chromosomes, cell division, meiosis and chromosome abnormalities

Question 1

outline eukaryotic chromosome structure

Answer 1

linear, telomeres to protect the end, centromere in the middle, can be heterochromatin or euchromatin

Question 2

what are telomeres

Answer 2

5’ - TTAGGG - 3’ repeats, hundreds of copies at the ends of the chromosomes, about 100bp lost at mitosis

Question 3

how is the lagging strand of DNA replicated

Answer 3

okazaki fragments, each fragment is primed by an RNA primer which is removed at the end of replication

Question 4

define mitosis

Answer 4

the separation of sister chromatids into two daughter cells

Question 5

prophase

Answer 5

Chromosomes condense
Nuclear membrane disappears
Spindle fibres form from the centriole

Question 6

metaphase

Answer 6

Chromosomes aligned at the equator of the cell
Attached by fibre to each centriole
Attachment of the microtubules to the centromere
Maximum condensation of chromosome

Question 7

anaphase

Answer 7

Sister chromatids separate at centromere
Separate longitudinally
Move to opposite ends of cell

Question 8

telophase

Answer 8

New nuclear membranes form

Each cell contains 46 chromosomes (diploid)

Question 9

cytokinesis

Answer 9

Cytoplasm separates
Two new daughter cells
Both inherit the full complement of DNA from the original parent cell

Question 10

what are centromeres

Answer 10

constricted region joining sister chromatids, site at which the kinetochore is assembled, repetitive DNA sequences, requred for chromosome separation during cell division, critical for the chromosomes to behave correctly and segregate into the 2 daughter cells

Question 11

what is a kinetochore

Answer 11

protein complex where the microtubules of the spindles attach

Question 12

heterochromatin

Answer 12

inactive, condensed structure, silenced genes

Question 13

euchromatin

Answer 13

transcriptionally active, open structure, active genes

Question 14

what are the 2 types of extragenic sequences

Answer 14

tandemly repeated DNA sequences, highly repeated interspersed DNA sequences

Question 15

what are tandemly repeated DNA sequences

Answer 15

○ Satellite DNA (14-500 bp repeats in 20-100 kb arrays, typically present in large amounts at centromeres and telomeres, present in heterochromatin)
○ Minisatellite DNA (15-100 bp repeats in 1-5 kb arrays, used for DNA fingerprinting as it is highly variable between individuals)

Question 16

what are Highly repeated interspersed DNA sequences,

Answer 16

~45% of the genome, ○ SINEs (short interspersed nuclear elements), 100-400 bp, most common are Alu elements which make up a huge proportion of our genomes)
○ LINEs (long interspersed nuclear elements), up to 6kb, most common are L1 elements)

Question 17

chromatin structure

Answer 17

• Nucleosomes consist of histones (8 histone molecules in a nucleosome), each nucleosome can wrap 146 base pairs around itself (1.8 turns)
  
  • DNA packaged with histone proteins to form chromatin
  • Histones have positive charge
  • Packaged into units called nucleosomes
  • Further wrapping of nucleosomes
    ○ About 6 nucleosomes per turn
  • Form solenoid structure (30nm fibre)
    Compacted DNA by about factor of 40

Question 18

what are the 4 levels of chromatin structure

Answer 18

1: nucleosome (beads on a string)
2: chromatin fibre (30nm solenoid structure)
3: fibre-scaffold complex
4: chromosome

Question 19

what is the purpose of packaging DNA

Answer 19

• -vely charged DNA neutralised by +ve charged histone proteins (contain lots of basic residues)
  
  • DNA takes up less space
  • Inactive DNA can be folded into inaccessible locations until required e.g. globin genes exist in euchromatin in genes where they will be expressed but in euchromatin in other types of genes

Question 20

chromosome structure

Answer 20

Visible banding patterns on metaphase chromosomes
Chromosomes consist of two sister chromatids held together at the centromere refion (allows arm to be designated as p or q)

Question 21

submetacentric

Answer 21

centromere displaced to one side

Question 22

acrocentric

Answer 22

centromere at one end of the chromosome, non-functional arm - no coding DNA

Question 23

fluorescent in situ hybridisation

Answer 23

technique used to pick out specific sequences
Take a cloned copy of DNA at a specific sequece, label it using fluorescent dye –> fluorescent copy of the gene, anneals to the cellular copy of the DNA, the gene can then be visualised

Question 24

types of fish probes

Answer 24

• Unique sequence probes
• Centromeric probes
• telomeric probes
• whole chromosome probes

Question 25

centromeric probes

Answer 25

Useful for determining chromosome number

Question 26

telomeric probes

Answer 26

○ Useful for detecting subtelomeric rearrangements

Often present in children with unexplained mental retardation

Question 27

whole chromosome probes

Answer 27

○ Cocktail of probes covering different parts of a particular chromosome
○ Used with different fluorescent dyes
○ Spectral karyotype
○ Useful for detecting translocations and rearrangements

Question 28

meiosis

Answer 28

• Cell division in germ cells
  
  • Diploid cells (in ovaries and testes) divide to form haploid cells
  • Chromosomes are passes on as rearranged (recombined) copies
    Creates genetic diversity

Question 29

gametogenesis

Answer 29

• Oogenesis = process of egg formation
  
  • Spermatogenesis = process of sperm formation
  • Both go through several stages, with different timing in males and females
    Sperms go through more cell divisions than eggs do therefore more chance of mutation

Question 30

fertilisation

Answer 30

• 2 haploid cells (egg, sperm) form 1 diploid cell (zygote) which develops into an embryo
  
  • Whether sperm contain an X or Y chromosome determines if the embryo is female (XX) or male (XY)
  • Embryo contains an assortment of genes from each original parent (more genetic diversity)
    Mitochondria (and their DNA) come only from the mother via the egg (maternal inheritance)