A1.2 Nucleic Acids (2.9-2.11)

Where is genetic information stored in DNA?

In the base sequence of one of the DNA strands.

How many possible bases are there in DNA?

Four: A, C, G, T.

How many possible sequences are there for n bases?

4ⁿ possible sequences.

Why does DNA with 10 bases already have so many possibilities?

Because 4¹⁰ = 1,048,576 combinations — DNA can store immense amounts of information even in short sequences.

Can DNA molecules be different lengths?

Yes, DNA molecules can vary in length, which increases the potential diversity of base sequences.

How wide is a DNA molecule?

2 nm

Why is DNA so space-efficient?

Because it is extremely thin and tightly coiled, allowing immense lengths of information to fit in a tiny nucleus.

What does the sequence of bases in DNA or RNA do?

It contains information in a coded form, which is decoded during protein synthesis.

What is a codon?

A group of three bases in DNA or RNA that has a meaning in the genetic code.

How many possible codons are there?

64 codons (4 × 4 × 4).

What does most codons specify?

One particular amino acid.

What is the start codon?

A codon that signals the start of protein synthesis.

What are stop codons?

Three codons that signal the end of protein synthesis.

Is the genetic code the same in all organisms?

Almost all living organisms and viruses use the same genetic code.

Why is the genetic code like a language?

Because codons are like "words" that specify amino acids or instructions (start/stop) for protein synthesis.

How are nucleotides linked in DNA or RNA?

The phosphate group of one nucleotide is linked to the sugar of the next nucleotide by a phosphodiester bond.

What does it mean that a DNA or RNA strand has directionality?

The nucleotides are oriented in the same way, giving the strand a 5′ end and a 3′ end.

What is the 5′ end?

The end of the strand with a phosphate group attached to the 5th carbon of the sugar.

What is the 3′ end?

The end of the strand with a hydroxyl (-OH) group on the 3rd carbon of the sugar.

What enzyme makes an RNA copy of a DNA base sequence?

RNA polymerase (during transcription).

In what direction is mRNA built during transcription?

5′ → 3′ direction.

What process reads the RNA base sequence to make proteins?

Translation (at the ribosome).

Why must nucleic acid strands face the correct direction in enzymes?

Because enzyme active sites only fit nucleotides oriented correctly.

In what direction does the ribosome read mRNA?

5′ → 3′ direction

In DNA replication, to which end are new nucleotides added?

The 3′ end of the growing strand.

What direction is DNA replication in?

5′ → 3′ direction.

What does "antiparallel" mean in DNA?

The two strands run in opposite directions — one 5′ → 3′, the other 3′ → 5′

What enzyme adds nucleotides during DNA replication?

DNA polymerase.

How are both strands used during replication?

Each original strand serves as a template for building a new complementary strand.

Why can DNA polymerase only build in one direction?

It can only add new nucleotides to the 3′ end of the growing strand.

What is the leading strand?

The strand built continuously in the 5′ → 3′ direction (same direction as unzipping).

What is the lagging strand?

The strand built in short fragments (Okazaki fragments) in the 5′ → 3′ direction.

What joins Okazaki fragments together?

DNA ligase

Why is replication on the lagging strand slower?

Because it's built in small sections due to the opposite strand direction.

Why can't both DNA strands be copied continuously during replication?

Because the two strands run in opposite directions (antiparallel).

On which strand does DNA polymerase move in the same direction as the replication fork?

The leading strand.

On which strand does DNA polymerase move opposite to the replication fork?

The lagging strand.

What are short fragments of DNA made on the lagging strand called?

Okazaki fragments.

Which DNA strand is used as a template for transcription?

The strand that runs 3′ → 5′ (the template strand).

What enzyme builds the RNA strand during transcription?

RNA polymerase.

How does the RNA strand relate to the DNA strands?

It's complementary to the template strand and identical (except for U instead of T) to the coding strand.

Why does transcription happen 5′ → 3′?

Because RNA polymerase can only add new nucleotides to the 3′ end of the growing RNA molecule.

What carries the sequence information for making a polypeptide?

A molecule of RNA (mRNA).

What structure carries out translation?

The ribosome.

In which direction does the ribosome move along mRNA?

Toward the 3′ end of the mRNA.

In which direction is translation carried out?

5′ → 3′ along the mRNA.

What is formed by linking amino acids together during translation?

A polypeptide (protein).