8.2 DNA and chromosomes

Question 1

Describe the structure of prokaryotic DNA.

Answer 1

• Shorter
• forms a circle
• are not associated with protein molecules
• no chromosomes

Question 2

Describe the structure of eukaryotic DNA.

Answer 2

• longer
• linear
• associated with histone proteins to form chromosomes
• mitochondria and chloroplasts contain DNA of the same structure as prokaryotes.

Question 3

Where is DNA found?

Answer 3

• it is only visible during cell division

- other than that, they are found in the nucleus

Question 4

Describe the structure of chromosomes during cell division.

Answer 4

• at the start, they appear as two threads joined by a single point
• each thread is called a chromatid because DNA has already replicated to give two identical molecules

Question 5

Describe the structure of DNA within chromosomes.

Answer 5

• the double helix is wound around histones to fix it in position
• the DNA-histone complex is then coiled and packed into a chromosome
• the number of chromosomes is the same for members of the same species but varies between species

Question 6

What is a homologous chromosome?

Answer 6

A chromosome which is made up of one side from the mother in the egg (maternal chromosomes) and one side from the father in the sperm (paternal chromosomes)

• this is true of sexually produced organisms
• the fusion of sperm and egg gives one complete set of chromosomes
• the total number of homologous pairs known as the diploid number; 46 in humans

Question 7

What is an allele?

Answer 7

One of many alternative forms of a gene.

• each gene exists in two or more different forms, and each of these is called an allele.
• each individual inherits a gene from each different parent
• each allele has a different sequence of bases, and therefore produces a different polypeptide.

Question 8

What happens when there is a change in the base sequence of a gene (mutation)?

Answer 8

• a new allele is produced
• a different amino acid sequence is coded for, and therefore a different polypeptide is produced.
• this may cause a protein to not function properly, or to not function at all.
• when this is for an enzyme, it may have a different shape which doesn’t fit the substrate