Review DNA replication, transcription, translation, RNA processing, and mutations.
Part of the free Knowledge Mountain biology library. Read an explanation, try a question, and use a memory cue to check what you can recall.
DNA has a sugar-phosphate backbone and paired bases: A with T, C with G. The two strands run in opposite directions, called antiparallel.
Hydrogen bonds connect paired bases between strands. Covalent bonds hold each strand’s backbone together. A sequence on one strand determines the complementary sequence.
A DNA strand is written 5′-ATGCC-3′. Write its paired strand directly beneath it with correct ends.
3′-TACGG-5′.
Apply A–T and C–G base pairing while keeping the strands antiparallel. If asked to rewrite the complement in the 5′ to 3′ direction, it would be 5′-GGCAT-3′.
DNA replication is semiconservative: each daughter DNA molecule contains one original strand and one newly synthesized strand. DNA polymerase adds complementary nucleotides to the growing strand.
New DNA grows 5′ to 3′. Because the templates run opposite ways, one new strand can be made continuously and the other is assembled from fragments.
Match these jobs to helicase, DNA polymerase, and ligase: separate strands; add DNA nucleotides; join DNA fragments.
Helicase separates strands; DNA polymerase adds nucleotides; ligase joins fragments.
Replication needs coordinated steps. Separating the templates does not copy them, and producing short fragments does not by itself create one continuous backbone.
Transcription uses a DNA template to make RNA. Translation uses mRNA codons to build a polypeptide. In eukaryotes, transcription occurs in the nucleus and translation occurs on ribosomes in the cytoplasm.
RNA polymerase makes RNA. Ribosomes, tRNAs, and other components perform translation. The DNA sequence is not directly read by a ribosome.
A DNA template segment is 3′-TAC GGA-5′. What mRNA segment is made?
5′-AUG CCU-3′.
RNA is complementary to the template and uses U instead of T. The first triplet here, AUG, is commonly the start codon and codes for methionine.
A eukaryotic pre-mRNA usually receives a 5′ cap and a poly-A tail. RNA splicing removes introns and joins exons. The mature mRNA can then leave through a nuclear pore.
Alternative splicing can combine exons in different ways, allowing one gene to contribute to multiple protein products. Exons are retained in mature RNA, though not every retained base is translated.
Where is a codon found, and what matches it during translation?
A codon is a three-nucleotide sequence in mRNA. A complementary tRNA anticodon matches it.
The tRNA carries an amino acid, and the ribosome helps link amino acids into a growing chain. Stop codons are recognized by release factors rather than ordinary amino-acid-carrying tRNAs.
A base substitution changes one nucleotide pair. It may leave the amino acid unchanged, replace an amino acid, or create a stop codon. Insertions or deletions can shift the reading frame if their size is not a multiple of three.
The effect depends on location and context. A mutation in a noncoding region can still matter if it changes regulation or RNA processing.
One nucleotide is deleted near the beginning of a protein-coding sequence. Why can it affect many later amino acids?
It can cause a frameshift, changing how later bases are grouped into codons.
Ribosomes read consecutive groups of three. Removing one base shifts those groups and can also produce an early stop. Deleting three bases usually preserves the frame, though it can still damage function.