Cell division Flashcards
Why do cells need to divide
For growth, reproduction and for replacement of dead cells
What are the two types of chromatin
Euchromatin and Heterochromatin
What is Euchromatin
Relatively uncondensed chromatin associated with active (expressed genes)
What is heterochromatin
Condensed chromatin associated with repetitive gene poor regions that are inactive (silenced)
What is a centromere
Region of a chromosome where two sister chromatids are held together
What are alpha satellites
Arrays of repeated sequences in centromeres of human chromosomes
What is the kinetocore
A protein structure located at the centromere that serves as an attachment point for mitotic spindles
How are chromosomes categorised based on centromere position
Metcentric (central centromere), submetacentric (centromere more towards one end), arcocentric (really short p arms/ top half of chromosome), telocentric (really long a arms/no p arms/ centromere at top)
What are telomeres
Specialised regions at the ends of chromosomes
What are the 2 functions of telomeres
1) Enable cells to distinguish a real chromosome end from an unnatural chromosome end caused by a chromosome break
2) solve the end replication problem
What is the end replication problem
During DNA replication the 3’ terminal portion of the chromosome cannot be copied resulting in a shortened chromosome and progressive shortening over successive replication cycles
What is the repeated sequence that makes up telomeres
5’- TTAGGG-3’
How do telomeres resolve the end replication problem
Uncopied region of the chromosome is recognised by the enzyme telomerase which adds multiple copies of ‘TTAGGG’ repeat (telomere sequence) to the template strand so the new DNA can be synthesised by replication machinery in its entirety
How many chromosomes does the human genome have
46 (23 pairs)
What are homologous chromosomes
Pairs of chromosomes that carry the same set of genes
What is a karyotype
Image of all chromosomes acquired through isolation and treatment of cells in anaphase
How are chromosomes seen in a karyotype
Opened and stained in anaphase to produce a banding pattern on the chromosomes, which enables homologous chromosomes to be paired (same banding)
What are the 4 techniques for staining DNA in karyotype
G banding, R banding, Q banding and C banding
What dye is used in G banding, R banding and C banding staining techniques
Giemsa
Describe the procedure and results of G banding
Mild proteolysis followed by giesma to produce dark bands AT rich
Describe the procedure and results of R banding
Heat denaturation and giesma to produce dark bands GC rich
Describe the procedure and results of Q banding
Stain with quinacrine and view under UV light to see AT and GC regions vary in brightness
Describe the procedure and results of C banding
Treat with alkali and giesma to produce dark banding at the centromeric region
Does gene number and chromosome number increase with genome size
No
What are the two major processes in eukaryotes for transmission of genes
Mitosis and meiosis
Where does mitosis occur
Somatic tissue
What does mitosis result in
2 identical daughter cells with identical number of chromosomes to the parent cell (diploid)
Why is mitosis necessary
Embryonic development, replacement of skin cells and wound healing
What are spindle fibres
Formed from micro tubules and they between the poles of the cell creating an axis for chromosome separation
What is a centriole
A cytoplasmic organelle composed of nine groups of micro tubules that serve as the foci for the generation of the mitotic spindle fibres
What is the centrisome
A region of the cytoplasm containing a pair of centrioles
What makes up interphase of the cell cycle
G1 (and checkpoint), s phase, G2 (and checkpoint)
What occurs in G1
Cell grows and makes proteins required for DNA synthesis
What occurs in s phase (synthesis phase)
DNA is replicated to produce chromosomes with two identical sister chromatids
What occurs in G2
Production of proteins required for mitosis
What is the importance of the G1 and G2 checkpoints
Ensures all proteins required for DNA replication (1) are present and DNA has been replicated accurately (2) before mitosis
What is cytokinesis
Division of the cytoplasm
What happens in prophase of mitosis
Chromosomes condense and the centrosomes move apart
What happens in prometaphase of mitosis
Chromosomes are recognisable double structures
Centrioles reach the opposite poles
Spindle fibres form and attach to the kinetochore
What occurs in metaphase of mitosis
Centromeres align on metaphase plate
Spindle assembly checkpoint to ensure all chromosomes are aligned and 1 sister chromatid is attached to each pole
What occurs in anaphase of mitosis
Centromeres split and the daughter chromatids migrate to opposite poles
What occurs in telophase of mitosis
Daughter chromosomes arrive at poles
Chromosomes decondense and nuclear envelope reforms
Cytokinesis
What is meiosis
Cell division to produce sex cells/gametes
What does meiosis produce
4 Haploid cells (n) that are not genetically identical
How many divisions are in meiosis
2
In meiosis 1, what are the 5 different stages of prophase 1
Leptotene, zygotene, pachytene, diplotene and diakinesis
What occurs in prophase 1 of meiosis that does not happen in mitosis
Crossing over to generate genetic variability
What happens in leptotene of prophase 1
Duplicated chromosomes condense
What happens in zygotene of prophase 1
Bivalent forms, synaptonemal complex forms and synapsis begins
What happens in pachytene of prophase 1
Synapsis and synaptonemal complex are complete, crossing over occurs
What happens in diplotene of prophase 1
Chiasma is visible, exchange of lengths of DNA, synaptonemal complex disappears
What happens in diakinesis of prophase 1
Nuclear envelope fragments, bivalent ready for metaphase
What are tetrads
Homologous chromosomes pairs
What is a dyad
1 half of each homologous chromosome pair
What is the synaptonemal complex
A nucleoprotein ‘zipper’ that forms between the paired homologous chromosome that plays an important part in crossing over (chiasmata formation)
What is a chiasma
A physical connection between non sister chromatids that results in genetic exchange and creates ‘mosaic’ chromosomes of the paternal and maternal chromosomes
How do you calculate the number of variations produced in meiosis
2 to the power of chromosome number e.g. 23
What is the difference between meiosis in human males and females
Divisions in oogenesis are not continuous like spermatogenesis as the first division in the ovary arrests in prophase 1 and only resumes many years later prior to ovulation and the second division only occurs after fertilisation
What happens when meiosis goes wrong
Nondisjunction (Aneuploidy, polyploidy, trisomy)
What is aneuploidy
Increase or decrease in number of individual chromosomes
What is polyploidy
An increase in number of chromosomes sets
What is a trisomy
3 copies of a particular chromosome rather than 2
What are the 4 usual human autosomal trisomies
Trisomy 21, 18, 13 and 8
What is Trisomy 21
Down syndrome (intellectual disability, broad flat face)
What is trisomy 18
Edward’s syndrome (intellectual disability, deformed feet, heart problems)
What is trisomy 13
Patau syndrome (intellectual disability, small head, cleft lip and palate, extra fingers and toes)
What is trisomy 8
Warkany syndrome 2. (Intellectual disability, contracted fingers and toes, malformed ears, prominent foreheads)
Which human autosomal trisomy is rarest
Trisomy 8/ warkany syndrome 2
What determines maleness
Presence of the Y chromosome
What is the klinefelter karyotype
More than one X chromosomes in males
What is the turner karyotype
Single X chromosomes in females
What causes klinefelter and turner syndrome
Aneuploidy (NJ)
What does the Y chromosome code for
Fewer genes than X but still codes for 26 unique proteins for male fertility
What are the 3 regions of the Y chromosome
Pseudoautosomal regions (PARs), male specific region (MSY) and sex determining region (SRY)
Describe the PARs of the Y chromosome
Shares homologous with X, synapse and recombine with X during meiosis
Describe the MSY region
Does not synapse with X chromosome during meiosis
Describe the SRY region
Produces testis determining factor (TDF) which triggers undifferentiated gonadal tissue of the embryo to form testes
What is the advantage of sexes
Sexual reproduction results in recombinant genotypes enabling the population better adapted to changes in environment
What is dosage compensation
Mechanism to keep X-linked gene product dosages similar between males and females as females have 2 X chromosomes so potential to produce double the amount of gene product
What is a Barr body
Inactivated X chromosome which results from dosage compensation
How do you calculate the number of Barr bodies
Xn-1
How many Barr bodies does a turner female have (X)
None
How many Barr bodies does klinefelter male (XXXY) have
2
What is lyonization
It is randomised whether the maternal or paternal X chromosome is inactive (Barr body), all progeny cells have same inactivated X chromosomes and inactivation occurs early in development
