Midsem test - topic 1 Flashcards
Sources of genetic variation
- independent assortment at metaphase 1
- crossing over at prophase 1
- fusion with 2 gametes
Human chromosome number
46 - 22 pairs and a pair of sex chroomosomes
Telocentric
centromere at one end (<1.7 ratio)
Acrocentric
centromere off centre (>1.7 ratio)
Metacentric
centromere in middle (>7 ratio)
Telomere role
stabilise the chromosomeW
What does nucleoli contain
rRna and components of ribosomes
Nucleolar organiser
- secondary constriction
- located in different positions in different species
- contains a cluster of genes that code for rRNA
Primary constriction
centromere
Why can horse and donkeys form a mule zygote
homologous pairs dont interact during mitosis but are similar enough to make zygote
Euchromatin
- loosely packed
- rich in genes
- increased recombination frequency
Heterochromatin
- tightly packed
- less genes
- decreased recombination frequency
Common chromosome stain
Giemsa
Chromatin
- DNA associated with histones
- not uniformly distributed
Nucleosomes
DNA packed around histones
Solenoid
nucleosomes organised into coils
Proteinaceous scaffold
final arrangement of solenoid coils
Prokaryotic genome size and density
- smaller
- more dense
Why are eukaryotic genomes less dense
- genes with multiple exons are spliced in multiple different ways
Major components of eukaryotic genome
- 1.5 % exons
- Unique = introns, non coding DNA, regulatory sequences, exons
- Repetitive DNA (L1 and Alu) -includes gene families, telomeric repeats, satellite repeats and transposable elements
Dispersed gene families
- DNA sequence of genes within family that have diverged to different functions
- all proteins coded by family of homologous genes
- some have become pseudogenes
- dispersed throughout genome
- Hemoglobin = alpha and beta globin gene families have multiple different genes at different stages of life
Tandem gene families
- multiple repeats of the same gene (duplication)
- organised as tandem repeats
- often share similar functions
- examples = histones and rRNA gene in nucleolar organiser
Satellite repeats
- highly repeated tandem sequences
- heterochromatic short AT rich tandem repeats in the centromere
- can be very abundant
- short DNA sequence repeats
- vary between individuals so can be used for DNA fingerprinting and paternity tests
Microsatellites
shorter repeated DNA sequence (2-6bp) at a particular locus on the chromosome
Minisatellites
longer repeats which doesnt code for proteins (15-100bp)
Transposed sequences
- mobile DNA sequences
- can insert themselves into many different locations in the genome
Telomeric repeats
- composed of tandem arrays
- sequence = TTAGGG
How can telomeres be visualised
probe chromosome with short sequence of fluorescent ssDNA that binds via complementary base pairing, ssDNA finds homologous chromosomal region during hybridisation
Genes
physical and functional unit of heredity
Locus
specific place where a gene is located
Alleles
different versions of the same gene
Homologous
paired chromosomes with same gene sequence and loci but may differ in alleles
Characteristics of mitosis
- in somatic cells
- one cell division results in 2 daughter cells
- chromosome number maintained
- one S phase per cell division
- no pairing of homologs
- no crossover
- centromeres divide at anaphase
- conservative
- can be diploid or haploid
Characteristics of meiosis
- cells in sexual cycle
- 2 cell divisions resulting in 4 cells
- chromosome number is halved
- one S phase for both divisions
- full interaction of homologs at prophase 1
- atleast 1 crossover per homolog during prophase 1
- centromeres divide at anaphase 1
- promotes variation
- cells
Ratio that occurs when genes are found on different chromosomes
9:3:3:1 = independent assortment/unlinked
Linked genes
found close together so they have a decreased chance of crossover/recombination
Test cross
- involves crossing a heterozygous individual with homozygous recssive
Trans test cross
dominant allele on different homolog, has decreased recombination
Cis test cross
dominant allele on the same homolog, increased recombination
Inter chromosomal recombination
genes on different chromosome so recombination occurs via independent assortment during anaphase
Intra chromosomal recombination
recombination of genes on the same chromosome
How does the recombination frequency change the farther apart the genes are
it gets closer to 50% but cannot become more than 50% as only 2 sister chromatids are involved
Advantages of 3 point test cross
- includes double crossovers
- allows the order of genes to be determined
- produces 2 parental and 6 recombinant progeny
Interference
- occurs when expected frequency is not equal to the observed
- one crossover is inhibiting the other crossover
- when I = 1 there is complete interference
Neurospora
- haploid fungi (n=7)
- asexually and sexually
- has 2 identical looking mating types which can be regarded as simple sexes
- one of the first model organisms
Neurospora life cycle
- when 2 different mating types come into contact their cell walls and nuclei fuse to form a diploid nuclei
- perithecia forms which contain asci
- diploid nuclei undergoes meiosis to produce 8 ascospores in one long asci
- each meiocyte produces a linear array of 8 ascospores
M1 pattern of segregation
crossover hasnt occured - 4:4 pattern
M2 pattern of segregation
crossover between gene and centromere - 2:2:2:2 pattern
How do different ascus arrangements occur
centromeres attaching to the spindle at random during the 2nd meiotic division
To calculate the map distance between a gene and centromere
(recombo/total x100) / 2
- HAVE TO DIVIDE BY 2 AS ONLY HALF OF THE PRODUCTS OF ANY MEIOSIS WITH A SINGLE CROSSOVER WILL BE RECOMBINANT
physical basis of recombination
- crossing over involves breakage and reunion of non-sister chromatids at the 4 chromatid stage
- chiasmata = sites of crossing over
Physical map of human chromosomes
- shows actual location of genes
- good resolution and accurate
- sequence based
- gene loci determined
- sources = somatic cell hybrid mapping and genome sequencing
Genetic map of human chromosomes
- created from recombination frequency data
- limited accuracy
- gene order and relative position determined
- source = recombination analysis of phenotypic and molecular markers
Why is mapping human genes difficult
inability to perform a testcross and humans have a small number of progeny
X chromosome mapping
- first human chromosome mapped
- suitable for mapping by recombination analysis because males are hemizygous
Techniques to construct human genome maps
- human and mouse somatic cell hybrids = different human chromosomes will be lost in different cell lines
- FISH = fluorescent hybridisation, tags chromosomes
- molecular markers = map gene of interest to a molecular marker
Structural chromosomal changes
- deletion
- duplication
- inversion
- translocaion
Deletions
- usually requires 2 chromosomal breaks
- results in the absence of a centromere
- terminal = at end
- interstitial = within chromosome arm
- intragenic = small deletion within a gene that inactivates the gene
- multigenic = involve several genes, creates dosage problem
- in meiosis chromosomes with deletion will form a loop to account for the loss
Duplications
- tandem = adjacent, inserted next to duplicate
- insertional = located elsewhere in genome
- a diploid cell with a duplication with have 3 copies which causes a dosage issue
- effective for increasing the number of copies and size
- to detect a duplication look at banding patterns and presence of loops at meisosi
Inversions
- involve 2 chromosome breaks in same chromosome, region is flipped then reinserted
- paracentric = centromere is outside the inversion
- pericentric = centromere is inside the inversion
- dont change the amount of genetic material
- has to have 1 centromere and 2 telomeres to survive
- acentric chromosome = lacks a centromere so it will not segregate in anaphase
- inversion loop forms during meiosis to pair with homolog
Translocations
- involves movement of genetical material between non-homologous chromosomes or within the same chromosome
- reciprocal = part of one chromosome is exchanged with one another
- non reciprocal = part of one chromosome moves to another without exchange
- can result in hybrid genes
- chimps have more chromosomes due to translocation and loss of chromosome
chimp and human chromosome difference - translocation
- human chromosome 2 is metacentric with G-banding patterns that match 2 acrocentric chromosomes in chimps
- human chromosome 2 has end to end fusion of 2 chromosomes
Numerical chromosomal changes
- euploidy
- aneuploidy
Euploidy
- change in number of sets
- common in plants = polyploidy
- major mechanism by which new plant species have evolved
- increased chromosome number = increased cell size
- autopolyploid = sets from one species
- allopolyploid = sets from 2 or more species that are closely related and partly homologous
Aneuploidy
- change in number of individual chromosomes
- can arise in chromosome is lost if centromere is deleted, small chromosomes generated by translocations are lost, gametes arise from nondisjunction due to failure of seperation
Meiotic nondisjunction
- happens in humans but most die in utero
- monosomy = 2n-1, non viable except turners chromosomes
- trisomy = 2n + 1, viable in 21 (down syndrome),13,18 and sex chromosomes
Mosaic
genetically distinct cells within an organism that are derived from a single zygote
- at birth = nondisjunction
- later in life = alterations
Chimera
genetically distinct cells within an organism that are derived from multiple zygotes
- birth = fusion of 2 zygotes
- later in life = transfusion of donor cells
Types of sex determination
- chromosomal = sex determined by genes located on the sex chromosomes
- genes
- environment
Gene dosage
number of copies in a gene present in a genome
Turners syndrome
- XO = sterile female
- aneuploidy
Klinefelters syndrome
XXY = sterile male
- anueploidy
SRY gene
- responsible for maleness
- encodes for a transcription factor
- protein binds to DNA and stimulates transcription of other genes which promotes development of testes
- turns on SOX 9
- SOX 9 cooperates with other genes to activate expression of AMH
- determined genetically and hormones
Androgen insensitivity syndrome
mutation on gene for androgen receptor on the X chromosome, embryo develops as female
Barr body
- inactivated X chromosome
- stable through mitosis but reactivated for meiosis in those that give rise to ova
- inactivation involves modification of DNA and histones
- multiple copies of RNA product of XIST attach and inactivate X
- females arent that affected by X linked diseases due to the 2 copies of X
Tortoiseshell cats
- mosaic of 2 type of cells = active X derived from male and active X derived from female
- occurs early in development
