1. Unit 5: Cellular & Organismal Reproduction
Topic 1: The Cell Cycle

2. Cell reproduction How do cells reproduce? When does a cell reproduce?
They divide into two cells!
First they copy all their parts (DNA, organelles, ribosomes)
When a multicellular organism is growing or needs to replace dead or damaged cells (mitosis)
To make gametes for sexual reproduction in eukaryotes (meiosis)
When a single-celled organism reproduces asexually (mitosis or binary fission)

3. Some Definitions Parent Cell Daughter Cell Sexual Reproduction
Variation
Asexual Reproduction
A parent cell is a cell that is getting ready to divide. It has completed DNA replication and copied its organelles.
When a parent cell divides, it splits into 2 daughter cells. These are more or less identical to the parent cell.
When we talk about reproduction of organisms, we distinguish between sexual reproduction and asexual reproduction. Sexual reproduction occurs between two individuals of a species. They combine special cells called gametes together. This results in a mixing of DNA. An advantage of sexual reproduction is the creation of genetic variation. Each time 2 individuals reproduce, a unique combination of genes is formed.
Asexual reproduction is based purely on cell division. Our cells reproduce asexually every day. In terms of organisms, some can create offspring on their own through asexual reproduction. An advantage of asexual reproduction is an organism does not have to expend energy finding a mate. A disadvantage is that all the offsprings are clones of the parents. 3

4. Asexual reproduction in bacteria (binary fission) Binary Fission
Unit 5 - Reproduction (Topics 3-4)
Asexual reproduction in bacteria (binary fission)
Binary
Fission 4
Figure 8.3A, p127 4

5. Other forms of asexual reproduction
Budding
Fragmentation
Regeneration
Advantages:
Don’t need to search for a mate
Can reproduce if damaged
Disadvantage:
No recombination of genetic material

6. Chromatin Unit 5 - Reproduction (Topics 3-4)
During most of the life of a cell, the DNA is not uniformly organized. It is stored safely in the nucleus as chromatin, which is a combination of DNA and histone proteins. Sometimes the chromatin is tightly coiled in some places and relaxed in others. Why does this happen and what are the consequences for the genes? (methylation and acetylation…turns genes on and off. On when relaxed because the transcription enzyme can access the gene, off when coiled because the gene is not accessible to the transcription enzyme.
(CLICK) Before a cell reproduces, DNA is packaged into condensed structures called chromosomes. Human DNA is packaged into 46 chromosomes (or 23 pairs). Almost all cells in the human body contain a full set of 46 chromosomes in the nucleus, and the chromosomes from cell-to-cell in a person are identical. Each set includes 44 autosomes and 2 sex chromosomes. All together these are called the genome. 6 6

7. Unit 5 - Reproduction (Topics 3-4)
You can see all 46 chromosomes in the karyotype here and in your packet. This is a picture of all the chromosomes in a cell. The chromosomes are taken from a part of the cell cycle in which the DNA has been replication in preparation for reproduction. We refer to the chromosomes as replicated chromosomes.
The replicated chromosomes in this karyotype are in homologous pairs. This means that one chromosome contains DNA with a specific set of genes inherited from one parent and the other chromosome contains the same set of genes, but inherited from the other parent. 7 7

8. Fun facts: number of chromosomes in a variety of organisms
Unit 5 - Reproduction (Topics 3-4)
Fun facts: number of chromosomes in a variety of organisms
Species
# chromosomes
Humans 46
Kangaroo 12
Cat 38
Cow 60
Dog 78
Turkey 82
Algae
148
Shrimp
254
Ophioglossum reticulatum (fern)
~1200! (highest known) 8

9. Each replicated chromosome is comprised of 2 sister chromatids, joined at the centromere. The two sister chromatids of a replicated chromosome are identical.

10. UNreplicated Chromosome
Unit 5 - Reproduction (Topics 3-4)
UNreplicated Chromosome
Replicated Chromosome
Sister Chromatids
A single, unreplicated chromosome is a condensed string of chromatin. This rarely exists in a cell at any given time, unless all the genes are turned off (this is the case only for one of the two x chromosomes in females).
A replicated chromosome consists of two identical strings of chromatin that are condensed and attached at a location called the centromere. Each condensed strand is referred to as a sister chromatid. Replicated chromosomes are present in a cell when it is getting ready to reproduce (split into two cells). During cell division, the sister chromatids will split apart and each will go into a different daughter cell.
A homologous chromosome pair consists of two chromosomes, each with the same types of genes. One of the chromosomes was inherited from the mother and the other was inherited from the father.
UNreplicated Chromosomes 10
Figure 8.4, p128 10

11. Chromosome Anatomy 11

12. Homologous chromosomes
Unit 5 - Reproduction (Topics 3-4)
Homologous chromosomes
Each chromosome has a homologue, or a chromosome that carries the same type of information as another chromosome
The chromosomes may have different versions of the genes but the genes code for the same type of information 12 12

13. Cell cycle Figure 8.5, p129 Unit 5 - Reproduction (Topics 3-4)
Interphase is the period of a cell’s lifespan that lasts the longest. During interphase, the cell does all its normal functions. During G1, the first stage of interphase, the cell grows while also doing its work. It copies its organelles and grows by adding on to its cell membrane.
During S phase the cell continues to grow and do its work, but it also replicates its DNA.
During G2, the last stage of interphase, the cell grows more and gets ready for division, which is called mitosis. Mitosis can be described in four major phases. 13
Figure 8.5, p129 13

14. Mitosis inquiry

15. Cell cycle Figure 8.5, p129 Unit 5 - Reproduction (Topics 3-4)
Interphase is the period of a cell’s lifespan that lasts the longest. During interphase, the cell does all its normal functions. During G1, the first stage of interphase, the cell grows while also doing its work. It copies its organelles and grows by adding on to its cell membrane.
During S phase the cell continues to grow and do its work, but it also replicates its DNA.
During G2, the last stage of interphase, the cell grows more and gets ready for division, which is called mitosis. Mitosis can be described in four major phases. 15
Figure 8.5, p129 15

16. Unit 5 - Reproduction (Topics 3-4)
The stages of mitotic cell division in an animal cell: G2 phase; prophase; prometaphase
In prophase and prometaphase (which we can lump together as prophase) the chromatin condenses into replicated chromosomes. The nuclear membrane/envelope breaks apart and the centrosomes (pairs of centrioles) move to opposite poles of the cell and make spindle microtubules, some of which connect to the kinetochores (protein structures on the outsides of the centromeres) of the sister chromatids and some of which stretch across to the opposite centrosome. 16
Figure 8.6, p130 16

17. Unit 5 - Reproduction (Topics 3-4)
The stages of mitotic cell division in an animal cell: metaphase; anaphase; telophase and cytokinesis.
In metaphase, the chromosomes have been lined up in middle of the cell by the spindle microtubules. Sister chromatids of replicated chromosomes are facing opposite poles of the cell.
In anaphase, the sister chromatids are pulled apart from each other. Now we just call them unreplicated chromosomes. The spindle microtubules pull the unreplicated chromosomes towards the poles of the cell. This happens because the centrosome shortens the microtubules. Meanwhile, the microtubules that are not attached to chromosome grow longer, pushing the poles further apart.
Telophase and cytokinesis occur simultaneously. Telophase is basically the opposite of prophase. The nuclear membrane/envelope reforms and the chromosomes begin to loosen up a little bit. Technically mitosis is the division of genetic material. Cytokinesis is the division of the cytoplasm and its contents. Cytokinesis happens gradually, with the cell membrane pinching inward forming a cleavage furrow. Eventually the parent cell is pinched into two daughter cells. 17
Figure 8.6, p131 17

18. Mitosis
Unit 5 - Reproduction (Topics 3-4) 18 18

19. Mitosis in an onion root
Unit 5 - Reproduction (Topics 3-4)
Can you find a metaphase? Early telophase? Late telophase? 19 19

20. Cytokinesis in animal and plant cells
Unit 5 - Reproduction (Topics 3-4)
Cytokinesis in animal and plant cells 20
Figure 8.7, p132 20

21. Cell turnover
The length of a cell cycle depends on the type of cell and its function in the body.
Some cells never divide after the first few months of life: brain, nerves
Some cells never divide at all: red blood cells
Cells that never divide after development enter the G0 phase after they are created.
Some divide every 20 hours or so: cells of organ linings and skin cells

23. Unit 5 - Reproduction (Topics 3-4)
Chromosomes 23 23