1. Prokaryotes have one circular chromosome without histone proteins, while eukaryotes have multiple linear chromosomes associated with histone proteins. 2. Some prokaryotes also contain plasmids, which are extra DNA molecules that can be transferred between cells and species. 3. Eukaryotic cells can be haploid, containing one of each type of chromosome, or diploid, containing two of each type of homologous chromosome. The number and type of chromosomes is characteristic of each species.

1. Topic 3.2 Chromosomes
IB Biology
R. Price
v. 1 2015

2. Allott 149

3. #1: Prokaryotes have one
chromosome consisting of
a circular DNA molecule
• One chromosome
• Circular DNA
• Not associated w/ proteins, so
“naked”
Allott 150

5. #2: Some prokaryotes also have plasmids but eukaryotes do not
• Extra DNA not in the chromosome
• May/may not replicate when
chromosome’s replicated
• May/may not be passed on during
cell replication
• Can be transferred to other cells
• Can be transferred to other species
• Natural method of gene transfer
between species
• Researchers use to transfer genes
artificially
Allott 150

6. Allott 150, 151

7. Allott 151

8. #3: Eukaryote chromosomes are linear DNA molecules associated with
histone proteins
Chromosomes in eukaryotes
• DNA + histone protein
• Histones = globular proteins
• Many histones in a chromosome
• DNA wind around histone

9. #4: In a eukaryote species there are different chromosomes that carry
different genes
In eukaryotes:
• At least two different types of
chromosomes
• In humans -> 23 types of
chromosomes
• Genes are found on a specific
position on a specific type of
chromosome (locus)
• Each chromosome carries specific
sequence of genes along linear
DNA molecule

10. #5: Homologous chromosomes carry the same sequence of genes but
not necessarily the same alleles of those genes
• Two chromosomes carry
same sequence of genes =
homologous genes
• Same sequence, but not
identical -> alleles

11. Allott 153

12. Allott 153, 154

13. Allott 154

14. #6: Haploid nuclei have one chromosome of each pair
• Haploid nuclei have one
full set of chromosomes
• Has only one
chromosome of each
type
• Some plant life cycles
have a haploid phase
(ferns)
• Gametes (sex cells) are
haploid

15. #7: Diploid nuclei have pairs of homologous chromosomes
• Diploid nuclei have two full sets
of chromosomes
• Has two chromosomes of each
type
• Except for sex chromosomes,
has two copies of each gene
• Advantage: dominant allele may
protect from a damaging allele
• Advantage: hybrid vigor

16. #8: The number of chromosomes is a characteristic feature of members
of a species
• Fundamental characteristic
• Organisms with different
number of chromosomes are
unlikely to interbreed
• # can change, but rare. Can
occur during evolution of species
• Decrease if chromosome fuse
• Increase if chromosomes split

17. Allott 155, 156

18. Allott 156

19. #9: A karyogram shows the chromosomes of an organism in
homologous pairs of decreasing length
• Cell in metaphase gives clearest
view of chromosomes
• Chromosomes treated with a stain
• Micrograph taken of chromosomes
• Computer used to sort
chromosomes
• Chromosomes sorted by size,
position of centromere, and
banding after staining
• Usually in homologous pairs
• Arranged longest to smallest

20. #10: Sex is determined by sex chromosomes and autosomes are
chromosomes that do not determine sex
Humans (rules may change for other
species)
• X chromosome large, centromere
near middle
• Y chromosome smaller, centromere
near end
• One Y chromosome (SRY or TDF)
starts development of male
features
• XX = female
• XY = male

21. Allott 158

22. Allott 158

23. Allott 159

24. Sources
Content
Allott, Andrew, and David Mindorff. Biology: Course Companion. 2014
ed. Oxford: Oxford UP, 2014. Print. Oxford IB Diploma Programme.
Walpole, Brenda. Biology for the IB Diploma. 2nd ed. Cambridge:
Cambridge UP, 2014. Print.
Images
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(www.pixabay.com) and used under the CC0 Public Domain license.