1. DNA replication: mitosis & meiosis
Biology
Human Biology

2. DNA replication Mitosis Meiosis DNA replication Interphase Prophase
Keywords
Keywords
Mitosis
Meiosis
DNA replication
Interphase
Prophase
Metaphase
Anaphase
Telophase
Chromatin
Chromatids
Chromosomes
Centromere
Centriole
Spindle fibres
Cytokinesis

3. Genetic information
Each species has a specific number of chromosomes, composed of a particular kind of DNA
Example: -
Humans have 46 chromosomes
Dogs have 78 chromosomes
Dandelions have 16 chromosomes
Strawberries have 14 chromosomes

4. Prokaryotes vs. eukaryotes
Single celled
Asexual reproduction
Binary fission
Eukaryotes
Multicellular
Mitosis
Sexual reproduction
Meiosis
Apoptosis

5. Prokaryotes

6. Asexual reproduction – binary fission
DNA replication of the plasmid
The plasmid (single chromosome) is duplicated
The 2 chromosomes segregate - each copy of the chromosome attaches to a different part of the cell membrane
Cytokinesis
The cell begins to divide - a wall forms across the cell and divides it into two
The end result is 2 (genetically) identically daughter cells

7. Eukaryotes

8. Mitosis
Cell division is responsible for growth, replacement and repair.
The result of mitosis is the formation of two genetically identical cells – the parent cell divides to form 2 identical daughter cells

9. Stages of mitosis
Interphase is a period when the cell functions as normal, but is also preparing for cell division.
The chromosomes are strung out in long chromatin threads within the nucleus.
Each chromatin thread is replicated over a period of time.
All the various organelles are also being replicated at this time.
Mitosis can be divided into 5 stages:
Interphase
Prophase
Metaphase
Anaphase
Telophase

10. Prophase Centrioles become visible in the cytoplasm
Chromatin condenses into chromosomes
Each chromosome consists of 2 identical strands called chromatids
These are attached to each other at a point call the centromere
Nuclear membrane disappears
centrioles
chromosome –consisting of 2 chromatids

11. Metaphase
During metaphase the chromatids line up at the equator of the cell.
The centromere of each pair is attached to a spindle fibre
spindle fibres
chromosomes – spindle fibre attached at the centromere
centrioles

12. Anaphase
During anaphase each pair of chromatids separate at the centromere
The chromatids are pulled apart towards the opposite poles of the cell by the spindle fibres
spindle fibre
chromatid
centriole

13. Telophase
During telophase the 2 sets of chromosomes form tight groups at each pole
The spindle disappears
Nuclear membranes and nuclei form
Centrioles divide
Chromosomes uncoil and disappear
Cytokinesis occurs during this phase
Cytokinesis is the division of the cytoplasm
centriole
cytokinesis
group of chromosomes

14. Stages of mitosis

15. Interphase – beginning of cell cycle

16. Just before prophase – chromosomes have been replicated

17. Meiosis There is another form of cell division called meiosis
Meiosis does not produce identical daughter cells that are copies of the original parent cell
Instead, meiosis produces the gametes (sex cells)
After meiosis, each gamete ends up with only half the number of chromosomes

18. Homologous pair of chromosomes
Meiosis
Meiosis
Mitosis
The process of meiosis involves 2 nuclear divisions, but the chromosomes duplicate only once
The first division of meiosis is similar to mitosis
However, during metaphase I the chromosomes pair off, rather than all of them lining up singly.
Metaphase I
Metaphase
Homologous pair of chromosomes
Metaphase II

19. Meiosis I & meiosis II

20. Diploid vs haploid cells
Term
Definition
Diploid
The number of chromosomes present in the body cells (somatic cells) of an individual (2n)
Haploid
The number of chromosomes present in the gametes (germline cells) (n)

21. Mitosis vs. meiosis

22. The importance of meiosis
Meiosis maintains chromosome number
Half from each diploid parent only; a complete set on fertilisation
Meiosis allows for recombination of alleles
Pairing of different alleles from different parents increases variation
Meiosis allows for the separation of alleles
One allele is found on each chromosome in a pair.
Separation allows alleles to move independently, thus increasing variability

23. What’s so special about meiosis?
Meiosis plays an important role in maintaining genetic diversity
There are a number of events that happen during meiosis that ensure the next generation has as much variation as possible
Random fertilisation
Crossing over
Independent assortment
Mutations
Non-disjunction

24. Random fertilisation Fertilisation of gametes is a random process
Each fertilisation event produces a unique zygote

25. Crossing over
Crossing over occurs during prophase I. This ensures genetic diversity in offspring
Homologous chromosomes pair up
Similar sections cross over and swap places
All gametes have different genetic content
Linked genes are those found on the same chromosome and usually inherited together are separated and can be inherited independently

26. Crossing over

27. Crossing over

28. Random assortment
After meiosis, each gamete has only one set of genes.
They unite randomly at fertilization, combining their genes to form a different genetic combination from either parent.
This is called random assortment
Sometimes it is also called independent assortment

29. (½)23 Random assortment Humans have 23 pairs of chromosomes:
there is an equal chance that the gametes will get either one of the pair.
½ chance of getting maternal chromosome 1
½ chance of getting maternal chromosome 2
And so on ...
Therefore that chance of getting ALL the maternal chromosomes is
(½)23
or ½ x ½ x ½ x ½ x times

30. Random assortment

31. Mutations

32. Chromosomal mutations
Duplication
Inversion
Deletion
Insertion
Translocation

33. Gene or point mutations

34. Results in too many chromosomes in one gamete and not enough in another.

35. Hybrid vigour increases the survival of the offspring of the cross between the two different parents

36. Apoptosis Programmed cell death
Cell death is vital to the health of an organism
Metamorphosis
Apoptosis is important during metamorphosis
Example: tadpole into a frog.
As a tadpole changes into a frog, the cells in the tadpole tail are induced to undergo apoptosis; as a consequence, the tail is lost.
Tissue maintenance
In eukaryotes, cell death exactly balances cell division, otherwise their tissues and organs would grow or shrink.

37. Apoptosis Cell volume increases and the cell membrane bulges outwards
Nucleus fragments as enzymes destroy it
Apoptic bodies form
Apoptic debris is engulfed by macrophages