Embryology LEC 2 :
( for images refer to the slides )
Chromosomes :
- Chromosomes are found in the cell as pair of sister chromatid (nucleosomes),it is a long chain of
.DNA(deoxyribonucleic acid),it's protein(amino acid),and the dna is a helix of 2 strands
- the DNA differ from one person to another and from cell to another in the same person
-dna has specific arrangement in each person which is very important
- chromosome consist of:short arm and long arm and there is a sintromer between them
- the number of chrormosmes in each human cell has 46 chromosmes (23 pairs),why do we call them 23
.pairs??because each couple of them are homologues
- the 23 pairs of chrormosomes are:
* 22 pairs autosome
* 1 pair sex chromosome,
- the autosom chromosome indicates the recips of the person
-the sex chrormosome from male we will find it (x,y) and from female we will find it (x,x
- the body contain's million of cells and divided into:
*soumatic cells which are found in all the body
*and the other type is gametes & gonads ,the gonads present in male as the testis and it is represented by
semi nefarious tube and this semi nefarious tube makes the sperms, while in females the gonans
represented by ovary that produce mature ovum ,each cell in the testis or ovary has a constant number
of chromosomes which equals 46 chromosome
- all cells of the body divide direct division called mitosis except the gonads in testis and ovaries that
divide reductive division called meiosis>> (so all cells .. mitosis … gonads meiosis )
- In mitosis the somatic cell divides & gives two cells similar to the mother cell, this means that the two
cells have the same number of chromosomes (46) & the same genetic traits
.
- the importance of mitosis: for example:-when the skin is injured so the injured skin will secrete protein
factory that stimulates the cells that surrounded the injured skin to divide & produce cells that contain
.the same number of chromosomes (46) and the same genetic traits so the injury will recover
another example: when u eat if you harm the mucosa oral cavity so the cells around it divide & produce
simillar cells & repair the damaged cells ,& the function of the new cells is the secretion of salica &
.mucus
- as we know that when skin cells divide, every cell gives 2 cells have 46 chromosomes and have the
.same hereditary genes
- the gonands (sperms,ovary) we called them spermatogonia in the testis or ogonia in the
ovary, the number of chromosomes in these cells are 46 chromosomes, they divide by meiosis to
give us primary spermatocyte and primary ocyte in the ovary, then they divides by mioses from primary
to secondary
- why does they cell divide by mioses in secondary ?
- because each cell we called it primary spermatocyte will divide and give 2 secondary spermatocyte
then each one of the secondary spermatocyte will divide to give 2 spermatocyte,so there will be 4 cells
.called spermatocyte (sperms),each one of these sperms have got 23 single chromosomes
- in males the 23 single chromosomes are 22 autosome and 1 sex chromosome(2 x and 2 y)
- in ovary, the ogonia divide to give primary oocyte then the primary divide into secondary by
meiosis, it gives us 1 ovam and 1 polar body, each one has 23 single chromosome,in seconady
division it gives us 1 mature ovum and 3 polar bodies which will be disintegrate.
- Humans have 35000 genes from the dna on 46 chromosomes
-these genes in somatic cell are 23 pairs and homologues and we call them 2n(amount of dna) that starts
.with 2n and ends with 2n but when a replication happens for dna it will be 4n
- diploid number is 46 chromosomes equals to 2n
- haploid number in gametes is 23 chromosomes equals to 1n
Cell Division :
- when a cell is in the interphase and we stimulate it to divide , it goes to prophase then metaphase then
.anaphase then telophase then goes back to interphase
- in interphase we have got: G1,S,G2 , G0 periods
G1
8-10 hours ,cell is metabolically active, duplication of organelles and replication
of centrosome
S
DNA is duplicated so the 2n will bs 4n
G2
4-6 hours,growth continues, enzyme & other proteins are
synthesized……..centrosome replicated & completed , while the replicaton of
centrosome begins in G1
G0
when the division is completed……the cell inters the interphase
- interphase: occurs between the division & it's the longest part of the cycle & occurs in stages
The comparesion between mitosis & meiosis:
- first : mitoisis :
*it has periods : prophase , metaphase ,anaphase & telophase .....
.* in the end it gives us daughter cell of mitosis wich has the same genetic recipes & the same number of
chromoromes 46 <2n>.......for example the cells in skin & stomach
- secondly: meiosis:
*has the same phases of mitosis.....but it is divided into two stages :1-meiosis(1):starts with 2n cell &
)ends in two haploid cells(1n
*meiosis(2):it's exactly same as mitosis ......here each cell gives two cells have the same number of
chromosomes wich is 23 single chromosome..& the meiosis 2 ends with 4 cells each of them called
daughter cell of meiosis 2 & each of them 1n
* in male: the 4 cells are sperms or we'll call them spermatid or spermatozoma when we study the
spermatogenisis in the next lecture
* in female : the 4 cells are 1 mature ovum and 3 polar bodies which will be disintegrate.
*in prophase(in mitosis): the chromosomes begin to coil, contract & condense ......at the beginning of
the prophase each chromosome consists of two parallel subunits called sister chromatid wich are joint in
centromere , through out the prophase the chromosomes continue to coil ,shorten & condesnse
* in metaphase: the chromosomes become as a row in the middle of the cell & each centriole goes to one
of the poles of the cell...........& become here the bedallar base wich pulls the chromosomes so each
chromosome is divided into its two subunits(sister chromatid) & each one of the two poles of the cell...&
the centromere is divided into two halves … and the microtubule extends from centromere to the
.......centriole forming mitotic spindle
* anaphase( the dr didin't say anything about it )
* telophase :in the end....we wil have cytokinesis , constriction in cytoplasm wich is mean that the
cytoplasm is divided into two halves....& the nuclear envelope that has experienced the disappearance
back surrounding the nucleus, each daughter cell recieves half of all doubled chromosome's material &
DNA... &maintain the same number of chromosomes & then back 2n (46 chrormosomes)
**meiosis***
- is the cell division that occurs in germ cell to generate male & female gametes wich are called sperms
& mature ovum
- the meiosis required two cell division (meiosis 1 & meiosis 2) to reduce the number of the
chromosomes into the half(23 chromosomes).
in another words , the "goal " of meiosis is to produce sperms & mature ovum have the haploid number
of chromosomes (1n)
-at the beginning the number of chromosomes in gonads is 46
- in contrast to mitosis :the homologous chromosomes aline themselves into pairs wich is called
.........synapses...
- when meiosis 1 begins the homologous chromosomes will align themselves side by side & we call this
)(pairing
- after that occurs wich called synapses or crossover… &
- crossover : one of the chromatian of one of the homologous chromosomes cross the other
chromatian of the other chromosome) this crossing so important. our god makes this crossing to
make the new baby has different recipes than the mother & father
- so the crossover is critical event in meiosis 1 because in crossover occurs interchange between
chromated segments & their homologous chromosomes & when these interchanges in DNA mean
that the arrangement of DNA is confused
- crossover is also known as : chiasma formation
- approximately 30-40 crossover 1 or 2 pair chromosome for each meiotic division are mostly frequent
.......between genes that are far apart only chromosomes
- in meiosis (1): each cell gives two cells that have the haploid number of chromosomes(23)......
- .in males: one of the two cell would be X & the other [(Y)] ...
- while in females :one of them (mature ovum) & the other is polar body
- meiosis (2):passes in the same phases of mitosis but here the DNA replication won't occur , but each
one of chromatin converted into chromosome , so the number of chromosomes still 23 chromosomes but
in the end each one of the two cells(that are produce from meiosis 2) will give two cells , so we will
have 4 cells each of them has 23 single chromosome
**the results of meiosis **
- in males:X X Y Y
- in females: 3 polar bodies & 1 mature ovum that has X chromosome--- genetic variability (is a result of crossover which is redistribute genetic material) & this is an important
point
- genetic variability is also enhanced through the random distribution of homologous chromosomes &
to the daughter cell
- so each germ cell has a haploid number of chromosomes & in fertilization when the sperm comes with
mature ovum & then it back to the diploid number of chromosomes
***note: please refer to the table in the slides to read the differences between mitosis & meiosis***
*****chromosomal abnormality*****
- occurs in number or structure
- if any of chromosomal ubnormality occurs ......when the sperm fertilizes the mature ovum, in the most
cases abortion accurs
- of the spontaneous abortion (which presents 50 % of the abortion)......is caused by chromosomal 25%
defects
- there's is many cases pregnancy continue even if there is a chromosomal abnormality & there's kinds of
abnormalities :
*disjunctional chromosomes:
the seperation between the two chromatids of the homologous chromosomes won't occur so we call this
chromosome <disjunctional chromosome> & the result of this chromosome that one cell has more one
chromosome & the other cell has less one chromosome............examples :trisomy syndrome (down) &
turner syndrome.
* dawn syndrome: (the mongolism)
on number 21 of chromosomes occurs non-disjunction so we will find a cell has 24 chromosome & the
other has 22 chromosome & if the sperm has the 24 chromosome or the mature ovum & makes the
fertilization with another normal gamete so the produced zygote will have 47 chromosomes & this is the
mongolism.
*but if the gamete that has the 22 chromosomes makes the fertilization with another normal gamete so
the produced zygote will have 45 chromosomes & this is abnormal too but this zygote will die rapidly
- normal somatic cell: has the diploid number of chromosomes (2n)
- normal gametes:have haploid number of (1n)
some terminology :
- euploid: refers to any exact multiple of (n) like:diploid,triploid.......etc....
"- aneuploid: refers to the number of chromosomes that's not "euploid**
it's usually applied when extra chromosomes present
- trisomy : the chromosomes are increased by 1 chromosome
- monosomy : the chromosomes decreased by 1 chromosome
- non-disjunction can occur in meiosis"1" or meiosis"2 or in mitosis also may occur in autosome
or sex chromosome but the important one is meiosis in the formation of embryo
- so …. as a result of any syndromes , we will find 22 or 24 chromosome
- when any of them fuses with the gametes (cell) that has 23 chromosome the resulting cell (zygote) &
)will be 45 chromosome (monosomy) or 47 chromosome (trisomy
- after the 35 years from the woman's age the possibility of chromosomal abnormalities increases in the
embryo
***translocation***
- the gene could translocate from one chromosome to another but without the lost of genetic material
‫ال تؤثر كثيرا‬
- unbalanced translocation between the long arm of chromosome 14 and 21 during meiosis 1 or 2
produce gametes with an extra copy of 21 , which is one of the cause of down syndrome else than the
non-disjunction.
" there is in the book some clinical abnormality , but the Dr said that we have to know the following
things in the abnormality of chromosomes :
1- name of syndrome
2- the place of the defect ( the num of chromosome )
3- number of chromosomes after the syndrome
4- some clinical features
e.g :
the down syndrome :
- may happened during meiosis or mitosis , caused by disjunction of chromosome number 21
-monosmic cell ( which is 21 chromosome = 2n-1 ) and in this cause the embryo will die.
- features : flat face and small , fold of skin on the angle of the eye , lower lip is thick , mental retarded ,
heart abnormality.
notice : look in the slides for other syndromes cause u need to see the pictures and remember the 4
points that u need to know 