3.4.1 DNA, genes and chromosomes

Which of the following best describes the DNA in prokaryotic cells?

• Long, linear and associated with histones

• Short, circular and not associated with proteins

• Short, linear and associated with proteins

• Long, circular and associated with histones

Answer explanation

The DNA in prokaryotic cells is typically short and circular, existing as a single chromosome. Unlike eukaryotic DNA, it is not associated with histones, making 'Short, circular and not associated with proteins' the correct description.

In eukaryotic cell nuclei, DNA molecules are:

• Short, circular and not associated with proteins

• Very long, linear and associated with histones

• Short, linear and associated with proteins

• Very long, circular and not associated with proteins

Answer explanation

In eukaryotic cell nuclei, DNA is very long and linear, and it is associated with histones, which help package the DNA into a compact structure. This distinguishes it from prokaryotic DNA, which is typically circular.

What is the name given to the proteins that DNA is associated with in eukaryotic chromosomes?

• Enzymes

• Histones

• Ribosomes

• Ligases

Answer explanation

In eukaryotic chromosomes, DNA is wrapped around proteins called histones. These proteins help in packaging DNA into a compact structure, allowing for efficient storage and regulation of genetic information.

Which organelles in eukaryotic cells contain DNA similar to that of prokaryotes?

• Ribosomes and lysosomes

• Mitochondria and chloroplasts

• Nucleus and endoplasmic reticulum

• Golgi apparatus and peroxisomes

Answer explanation

Mitochondria and chloroplasts contain their own DNA, which is similar to prokaryotic DNA. This is due to their evolutionary origin from free-living bacteria, unlike other organelles that do not have their own DNA.

A gene is best defined as:

• A protein that codes for DNA

• A base sequence of DNA that codes for a polypeptide or functional RNA

• A sequence of amino acids

• A type of histone

Answer explanation

A gene is defined as a base sequence of DNA that codes for a polypeptide or functional RNA, making the second choice correct. It is not a protein, amino acid sequence, or histone.

What is the fixed position that a gene occupies on a DNA molecule called?

• Codon

• Locus

• Exon

• Intron

Answer explanation

The fixed position that a gene occupies on a DNA molecule is called a locus. This term refers to the specific location of a gene on a chromosome, distinguishing it from terms like codon, exon, and intron, which refer to different aspects of gene structure.

A sequence of three DNA bases that codes for a specific amino acid is called a:

• Codon

• Triplet

• Exon

• Locus

Answer explanation

A sequence of three DNA bases that codes for a specific amino acid is called a triplet. While 'codon' is also used, 'triplet' specifically refers to the three-base sequence, making it the correct choice.

Which of the following statements about the genetic code is correct?

• It is unique to humans

• It is universal, non-overlapping and degenerate

• It is overlapping and specific to each species

• It is not degenerate

Answer explanation

The genetic code is universal across all living organisms, meaning it is the same in nearly all species. It is also non-overlapping, as each codon specifies one amino acid, and degenerate, as multiple codons can code for the same amino acid.

In eukaryotes, much of the nuclear DNA:

• Codes for polypeptides only

• Does not code for polypeptides

• Codes only for tRNA

• Is found in mitochondria

Answer explanation

In eukaryotes, a significant portion of nuclear DNA does not code for polypeptides. This includes non-coding regions, introns, and regulatory sequences, which play crucial roles in gene expression and regulation.

What are non-coding multiple repeats of base sequences between genes called?

• Exons

• Introns

• Non-coding repeats

• Codons

Answer explanation

Non-coding multiple repeats of base sequences between genes are referred to as non-coding repeats. Exons and introns are related to coding sequences, while codons are units of genetic code, making 'non-coding repeats' the correct choice.