1. 3.2 Chromosomes
Understanding:
Prokaryotes have one chromosome consisting of a circular DNA molecule
Some prokaryotes also have plasmids but eukaryotes do not
Eukaryote chromosomes are linear DNA molecules associated with histone proteins
In a eukaryote species there are different chromosomes that carry different genes
Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes
Diploid nuclei have pairs of homologous chromosomes
Haploid nuclei have one chromosome of each pair
The number of chromosomes is a characteristic feature of members of a species
A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length
Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex
Applications:
Cairns’s technique for measuring the length of DNA molecules by autoradiography
Comparison of genome size in T2 phages
Comparison of diploid chromosome numbers
Use of karyotyes to deduce sex and diagnose Down Syndrome in humans
Skills:
Use of online databases to identify the locus of a human gene and its protein product
Nature of science:
Developments in scientific research follow improvements in techniques: autoradiography was used to establish the length of DNA molecules in chromosomes

2. Prokaryotes Contain a circular DNA molecule Some also contain plasmids
Understanding:
Prokaryotes have one chromosome consisting of a circular DNA molecule
Some prokaryotes also have plasmids but eukaryotes do not

3. Plasmids
Contain SPECIAL genes that are not needed for everyday life purposes
E.g. Antibiotic resistance
Useful when antibiotics around
Not needed all the time
Understanding:
Prokaryotes have one chromosome consisting of a circular DNA molecule
Some prokaryotes also have plasmids but eukaryotes do not

4. Plasmids can be transferred from one cell to another
Across a population
To a different species
Artificial
One species containing plasmid dies – absorbed by another species
Understanding:
Prokaryotes have one chromosome consisting of a circular DNA molecule
Some prokaryotes also have plasmids but eukaryotes do not

5. Eukaryotes Linear DNA molecules Associated with histone proteins
(DNA winds around histones)
Understanding:
Eukaryote chromosomes are linear DNA molecules associated with histone proteins

6. Different chromosomes carry different genes
Length
Position of centromere
Humans have 23 pairs
Understanding:
In a eukaryote species there are different chromosomes that carry different genes

7. Homologous Chromosomes
Two chromosomes with same sequence of genes
Not completely identical as they may have different alleles
E.g. Both chromosomes have hair colour genes. Each codes for a different colour
Understanding:
Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes

8. Online databases can find the exact locus of a gene.
Follow the instructions on page 154/kognity to search for different gene loci
Skills:
Use of online databases to identify the locus of a human gene and its protein product

9. Measuring DNA length Cairn’s technique
Grow E.coli cells for 2 generations in a medium with tritiated thymidine.
Thymidine contains base Thymine linked to deoxyribose – used by E.coli to make nucleotides in DNA replication
Tritiated = contains tritium – a radioactive isotope of hydrogen. E.coli produce radioactively labeled DNA
Applications:
Cairns’s technique for measuring the length of DNA molecules by autoradiography

10. Measuring DNA length Cairn’s technique
Cells placed onto dialysis membrane – cell walls digested using enzyme lysozyme
Cells gently burst to release DNA onto surface of dialysis membrane
Applications:
Cairns’s technique for measuring the length of DNA molecules by autoradiography

11. Measuring DNA length Cairn’s technique
Photographic emulsion applied to surface and left in darkness for 2 months
Tritium emits high energy electrons which react with the film
Applications:
Cairns’s technique for measuring the length of DNA molecules by autoradiography

12. Measuring DNA length Cairn’s technique
After 2 months the film is developed and examined with a microscope
Dark grains where tritium decayed – indicating the position of the DNA
Applications:
Cairns’s technique for measuring the length of DNA molecules by autoradiography

13. Measuring DNA length Cairn’s technique
Showed a single circular DNA molecule with a length of 1,100 um.
Micrometre
Applications:
Cairns’s technique for measuring the length of DNA molecules by autoradiography

14. Cairn’s Technique Measuring DNA length2. 3. 4. 5. 6. 7.

15. Haploid Nuclei Product of meiosis Humans: 23 single chromosomes
Gametes (egg and sperm)
Not all eggs/sperm have the same chromosome as meiosis increases variation
Alleles are different in each egg and sperm
Gets more complicated than that later
Understanding:
Diploid nuclei have pairs of homologous chromosomes
Haploid nuclei have one chromosome of each pair

16. Diploid nuclei Has two chromosomes of each type
Humans: 46 single chromosomes (23 pairs)
Haploid cells fuse together during fertilisation producing a diploid zygote
Understanding:
Diploid nuclei have pairs of homologous chromosomes
Haploid nuclei have one chromosome of each pair

17. Number of chromosomes Fundamental characteristic of an organism
Different numbers of chromosomes = unlikely to interbreed
Understanding:
The number of chromosomes is a characteristic feature of members of a species

18. Number of chromosomes Organism Number of chromosomes Threadworm 4
Humans 46
Chimp 48
Dog 78
Applications:
Comparison of diploid chromosome numbers

19. Determining sex Autosomes = chromosomes that do not determine sex
Sex chromosomes = determine sex
Understanding:
Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex

20. Determining sex X = large with centromere near the middle
Y = smaller with centromere near the end
Understanding:
Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex

21. X chromosome Many genes essential in both males and females.
All humans have at least one X chromosome
Y chromosome contains genes that are not needed for female development
Understanding:
Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex

22. Contains TDF/SRY gene which induces the development of male features
Y chromosome
Contains TDF/SRY gene which induces the development of male features
Testes
Testosterone
If no TDF/SRY gene – ovaries develop and a female is produced
Understanding:
Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex

23. Determining Sex
Females always pass on an X chromosome (all eggs contain an X chromosome)
The male determines the sex of their offspring – sperm can either contain an X chromosome or a Y chromosome
Understanding:
Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex

24. Draw and complete the punnett square for determining sex:Y

25. Karyogram vs karyotype
Image of chromosomes arranged in size order
Karyotype
Property of an organism – number and type of chromosomes
Description of the karyogram
23 pairs of chromosomes
Homologous pairs
X and Y chromosomes
Male individual
Understanding:
A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length

26. Needle passes through mother’s abdomen wall
Ultrasound guides the needle
Needle withdraws a sample of amniotic fluid containing foetal cells
Risk of miscarriage 1%
Tool enters the vagina and obtains cells from the chorion (part of the placenta)
Can be done earlier
Risk of miscarriage 2%

27. Use of karyograms Karyotypes are determined by looking at karyograms:
Male or female (XX or XY)
Down syndrome female
(3 copies of chromosome 21 instead of 2)
Applications:
Use of karyotyes to deduce sex and diagnose Down Syndrome in humans

28. Male with no down syndrome

29. Male with down syndrome

30. Female with no down syndrome

31. Female with down syndrome

32. Presentations Inherited diseases: Cystic fibrosis Huntingdon’s
Red-green colour blindness
Haemophilia

33. Start to make presentations
Inherited Diseases
How common is the disease?
Where is it most common in the world?
Is it a dominant or recessive allele that causes it?
Which chromosome is the gene located on?
What are the symptoms?
Can you die from this disease?
When do the symptoms start? What age?
Is it mainly in males or females or both?
Is there a cure?
How can the symptoms be eased?
Include a Punnett square to show how the disease could be inherited
Is this a sex-linked disease? (on an X or Y chromosome?)
Thursday 30th August:
Start to make presentations
Monday 3rd/Tuesday 4th:
Give presentations

34. Inherited Diseases Presentations
How common?
Dominant or recessive?
Chromosome
Symptoms/ can you die?
Does it affect one sex over another?
Treatment or cure?
Is it sex linked?
Cystic fibrosis
Huntington's
Red green colorblindness
Hemophilia