ntry points of glucogenicamino acids after transamination are indicated by arrows extended from circles the key gluconeogenicenzymes are enclosed in double bordered boxes. oated fashion. They are interdependent; each forms a strand in the web of life. Parasitology is the science that deals with organisms living in the human body (the host) and the medical significance of this host-parasite relationship. ASSOCIATION BETWEEN PARASITE AND HOST A parasite is a living organism, which takes its nourishment and other needs from ahost; the host is an organism which supports the parasite. The hosts vary depending on whether they harbor the various stages in parasitic development DIFFERENT KINDS OF PARASITES Ectoparasite – a parasitic organism that lives on the outer surface of its host, e.g. lice, ticks, mites etc. Endoparasites parasites that live inside the body of their host, e.g. Entamoeba histolytica. Obligate Parasite- This parasite is completely dependent on the host during a segment or all of its life cycle, e.g. Plasmodium spp. Facultative parasite – an organism that exhibits both parasitic and non-parasitic modes of living and hence does not absolutely depend on the parasitic way of life but is capable of adapting to it if paced on a host. E.g. Naegleria fowleri Accidental parasite – when a parasite attacks an unnatural host and survives. E.g. Hymenolepis diminuta (rat tapeworm). Erratic parasite - is one that wanders in to an organ in which it is not usually found. E.g. Entamoeba histolytica in the liver or lung of humans. Most of the parasites which live in/on the body of the host do not cause disease (non-pathogenic parasites). In Medical parasitology we focus on most of the disease causing (pathogenic) parasites. However, understanding parasites which do not ordinariy produce disease in heathy (immunocompetent) individuals but do cause illness in individuals with impaired defense mechanism (opportunistic parasites) is becoming of paramount importance because of the increasing prevaence of HIV/AIDS in our country. DIFFERENT KINDS OF HOSTS Definitive host – a host that harbors a parasite in the adult stage or where the parasite undergoes a sexual method of reproduction. Intermediate host - harbors the arval stages of the parasite or an asexual cycle of development takes pace. In some cases, larval development is completed in two different intermediate hosts, referred to as first and second intermediate hosts. Paratenic host – a host that serves as a temporary refuge and vehicle for reaching an obligatory host, usually the definitive host, i.e. it is not necessary for the completion of the parasites life cycle. Reservoir host – a host that makes the parasite available for the transmission to another host and is usually not affected by the infection. Natural host a host that is naturally infected with certain species of parasite. Accidental host – a host that is under normal circumstances not infected with th

1. Meiosis
Meiosis is the process of cell division in which gametes are formed and the number
of chromosomes is halved. So that sexual reproduction and zygote formation can
occur.
Zygote- Fertilized egg which has a diploid number of chromosomes.

2. Meiosis
• Sexual reproduction consists of two processes
• The first is meiosis, which leads to gametes in which chromosome number is
reduced by half
• The second process is fertilization, in which two haploid gametes fuse and
restore chromosome number to its original diploid value
• Meiosis and mitosis differ in the following ways:
• Mitosis consists of a single nuclear division and is usually accompanied by a
single cell division
• Meiosis, on the other hand, consists of two divisions
• After mitosis, chromosome number in newly formed cells is the same as that in
the original cell, whereas meiosis causes chromosome number in the newly
formed cells to be reduced by half
• Finally, mitosis produces genetically identical cells, whereas meiosis produces
genetically variable cells. Let’s see how these differences arise
• Like mitosis, meiosis is preceded by an interphase stage that includes G1, S, and
G2 phases
• Meiosis consists of two distinct processes: meiosis I and meiosis II, each of
which includes a cell division

3. Meiosis
• The first division, which comes at the end of meiosis I, is termed the reduction
division because the number of chromosomes per cell is reduced by half
• The second division, which comes at the end of meiosis II, is sometimes termed
the equational division
• The events of meiosis II are similar to those of mitosis
• However, meiosis II differs from mitosis in that chromosome number has already
been halved in meiosis I, and the cell does not begin with the same number of
chromosomes as it does in mitosis

5. Meiosis I
Interphase-
During interphase, the chromosomes are relaxed and visible as diffuse
chromatin
Chromosomes replicate
Each chromosome consists of 2 identical sister chromatids
Prophase I
• This is a lengthy stage, divided into five sub-stages
• In leptotene, the chromosomes contract and become visible
• In zygotene, the chromosomes continue to condense; homologous
chromosomes pair up and begin synapsis, a very close pairing association
• Each homologous pair of synapsed chromosomes consists of four chromatids
called a bivalent or tetrad
• In pachytene, the chromosomes become shorter and thicker, and a three-part
synaptonemal complex develops between homologous chromosomes
• The function of the synaptonemal complex is unclear, but the chromosomes of
many cells deficient in this complex do not separate properly
• Crossing over takes place in prophase I, in which homologous chromosomes
exchange genetic information

6. Prophase I
• The centromeres of the paired chromosomes move apart in diplotene; the two
homologs remain attached at each chiasma (plural, chiasmata), which is the
result of crossing over
• In diakinesis, chromosome condensation continues, and the chiasmata move
toward the ends of the chromosomes as the strands slip apart; so the homologs
remain paired only at the tips
• Near the end of prophase I, the nuclear membrane breaks down and the
spindle forms, setting the stage for metaphase I

7. Stages of prophase I

8. Metaphase I
• This is initiated when homologous pairs of chromosomes align along the
metaphase plate
• A microtubule from one pole attaches to one chromosome of a homologous
pair, and a microtubule from the other pole attaches to the other member of
the pair
Anaphase I
• This is marked by the separation of homologous chromosomes
• The two chromosomes of a homologous pair are pulled toward opposite poles
• Although the homologous chromosomes separate, the sister chromatids
remain attached and travel together
• Spindle fibers attach to the centromeres of each pair.
• Now each cell will get one chromosome from each homologous pair.

9. Telophase 1
• The chromosomes arrive at the spindle poles and the cytoplasm divides
• Spindle fibers break down
• Chromosomes uncoil
• Cytoplasm divides
• Another cell division is needed because the number of
chromosomes has not been reduced
• After telophase I there maybe a short interphase, but not always. It
is important to note that if a cell does have a second interphase,
there is No replication of chromosomes.

11. Meiosis II
• The period between meiosis I and meiosis II is interkinesis, in which the nuclear
membrane re-forms around the chromosomes clustered at each pole, the
spindle breaks down, and the chromosomes relax
• These cells then pass through prophase II, in which the events of interkinesis
are reversed: the chromosomes recondense, the spindle re-forms, and the
nuclear envelope once again breaks down
• In interkinesis in some types of cells, the chromosomes remain condensed, and
the spindle does not break down
• These cells move directly from cytokinesis into metaphase II, which is similar to
metaphase of mitosis: the individual chromosomes line up on the metaphase
plate, with the sister chromatids facing opposite poles
• In anaphase II, the kinetochores of the sister chromatids separate and the
chromatids are pulled to opposite poles
• Each chromatid is now a distinct chromosome
• In telophase II, the chromosomes arrive at the spindle poles, a nuclear envelope
re-forms around the chromosomes, and the cytoplasm divides
• The chromosomes relax and are no longer visible

16. 1. Outline the processes that takes place in prophase 1 of meiosis (8 marks)
1. Define aneuploidy and give the syndromes that results from it (8 Marks)
2. Using a well labelled diagram, describe the elements of a functional structure of
chromosomes (8 Marks)