Chromosomes in Prokaryotes
1 single, circular chromosome
•Reproduce by binary fission (asexually— can “conjugate” but that is not
considered sexual reproduction
•In sexual reproduction, have two parents,
so chromosomes are paired (homologues)
•Prokaryotes only have one parent, so only
one chromosome
Some prokaryotes have plasmids
Small loops of DNA
•Extra copies of SOME of the genetic info
•Found in some Archaea and Eubacteria
•Replicate independently, can have several plasmid copies per cell
•Used in genetic engineering
Eukaryotic Chromosomes
Linear DNA
•At least one PAIR of chromosomes
(usually more)
•Two copies of each gene (and alleles)
•No so easily used in genetic engineering—other techniques are required that we will cover later
Prokaryote/Eukaryote
Number of
chromosomes
Shape
Histones
Presence of
1
2 or more
(*Except male
bees,
wasps, ants)
Circular
Linear
Not present (Except in
Archaean DNA) (Bacteria
have DNA Factor
proteins)
Present
Sometimes
Never
Plasmids
Organized into
Pairs
No
Yes
HomologousChromosome
s
•Homologous = similar in shape
& size
• Means that the two chromosomes carry
the same genes
• Not identical because the alleles for the
genes from each parent could be
different
Definitions
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This image shows a pair of homologous chromosomes.
Name and annotate the labeled features.
4.3 Theoretical Genetics
8
Definitions
Genotype
The combination of alleles
of a gene carried by an organism
Phenotype
The expression of alleles
of a gene carried by an organism
Centromere
This image shows a pair of homologous chromosomes.
Name and annotate the labeled features.
Homozygous dominant
Having two copies of the same
dominant allele
Homozygous recessive
Having two copies of the same
recessive allele. Recessive alleles are
only expressed when homozygous.
Joins chromatids in cell division
Codominant
Alleles
Different versions of a gene
Dominant alleles = capital letter
Recessive alleles = lower-case letter
Carrier
Heterozygous carrier of a
recessive disease-causing allele
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Pairs of alleles which are both
expressed when present.
Heterozygous
Having two different alleles.
The dominant allele is expressed.
Gene loci
Specific positions of genes on a
chromosome
4.3 Theoretical Genetics
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• Diploid=cells that have two copies of each chromosomes (one
homologous chromosome from each parent = 2n
• Somatic (body) cells – most cells
• Sex cells – gametes are HAPLOID (n) – One copy of a chromosome
• Humans n = 23 (gametes), 2n = 46
Making Babies
1. Count the chromosomes in your envelope - there
should be 46 in total.
2. Shuffle the chromosomes, so that they are well
mixed up. Which aspects of meiosis and sexual
reproduction give genetic variation?
• Crossing-over in prophase I
• Random orientation in metaphase I and II
• Random fertilisation
3. Now arrange them in a karyotype (don't turn them
over - leave them as they were).
4. What is the gender of your baby?
Explain how gender is inherited in humans.
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Activity from:
http://www.nclark.net/Genetics
4.3 Theoretical Genetics
12
Making Babies
•
•
•
Crossing-over in prophase I
Random orientation in metaphase I and II
Random fertilisation
List all the traits in a table. Use the key above to determine the genotypes and phenotypes
of your offspring. Draw a picture of your beautiful child’s face!
HL identify traits which are polygenic, involve gene interactions and some which are linked.
Activity from:
http://www.nclark.net/Genetics
http://sciencevideos.wordpress.com
4.3 Theoretical Genetics
13
Autoradiography