Chromosomes. Chromosome Tightly coiled DNA form Found during mitosis and meiosis Made mostly of DNA...
-
Upload
theresa-colleen-thompson -
Category
Documents
-
view
216 -
download
0
Transcript of Chromosomes. Chromosome Tightly coiled DNA form Found during mitosis and meiosis Made mostly of DNA...
Chromosomes
Chromosome• Tightly coiled DNA form • Found during mitosis and
meiosis• Made mostly of DNA and
proteins• Centromere- point of
attachment• Chromatids- each arm of a
chromosome in replicated form
• Banding▫ Heterochromatin- dark
area with repetitive sequences
▫ Euchromatin- lighter protein-encoding area
Chromosome Structure: Telomeres
• Chromosomal tips
• Human chromosomes have repeats of TTAGGG
• Telomeres shorten during each cell division in most cells
Chromosome Structure: Centromere• Constricted region of
chromosome• Region of attachment of
spindle fibers during mitosis and meiosis
• Contains many repeats of a 171-base sequence called the alpha satellite
• Centromere associated proteins▫ Some form kinetochore
during mitosis and meiosis▫ Centromere protein A
(CENP-A)- surround centromere; are divided and continue with each chromatid during anaphase
Chromosome Structure• Moving away from the centromere
▫ Gene-rich regions▫ Subtelomeres
Gene start to taper off More repeats start to occur
Chromosome Shorthand• Shorthand used to locate
gene on chromosome• First number is
chromosome number• Letter (p or q) refers to
the arm• Second number refers to
the band on that arm▫ Number gets higher as
the band is farther from the centromere
Chromosomal Differences
•Size Difference▫Chromosomes are numbered from 1
(largest)-22 (smallest)•Banding differences•# of Genes
▫Some chromosome have very few genes for their size, and some have a lot of genes for their size
•Location of Centromere
Location of the Centromere• Metacentric- located in the
middle; divides chromosomes into two equal arms
• Submetacentric- located slightly off of center; divides the chromosome into a long (q) arm and a short (p) arm
• Acrocentric- located near one end of the chromosome; divides the chromosome into a long (q) arm and a chort (p) arm
• Telocentric- located at the end; humans do not have these
Karyotype• Pictorial representation of
chromosomes• Autosomal chromosomes
are lined up by number (from largest to smallest)
• Sex Chromosomes are arranged last
• What does it tell you?▫ Sex of individual▫ Presence of chromosomal
abnormality
Obtaining a Sample• Blood sample or cheek cell
sample• Embryo
▫ Amniocentesis▫ Chorionic Villus Sampling▫ Fetal Cell Sorting▫ Pre-implantation Testing
Creating a Karyotype•Cell division is stopped with colchicine•Cell is ruptured•Cell with the most spread out chromosomes is
used•Chromosomes are identified with stain or
FISH▫Staining- use chromosome specific stains▫FISH (fluorescence in situ hybridization)-
fluorescent probes are attached to chromosomes; each chromosome fluoresces a different color
Karyotype Analysis Shorthand• 46,XX• First number- number of
chromosomes• Sex chromosomes• Extra information
▫ Extra chromosomes (+#)
Chromosomal Abnormality: Polyploidy• Extra set of chromosomes• Triploid (3 sets)
▫ Ovum is fertilized with two sperm
▫ One gamete remains diploid after meiosis
• Usually spontaneous abortion
Chromosomal Abnormailty: Aneuploidy• Have a missing or extra
chromosome• Most are spontaneously
aborted• Some types can survive
until birth• Monosomy- missing one
chromosome▫ Only one viable is XO
(Turner Syndrome)• Trisomy- extra
chromosome
Nondisjunction• Most aneuploidy results
from nondisjunction• Chromosomes fail to
separate during meiosis• More commonly seen in
females (especially with older maternal age)
• Results in some gametes missing chromosomes and some having extra
Mosaics• Aneuploidy occurs
during mitosis• Individual has some
cells that are normal and some that are aneuploid
• Severity of disorder depends on when during development that it happens
Common Aneuploids
•Autosomal▫47, X__ +21– Down Syndrome▫47, X__ +13- Patau Syndrome▫47, X__ +18- Edward Syndrome
•Sex Chromosome▫45, XO- Turner Syndrome▫47, XXX- triplo-X▫47, XXY- Klinefelter’s Syndrome▫47,XYY- Jacobs Syndrome
Chromosomal Alterations
•Structural problems of chromosomes•Include
▫Deletions▫Duplications▫Translocations▫Inversions
Deletions• Deletion of part of a
chromosome• The larger the deletion,
the more severe the disorder
• Microdeletions- small deletions that can create symptoms (ex: impair fertility if on Y chromosome)
• Cri-du-chat- deletion of part of the short arm of chromosome 5 (5p-)
Duplication• Chromosome with
repeated sections• Usually repeats have
to be large to be symptomatic
• Fragile X syndrome- expanding triplet repeat adds extra material to X
Translocations• Two nonhomologous chromosomes exchange or
combine parts• Robertsonian translocation- short arms are
removed from two chromosomes; long arms stick together to form one long chromsome▫ Problems for future generations
• Reciprocal translocation- two different chromosomes exchange parts▫ Problem for individual if gene is broken▫ Problem for future generations
• Translocation carrier- asymptomatic carrier of translocation
Robertsonian Translocation
Reciprocal Translocation
Inversion• Chromosome with some genes inverted• Paracentric inversion-
▫ Does not include centromere▫ Crossing over can result in
Normal chromosomes Dicentric chromosome (two centromeres) Acentric chromosome (no centromere)
• Pericentric inversion-▫ Does include centromere▫ Crossing over can result in▫ Normal chromosomes▫ Abnormal chromosomes with one centromere
Inversion
Isochromosomes and Ring Chromosomes• Isochromosome
▫ Chromosome splits the wrong way during meiosis
▫ Chromosome end up with identical arms
• Ring Chromosome▫ Removal of telomeres
creates sticky end ▫ Sticky ends attach to
each other creating a ring
Uniparental Disomy•Individual has a chromosome set where
both chromosomes come from one parent •Can cause problems with recessive alleles
or imprinting•Occurs if
▫One parent has nondisjunction resulting in a gamete with that chromosome missing
▫One parent has nondisjunction resulting in a gamete with both homologs of that chromosome
▫These two fertilize each other