8.2 - DNA and chromosomes Flashcards
What are the differences between the DNA of a eukaryotic cell and a prokaryotic cell
- prokaryote: DNA molecules are shorter, form a circle and are not associated with protein molecules. Therefore = don’t contain chromosomes
- eukaryote: DNA molecules are longer, form a line (are linear) and occur in association with proteins called histones to form structures called chromosomes.
- The mitochondria/chloroplasts if a eukaryotic cell also contain DNA which (like the DNA of prokaryotes) is short, circular and not associated with proteins
When is the only time that chromosomes can be visible as distinct structures
When a cell is dividing
—> rest of time = widely dispersed throughout the nucleus
Describe the structure of a chromosomes when they are visible
- at start of cell division: chromosomes appear as 2 threads joined to a single point
- each thread is called a chromatid because the DNA has already replicated to give 2 identical DNA molecules
- The DNA in chromosomes is held by histones
How is a DNA packed into a chromosome
You can see that this single DNA molecule has many genes along its length, each gene occupying a specific position (locus) along the DNA molecule
Describe how a DNA molecule is compacted into forming a chromosome
- double helix of DNA molecule is wound around histones to fix it into position
- This DNA-histones complex is then coiled
- the coil, in turn, is looped and further coiled before being packed into the chromosome
—> therefore a chromosome contains just a single molecule of DNA
In sexually produced organisms, where is each pair of chromosome derived from. What are these known as?
- chromosomes provided by mother in the egg (maternal chromosomes)
- other is derived from chromosomes provided by father in the sperm (paternal chromosomes)
- known as = homologous pair, their diploid number in humans = 46
What is a homologous pair
2 chromosomes that carry the same genes but not necessarily the same alleles of the genes
What is an allele
- one of a number of alternative forms of a gene
—> genes exists in 2 (occasionally more) different forms
—> each of these forms are called an allele
Describe the alleles an individual receives from their parent
- inherits 1 from each of its parents
- the two alleles may be the same or they may be different
- if different = each allele has a different base sequence, therefore a different amino acid sequence, so produced a different polpeptide
What happens if there is any changes in the base sequence of a gene
- produces a new allele of that gene (=mutation) and results in a different sequence of amino acids being coded for
- this different amino acid sequence will lead to the production of a different polypeptide, and hence a different protein
- sometimes this protein may not function properly o have no function at all
- when this protein is an enzyme, it will have a different shape (may not fit the enzyme’s substrate) and therefore cannot function = can have serious consequences for the organism
Describe the difference between Prokaryote DNA and Eukaryote DNA in terms of the structure of the DNA
Prokaryote
- DNA is naked (no Histone proteins)
- DNA is circular in shape
- usually has no introns
Eukaryote
- DNA is wrapped around histone proteins
- DNA is linear in shape
- DNA usually has introns
Describe the difference between Prokaryote DNA and Eukaryote DNA in terms of organelles
Prokaryote
- no nucleus
- no membrane bound organelles
- 70S ribosomes
Eukaryote
- has a nucleus
- has membrane bound organelles
- 80S ribosomes
Describe the difference between Prokaryote DNA and Eukaryote DNA in terms of reproduction
Prokaryote
- Binary fission
- Single chromosome — Haploid
Eukaryote
- Mitosis and meiosis
- chromosomes paired in homologous pairs —> diploid or more
Describe the difference between Prokaryote DNA and Eukaryote DNA in terms of average size
Prokaryote
- smaller (1-5 micrometers)
Eukaryote
- larger (10-100 micrometers)
