3.2 Chromosomes Flashcards
describe bacterial DNA
- one chromosome which is a circular DNA molecule
- not assosciated with proteins so “naked”
- only one copy of each gene
- some have plasmids which are small rings of DNA, accessory DNA
- no homologous pairs so easy to predict
how are plasmids replicated or passed on?
Plasmids are not always replicated at the same time as the cell. This means that there may be more than one copy of a plasmid in a cell or a plasmid may not be passed on in cell division.
plasmids can also be transferred from one species to another or from one cell to another. This allows for faster evolution. Plasmids are also used by biologist to transfer genes between species artificially.
how can you measure the length of a DNA molecule?
John Cairns used this technique on E coli:
- cells grown for two generations in culture medium with tritiated thymidine ( has thymine linked with deoxyribose which Bacteria uses to make nucleiotides for DNA replication) it also has tritium a radioactive hydrogen isotope
- Cells placed in dialysis membrane and cells walls digested using enzyme lysozyme, cells burst to release DNA onto surface of dyalisis membrane
- photographic emulsion applied to surface of membrane and left in darkness for 2 months, tritium decays and emits high energy electrons which react with film
- film is developed and examined under microscope, dark grain shows tritium which shows DNA
shows that
- e coli has circular DNA
- LENGTH is 1 100 micrometers, length of e coli is 2 micrometers
- used by other researchers on eukaryotic DROSOPHILIA MELANOGASTER which had a chromosome 12 000 micrometers long , was linear
describe eukaryotic DNA
- linear
- has 8 ( in one bead) histone proteins which are globular in shape and wider than the DNA
- adjacent histone proteins are seperated by short stretches of DNA molecule - linker DNA
- –> string bead appearance during interphase
what are homologous chromosomes
some other definitons about chromatids etc
have the same sequence of genes but no necessarily the same alleles
each gene has a specific locus so DNA can be swapped
made of two chromatids just before division
PICTURE
in part 1 they are not dividing - we never see this
different chromatids (mom and dad) never join together at centromere
compare genome sizes
genomes vary a lot, viruses smallest
correlated with complexity of organism but not directly proportional as the proportion of DNA that act as functional genes is variable and the amount of gene duplication varies.
Organism - Genome size (million base pairs)
T2 PHAGE virus that attacks E coli - 0.18
ESCHERICHIA COLI gut bacteria - 5
DROSOPHILIA MELANOGASTER druit fly - 140
HOMO SAPIENS - 3 000
PARIS JAPONICA woodland plant - 150 000
haploid vs diploid
haploid
- one chromosome of each type but full set of chromosomes
- gametes are haploid
diploid
- two chromosomes of each type
- zygote
- animal and plants mostley made of entirely diploid cells
- two copies of each gene so harmful mutation on recessive allele does not affect it
- ogansims often more vigorous with 2 alleles not just one - hybrid vigour e.g. strong growth of F1 in hybrid crop plants
compare number of chromosomes
number can change in evolution is chrosomes fuse or split
number of chromosome is not indicative of amount of information in it
all eukaryotes have at least 4 chromosomes (diploid) and in diploid must always be an even number
name - diploid chromosome number
PARASCARIS EQUORUM horse threadworm - 4
ORYZA SATIVA rice - 24
HOMO SAPIENS - 46
PAN TROGLYDOTES chimpanzee - 48
CANIS FAMILIARIS dog - 78
IN HUMANS CHROMOSOME 21 IS THE SMALLEST
role of sex chromosome and describe them
X is large and has its centromere near the middle
Y is smaller and has centromere near end
stuff on Y is sometimes similar to X but mainly new
on Y chromosome SRY or TDF gene cause embryo to be male as intiates development of e.g. testes and testosterone
karyograms what are they
a karyogram shows the chromosomes of an organism in homologous pairs of decreasing length
in METAPHASE we get clearest view of chromosomes using stains
arranges according to
- size
- position of centromere
- banding pattern
karyotypes and down syndrome
a karyotype is a property of an organism - the number and type of chromosomes of an organism
found from karyograms
uses
- deduce wether individual is male or female
- diagnose down syndrome and other chromosomal abnormalities
Take fetal cells from uterus, if three copies of chromosome 21 then down syndrome or trisomy 21
features
- hearing loss
- heart disorders
- vision disorders
- mental retardation
- growth retardation
define
genotype
genome
genotype - the set of alleles that an individual possess
genome - all genes of an organism
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