IB DP Biology Topic 3.2 Chromosomes

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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