Free SACE Biology Flashcard Review

A phosphate group, a deoxyribose sugar and one nitrogenous base. The four DNA bases are adenine, thymine, cytosine and guanine. Nucleotide order carries sequence information; the repeated sugar–phosphate backbone supports the strand rather than supplying a different code at each position.

Covalent bonds connect neighbouring nucleotides along each sugar–phosphate backbone. Hydrogen bonds connect complementary bases between the two strands. Separating the strands for replication disrupts the between-strand interactions; it does not require cutting every backbone into individual nucleotides.

A pairs with T and C with G. Each parental strand therefore specifies the sequence of its new complementary strand. A template provides a sequence constraint, although replication is not infallible and errors can still produce mutations.

The two strands run in opposite chemical directions: one is 5′ to 3′ alongside the other 3′ to 5′. Direction labels matter when converting a template into a complementary sequence. Simply replacing letters while leaving every label unchanged can produce an incorrectly oriented answer.

Its aligned complementary strand is 3′-TCAG-5′. If the answer must instead be written in the conventional 5′ to 3′ direction, it is 5′-GACT-3′. State direction explicitly so the aligned complement is not confused with the reverse complement.

Each daughter double helix contains one parental strand and one newly synthesised strand. It does not preserve one entire old double helix while making a wholly new second helix, and it does not distribute short old fragments randomly throughout both daughter strands.

Replication produces sister copies of each chromosome so division can distribute a complete genetic set to each daughter nucleus. Replication doubles the DNA amount without automatically doubling the chromosome number when chromosomes are counted by their centromeres.

A typical bacterial chromosome is circular and lies in a nucleoid without a surrounding nuclear membrane. Eukaryotic nuclear chromosomes are linear and associated with histones. This comparison describes the usual course model; it should not be turned into a claim that every prokaryote has exactly one chromosome.

Mitochondria contain their own DNA, and chloroplasts do too in cells possessing them. These organelles have genomes distinct from the nuclear chromosomes. An animal cell normally lacks chloroplasts, so the possible locations must be matched to the cell type in the question.

If both original strands are labelled and new nucleotides are unlabelled, each daughter double helix has one labelled and one unlabelled strand. Follow whole parental strands through the model. The result supports semi-conservative inheritance rather than an all-old and an all-new pair of molecules.

Some genes produce functional RNA rather than a translated polypeptide, including tRNA, rRNA and regulatory RNA. A gene is a DNA sequence whose expression produces a functional product. Treating every RNA as merely an intermediate on its way to becoming protein misses these roles.

An RNA molecule complementary to the DNA template strand is synthesised. In a eukaryotic nuclear gene, transcription occurs in the nucleus and RNA processing can precede export of mature mRNA. Transcription copies information into RNA; it does not assemble amino acids into a protein.

RNA is complementary and antiparallel to the template strand. The coding strand has the same base sequence as the RNA in the corresponding 5′ to 3′ direction, with T in DNA where RNA has U. Identify the supplied strand before applying base-pairing rules.

The mRNA is 5′-AUGCCU-3′. Pair T with A, A with U, C with G and G with C while preserving the opposite directions. Do not retain thymine in the RNA or copy the template directly as though it were the coding strand.

Introns are removed and retained exon sequences are joined by splicing. For a protein-coding transcript, the translated region lies in the mature RNA. Not every retained nucleotide must encode an amino acid, because mature mRNA also contains untranslated regions.

mRNA supplies the codon sequence, tRNA pairs its anticodon with an appropriate codon while carrying an amino acid, and rRNA forms a central structural and catalytic part of the ribosome. These are different roles; tRNA is not the long message copied from the entire gene.

Use the mRNA codon in the table’s stated orientation, normally 5′ to 3′. Convert a supplied DNA template or tRNA anticodon appropriately first. A stop codon signals termination rather than naming an additional amino acid to append to the polypeptide.

They pair complementarily and antiparallel at the ribosome. For codon 5′-AUG-3′, the aligned anticodon is 3′-UAC-5′. If an anticodon is written in the opposite convention, reverse its order before comparing; direction labels prevent an apparent mismatch.

More than one codon can specify the same amino acid. It does not mean a given codon unpredictably specifies several different amino acids under the usual code. Degeneracy helps explain why some base substitutions leave the amino-acid sequence unchanged.

The message is read as successive non-overlapping groups of three nucleotides from the established start. Changing that grouping changes downstream codons. A sequence of the correct letters can therefore give the wrong predicted protein if it is divided into triplets from the wrong starting position.