1. By
Dr Samina Anjum

2. CELL DIVISION
Is the process by which a parent cell divides into two or more daughter cells.

3. CHROMOSOMES
Are structures that transmit genetic information to next generation.

4. Chromatid
Two copies of the same chromosome attached together
Centromere
Is the primary constriction where the sister chromatids are attached

5. KINETOCHORE
Is the protein structure that assembles on the centromere and attach sister chromatids to mitotic spindle; that move chromosomes during mitosis & meiosis.

6. KARYOTYPE
A karyotype is the complete set of chromosomes from an individual which can be compared to a "normal" Karyotype for the species via genetic testing.
It describes the number of chromosomes, and what they look like under a light microscope.

7. CELL CYCLE
The cell cycle is an ordered set of events, culminating in cell growth and division into two daughter cells. Non-dividing cells not considered to be in the cell cycle.

8. MITOSIS
Is nuclear division plus cytokinesis, and produces two identical daughter cells
Mitosis occurs in all somatic cells ---diploid (2n) cells

9. prometaphase

11. MEIOSIS
Meiosis is the cell division that takes place in germ cells only.
Requires two cell divisions
Diploid germ cells give rise to haploid (n) gametes.

12. Primary oocyte or primary spermatocyte
46 double structured chromosomes
Meiosis I
Meiosis I separates homologous chromosomes, producing two haploid cells (23 chromosomes, N in humans), so meiosis I is referred to as a reductional division. A regular diploid human cell contains 46 chromosomes and is considered 2N because it contains 23 pairs of homologous chromosomes. However, after meiosis I, although the cell contains 46 chromatids it is only considered as being N, with 23 chromosomes, because later in anaphase I the sister chromatids will remain together as the spindle pulls the pair toward the pole of the new cell. In meiosis II, an equational division similar to mitosis will occur whereby the sister chromatids are finally split, creating a total of 4 haploid cells (23 chromosomes, N) per daughter cell from the first division.
During prophase I, DNA is exchanged between homologous chromosomes in a process called homologous recombination. This often results in chromosomal crossover. The new combinations of DNA created during crossover are a significant source of genetic variation, and may result in beneficial new combinations of alleles. The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. At this stage, non-sister chromatids may cross-over at points called chiasmata plural; singular chiasma).
Prophase I
Leptotene
The first stage of prophase I is the leptotene stage, from Greek words meaning "thin threads“. During this stage, individual chromosomes begin to condense into long strands within the nucleus. However the two sister chromatids are still so tightly bound that they are indistinguishable from one another.
Zygotene
The zygotene stage, from Greek words meaning "paired threads“, occurs as the chromosomes approximately line up with each other into homologous chromosomes. This is called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. At this stage, the synapsis (pairing/coming together) of homologous chromosomes takes place.
The pachytene stage, from Greek words meaning "thick threads",[1] contains the following chromosomal crossover. Nonsister chromatids of homologous chromosomes randomly exchange segments of genetic information over regions of homology. (Sex chromosomes, however, are not wholly identical, and only exchange information over a small region of homology.) Exchange takes place at sites where recombination nodules (the aforementioned chiasmata) have formed. The exchange of information between the non-sister chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope.
Pachytene
Diplotene
During the diplotene stage, from Greek words meaning "two threads“, the synaptonemal complex degrades and homologous chromosomes separate from one another a little. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed in Anaphase I.
In human fetal oogenesis all developing oocytes develop to this stage and stop before birth. This suspended state is referred to as the dictyotene stage and remains so until puberty. In males, only spermatogonia (spermatogenesis) exist until meiosis begins at puberty.
Diakinesis
Chromosomes condense further during the diakinesis stage, from Greek words meaning "moving through“. This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form.
Synchronous processes
During these stages, two centrosomes, containing a pair of centrioles in animal cells, migrate to the two poles of the cell. These centrosomes, which were duplicated during S-phase, function as microtubule organizing centers nucleating microtubules, which are essentially cellular ropes and poles. The microtubules invade the nuclear region after the nuclear envelope disintegrates, attaching to the chromosomes at the kinetochore. The kinetochore functions as a motor, pulling the chromosome along the attached microtubule toward the originating centriole, like a train on a track. There are four kinetochores on each tetrad, but the pair of kinetochores on each sister chromatid fuses and functions as a unit during meiosis I.
Microtubules that attach to the kinetochores are known as kinetochore microtubules. Other microtubules will interact with microtubules from the opposite centriole: these are called nonkinetochore microtubules or polar microtubules. A third type of microtubules, the aster microtubules, radiates from the centrosome into the cytoplasm or contacts components of the membrane skeleton.
Metaphase I
Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line.
Anaphase I
Kinetochore microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome has only one functional unit of a pair of kinetochores[3], whole chromosomes are pulled toward opposing poles, forming two haploid sets. Each chromosome still contains a pair of sister chromatids. Nonkinetochore microtubules lengthen, pushing the centrioles farther apart. The cell elongates in preparation for division down the center.
Telophase I
The last meiotic division effectively ends when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. Sister chromatids remain attached during telophase I.
Cells may enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage.
23 double structured chromosomes

13. SPECIAL EVENTS IN MEOSIS
Pairing of homologous chromosomes length wise is called synapsis. Pairing is exact and point to point except for X & Y chromosome
Cross overs or interchange of chromatid segments between paired homologous chromosomes
Chiasma formation: As homologous chromosomes separate points of interchange are temporarily united and form an X like structure called chiasma.
dyad/tetrad

15. No DNA synthesis or replication
Meiosis II
Meiosis II is the second part of the meiotic process. Much of the process is similar to mitosis. The end result is production of four haploid cells (23 chromosomes, 1N in humans) from the two haploid cells (23 chromosomes, 1N * each of the chromosomes consisting of two sister chromatids) produced in meiosis I. The four main steps of Meiosis II are: Prophase II, Metaphase II, Anaphase II, and Telophase II.
Prophase II takes an inversely proportional time compared to telophase I. In this prophase we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division.
In metaphase II, the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes (centrioles) at each pole. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate.
This is followed by anaphase II, where the centromeres are cleaved, allowing microtubules attached to the kinetochores to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles.
The process ends with telophase II, which is similar to telophase I, and is marked by uncoiling and lengthening of the chromosomes and the disappearance of the spindle. Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes. Meiosis is now complete and ends up with four new daughter cells.
23 single structured chromosomes

16. SIGNIFICANCE OF MEIOSIS:
Provides constancy of the chromosome number from generation to generation by reducing the chromosome number from diploid to haploid, thereby producing haploid gametes.
Allows random assortment of maternal and paternal chromosomes between the gametes.
Relocates segments of maternal and paternal chromosomes by crossing over of chromosome segments, which "shuffles" the genes and produces a recombination of genetic material.
Meiosis facilitates stable sexual reproduction. Without the halving of ploidy, or chromosome count, fertilization would result in zygotes that have twice the number of chromosomes as the zygotes from the previous generation. Successive generations would have an exponential increase in chromosome count. In organisms that are normally diploid, polyploidy, the state of having three or more sets of chromosomes, results in extreme developmental abnormalities or lethality [4]. Polyploidy is poorly tolerated in most animal species. Plants, however, regularly produce fertile, viable polyploids. Polyploidy has been implicated as an important mechanism in plant speciation.
Most importantly, recombination and independent assortment of homologous chromosomes allow for a greater diversity of genotypes in the population. This produces genetic variation in gametes that promote genetic and phenotypic variation in a population of offspring.

17. CHROMOSOMAL ABNORMALITIES

18. INCIDENCE FOR MAJOR CHROMOSOMAL ABNORMALITIES
50% of conceptions end in spontaneous abortions and 50% of these abortions have major chromosomal abnormalities
Thus approx. 25% of conceptuses have major chromosomal defects
Chromosomal abnormalities account for 7% of major birth defects; Commonest is Turner’s syndrome
Gene mutations account for an additional 8% cases

19. Ploidy: Is the number of sets of chromosomes in a biological cell
Haploid = n (in normal gametes)
Diploid=2n (in Normal somatic cell)
Euploid = An exact or multiple of n or of the monoploid number.
e.g. ( 2n, 3n,4n etc)
Polyploid=A chromosome number that is a multiple of haploid number of 23 other than the diploid number eg. 69 i.e. more than two sets of homologous chromosomes.
Polyploidy refers to a numerical change in the whole set of chromosomes.
Aneuploidy =Is any chromosome number that is not euploid
Aneuploidy refers to a numerical change in part of the chromosome set

20. ANEUPLOID
Aneuploidy is an abnormal number of chromosomes such as having a single extra chromosome (47), or a missing chromosome (45).
Aneuploid (not good) karyotypes are given names with the suffix -somy (rather than -ploidy, used for euploid karyotypes), such as trisomy and monosomy.

21. CHROMOSOMAL ABNORMALITIES
Can occur during meiotic or mitotic divisions
Two types:
Numerical
Structural

22. NUMERICAL CHROMOSOMAL ABNORMALITIES
Meiotic Non disjunction
Mitotic Non disjunction
Chromosomal translocations

23. MEIOTIC NON DISJUNCTION
May involve autosomes or sex chromosomes
In females incidence increases with age 35yrs or more.
Meiosis I: Two members of homologous chromosomes fails to separate and both members of a pair move into one cell.
Meiosis II: When sister chromatids fail to separate.

25. MITOTIC NONDISJUNCTION
Mosaicism:
Some cells have abnormal chromosomal number and others have normal
Occurs in the earliest cell divisions
Affected individuals exhibit characteristics of a particular syndrome for e.g. Down syndrome -1% cases

26. CHROMOSOMAL TRANSLOCATIONS
When a portion of one chromosome is transferred to another non homologous chromosome, a fusion gene is created.
There are two main types of translocations:
Balanced: An even exchange of material with no genetic information is extra or missing, and individual is normal.
Unbalanced: Where the exchange of genetic material is unequal and part of one chromosome is lost & altered phenotype is produced.

27. BALANCED TRANSLOCATION
If no genetic material is lost during the exchange, the translocation is considered to be a balanced translocation.

28. UNBALANCED TRANSLOCATIONS
An entire chromosome has attached to another at the Centromere
long q arms of two chromosomes (14 & 21) become joined at a single centromere.
Unbalanced translocation can occur during meiosis I or meiosis II in 4% cases of Down syndrome.
s particular translocation is interesting because 28

29. DOWN’S SYNDROME Causes: Meiotic nondisjunction -95% (trisomy 21)
Unbalanced translocation-4% b/w 21 and 13,14,15
Mosaicism due to mitotic non dysjunction-1%
Incidence:
Female under : 2000
At : 300
At :40

32. KLINEFELTER’S SYNDROME
Have 47 chromosomes (XXY) & a sex chromatin Barr body or 48(XXXY); more the number of X more the chances of mental impairment
Cause:
Non disjunction of XX homologue
Found only in males, detected at puberty
Incidence ---1 in 500 males
S/S
Sterility, testicular atrophy,
hyalinization of seminiferous tubules, gynecomastia.

33. TURNER SYNDROME 45 X karyotype Only monosomy compatible with life
Cause
Non disjunction in male gamete
Structural abnormalities of X chromosome
One X chromosome is missing
Mitotic non disjunction

34. STRUCTURAL CHROMOSOMAL ABNORMALITIES
Occur when the chromosome's structure is altered, this can take several forms: Translocation, deletion or duplication of chromosomes
Chromosome breaks occur either as a result of damage to DNA (by radiation or chemicals) or as part of the mechanism of recombination.
However, the total number of chromosomes is usually normal.

35. CHROMOSOMAL DELETION A part of a chromosome is missing or "deleted."
Breaks are caused by environmental factors
A very small piece of a chromosome can contain many different genes.
When genes are missing, "instructions" are missing resulting in errors in the development of a fetus.

36. CRI-DU-CHAT SYNDROME Partial deletion of chromosome 5 S/S
High pitched cat like cry, a small head size , low birth weight, mental retardation and congenital heart disease.

37. ANGELMAN’S SYNDROME
Micro deletion (span few contiguous genes) on long arm of chromosome 15.
Inherited on maternal chromosome
S/S
Mentally retarded,
Cannot speak
Prolonged periods of laughter

38. PRADER-WILLI SYNDROME
Micro deletion occurs on long arm of chromosome 15
Inherited on paternal chromosome
S/S
Obesity
Mental retardation
Hypogonadism
Cryptorchidism

39. FRAGILE X SYNDROME
Fragile X is a genetic disorder that is caused by a break or weakness on the long arm of the X chromosome.
Syndrome occurs in 1:5000 individuals with males affected more than females.
Is the 2nd most common inherited cause of mental retardation due to chromosomal abnormalities
S/S
Mental retardation, large ears, prominent jaw and pale blue irises

40. Thank you