is the enzyme that reads the DNA template strand and synthesizes the complementary mRNA strand during transcription.

  2. In mRNA, thymine (T) pairs with adenine (A) on the DNA template strand.

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    Answer: False

    In mRNA, uracil (U) replaces thymine. So adenine in the DNA template pairs with uracil in the mRNA strand, not thymine.

  3. A sequence of three mRNA bases that codes for one amino acid is called a ___.

    Show answer

    Answer: codon

    A codon is a triplet of mRNA bases (e.g., AUG) that specifies a particular amino acid or signals the start or end of translation.

  4. Where does translation take place in a eukaryotic cell?

    1. Nucleus
    2. Mitochondria
    3. Ribosomes
    4. Golgi apparatus
    Show answer

    Answer: Ribosomes

    Translation occurs at ribosomes, which are found in the cytoplasm (and on the rough endoplasmic reticulum) — not in the nucleus where the DNA is stored.

  5. Guanine always pairs with cytosine in a DNA double helix.

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    Answer: True

    Guanine (G) and cytosine (C) always form a base pair in DNA, connected by three hydrogen bonds. This is one of the two complementary base pairing rules.

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