Topic 3.2 ChromosomesIB Biology

R. Price

v. 1 2015

Allott 149

#1: Prokaryotes have one chromosome consisting of a circular DNA molecule

• One chromosome• Circular DNA• Not associated w/ proteins, so

“naked”

Allott 150

#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

artificiallyAllott 150

Allott 150, 151

Allott 151

#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

#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

#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

Allott 153

Allott 153, 154

Allott 154

#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

#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

#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

Allott 155, 156

Allott 156

#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

#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

Allott 158

Allott 158

Allott 159

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.

ImagesUnless otherwise noted, images are obtained from Pixabay (www.pixabay.com) and used under the CC0 Public Domain license.