Cell reproduction including the cell cycle, mitosis and meiosis.

1. 02. Cell reproduction.
Ian Anderson
Saint Ignatius College Geelong

2. Cell reproduction.
 All cells are derived from pre-existing cells.
 Enables the genes and cell components of parent cell
to passed onto the daughter cells.
 Cells must reproduce for
 growth of the organism.
 repair of damaged cells.
 replacement of dying cells.
 reproduction of the species.

3. Cell cycle.
 The series of event that take place in a cell leading to
its division (i.e. interphase + mitosis).
Source: http://www.scq.ubc.ca/the-cell-cycle-a-universal-cellular-division-program/

4. Cell cycle.
 Interphase.
 Cells spend most of their lives in interphase.
 Three stages:
 G1 (Gap 1)
 Cell growing.
 S (Synthesis)
 DNA replication occurs  chromosomes duplicating.
 G2 (Gap 2)
 Cell grows & preparation for the next cellular division.
 Mitosis.

5. DNA replication.
 Why is DNA replication necessary?
 When cells divide new daughter cells are produced.
 These daughter cells need to have functioning DNA in
order to survive.
 DNA of the parent cell is therefore copied and each
daughter cell gets a copy.
Source: http://updateyourself-2012.blogspot.com.au/

6. DNA replication.
Steps of DNA replication.
1. DNA helicase unwinds DNA by breaking the weak
hydrogen bonds holding the base pairs together.
2. DNA polymerase attaches to each of the two strands
and pairs each exposed nucleotide with a new
complimentary nucleotide.
 Replication can only proceed in the 3’ to 5’ direction of the
original molecule (i.e adds nucleotides in the 5’ to 3’
direction of the new strand).
 Leading strand = bases added continuously.
 Lagging strand = bases added in short fragments
(Okazaki ) & joined by DNA ligase.

7. 3. When replication is nearing completion, the two new
molecules of DNA separate and become individual
chromosomes.
 The two new molecules of DNA (chromosomes) are
identical to each other.
 DNA replication is semi-conservative.
DNA replication.
Source:
http://www.biologycorner.com/bio1/DNA

8. Source: http://www.biologycorner.com/
DNA replication.
DNA replication animation.

9. Mitosis.
 Occurs in eukaryotic organisms.
 Single celled eukaryotes e.g. paramecium.
 Regions of growth & repair in animals.
 Roots & shoot tips of plants (+ other meristemic tissue).
 Mitotic cell division is used
 To provide new cells for growth.
 Repair and maintenance of tissues.
 Asexual reproduction.

10. Mitosis.
 Occurs in somatic cells.
 Involves one nuclear division.
 Results in two genetically identical daughter cells.
 Both daughter cells are diploid (2n).
 Four stages (although process is continuous).
 Prophase, Metaphase, Anaphase & Telophase.

11. Mitosis.
Source: http://www.britannica.com/EBchecked/topic-art/386154/66085/Stages-of-mitosis

12. Mitosis.
 Interphase:
 Not really part of mitosis [but often included].
 Cell looks like it normally does when not dividing.
 Nuclear membrane present.
 Chromosomes are not clearly visible – chromatin.
 DNA replication occurs (during S stage).

13. Mitosis.
 Prophase:
 Chromosomes condense & become visible (consisting of
two sister chromatids joined at centromere).
 Nuclear membrane breaks down.
 Centrioles move to opposite poles of the cell & extend
spindle fibres across cell.
 Centrioles not present in plants.

14. Mitosis.
 Metaphase:
 Centromeres (point of attachment for the two sister
chromatids) attach to spindle fibres.
 Chromosomes line up along the equator.
 Anaphase:
 Chromatids separate and move to opposite poles (called
chromosomes again from now on).

15. Mitosis.
 Telophase:
 Nuclear membrane reforms around chromosomes.
 Spindle fibres disperse.
 Cell cleavage
 Animal cells – cytoplasm constricts to form two separate cells.
 Plant cells – the rigid cell wall requires that a cell plate be
synthesised between the two daughter cells.
 Interphase:
 Cells return to interphase.

16. Meiosis.
 Occurs in eukaryotic organisms.
 Single celled eukaryotes.
 Gonads of animals.
 Flowers (stamens & pistil) of plants.
 Meiotic cell division is essential for sexual
reproduction
 Preserves genome size post fertilisation of gametes.
 Increases variability within a species.

17. Meiosis.
 Involves two divisions.
 Results in 4 daughter cells – gametes/spores.
 Daughter cells are all haploid (n) – half the number of
parent cell.
 Each division has four stages.
 Prophase I, Metaphase I, Anaphase I & Telophase I.
 Prophase II, Metaphase II, Anaphase II & Telophase II.

18. Meiosis.
Source: http://www.palaeos.com/Fungi/Lists/Glossary/GlossaryM.html

19. Meiosis.
 Interphase:
 Not really part of meiosis [but often included].
 Cell looks like it normally does when not dividing.
 Nuclear membrane present.
 Chromosomes are not clearly visible – chromatin.
 DNA replication occurs (during S stage).

20. Meiosis.
 Prophase I:
 Chromosomes condense & become visible.
 Chromosomes line up together (called synapsis) and
form homologous pairs.
 Crossing over can occur.
 Nuclear membrane breaks down.
 Centrioles move to opposite poles of the cell & extend
spindle fibres across cell.

21. Meiosis.
 Metaphase I:
 Centromeres (point of attachment for the two sister
chromatids) attach to spindle fibres.
 Homologous chromosomes line up along the equator.
 Mendel’s second Law of Independent Assortment.
 Anaphase I:
 Homologous pairs separate and move to opposite poles.

22. Meiosis.
 Telophase I:
 Nuclear membrane reforms around chromosomes.
 Spindle fibres disperse.
 Cytokinesis occurs (or partly occurs).
 Results in two daughter cells, each containing only one
of the homologous pairs of chromosomes (n).
 Interphase:
 Cells return to a brief interphase.
 DNA does not duplicate again.

23. Meiosis.
 Prophase II:
 Chromosomes condense & become visible as two
chromatids joined at centromere.
 Nuclear membrane breaks down.
 Centrioles move to opposite poles of the cell & extend
spindle fibres across cell.

24. Meiosis.
 Metaphase II:
 Centromeres (point of attachment for the two sister
chromatids) attach to spindle fibres.
 Chromosomes line up along the equator.
 Anaphase II:
 Sister chromatids separate and move to opposite poles.

25. Meiosis.
 Telophase II:
 Nuclear membrane reforms around ‘new’ chromosomes.
 Spindle fibres disperse.
 Cytokinesis occurs.
 Final result is four haploid (n) daughter cells.
 In humans – sperm (4 gametes) v ovum (1 egg & 3 polar
bodies).

26. Meiosis.
Crossing over.
 The exchange of equivalent portions of DNA between
homologous chromosomes.
 Occurs during Prophase I.
 Chromatids of the homologs are in close contact (synapsed)
with each other.
 Homologous, non-sister chromatids become entangled and
then exchange segments.
 Crossing over location = chiasma.
 Crossing over can occur at a number of locations (chiasmata)
on a chromosome.
 On average 2-3 crossovers occur per human chromosome.

27. Meiosis.
Crossing over.
Source: http://highered.mcgraw-hill.com/sites/dl/free/0071402357/156709/figure56_4.jpg

28. Meiosis.
Crossing over.
 Allows new combinations of genetic information (called
genetic recombination) and results in increased genetic
variability.
 Without crossing only two kinds of genetically different
gametes are formed, however with crossing over, four
genetically different gametes are formed.
Source: Enger et al. (2012)
Synapsis and crossing over at two locations.

29. Meiosis.
Independent assortment of chromosomes.
 The random orientation of pairs of homologous
chromosomes during Meiosis I.
 Each pair of homologous chromosomes is positioned
independently of the other pairs.
 The number of possible combinations when
chromosomes sort independently = 2n (where n =
haploid number of cell).
 e.g. In humans (n=23)  8.4 million (223) possible
chromosome combinations for each gamete.

30. Meiosis.
Independent assortment of chromosomes.
Source: Reece et al. (2011)

31. Meiosis.
Significance of meiosis.
 Results in the production of gametes, each with the
haploid number (n) of chromosomes.
 Diploid number (2n) is restored when two gametes fuse
during sexual reproduction.
 Provides opportunities for new combinations of gene
alleles to occur in the gamete cells  increased variation
within the species.
 Crossing over (Prophase I).
 Independent assortment of chromosomes (Metaphase I).
 Random combination of gametes during fertilisation.

32. Mitosis v Meiosis.
Mitosis Meiosis
Produces cells for growth
and repair; and in some
species asexual reproduction
Produces gametes, reduces
number of chromosomes by
half and introduces genetic
variability among gametes
One cell division Two cell divisions
Chromosomes line up
individually
Chromosomes line up in
homologous pairs
Produces two diploid (2n)
daughter cells, each
genetically identical from
the parent cell
Produces four haploid (n)
daughter cells, each
genetically different from the
parent cell
Daughter cells are genetically
identical
Daughter cells are not
genetically identical

33. Binary fission.
 Occurs in all prokaryotic & some eukaryotic
organisms.
 Not mitosis!
 Asexual reproduction.
 Prokaryotes have single circular chromosome.
 Chromosome is duplicated and attaches to cell
membrane.
 Cell membrane divides with one strand going into
each daughter cell.

34. Binary fission.
Source: http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect06.htm

35. Apoptosis.
 Cells have a limited life span
 They age and eventually become unable to effectively
carry out their role.
 They become infected or ‘sick’.
 Due to development of the organism, they no longer
have a functional role.
 e.g. full webbing between the fingers & toes of an embryo is
not needed if they are to become independent digits.

36. Apoptosis.
 Cells are therefore pre-programmed to age and die
 Apoptosis = programmed cell death.

37. Apoptosis.
 Controlled by genes that are activated
 During specific stages of development (e.g. a tadpole
losing its tail).
 In response to cell damage or viral invasion.
 During apoptosis the cell is broken down into
fragments and wrapped in a membranes. These
fragments are then engulfed by phagocytes.
 Cancerous cells avoid apoptosis and therefore are able
to grow and reproduce unchecked.

38. Apoptosis.
Source: http://www.microbiologybytes.com/virology/kalmakoff/baculo/baculohostinteract.html