Lecture 14: Chromosomes: The stuff of heredity Flashcards
Chromosomes
The genetic units divided by mitosis
Where and when does DNA replication occur?
PROKARYOTIC CELLS
Undergo cycle of binary fission
•Coordinated cytoplasmic growth, DNA replication, and cell division
•Produces two identical daughter cells
- It begins at a single site called the origin of replication (ori)
- Catalyzed by enzymes located in the middle of the cell
- Once the ori is duplicated, the two origins migrate to the two ends of the cell
Ori
Origin of replication
•Catalyzed by enzymes located in the middle of the cell
•Once the ori is duplicated, the two origins migrate to the two ends of the cell
What are chromosomes made of?
CHROMATIN
•Structural building block of a chromosome
•Complex of DNA and its associated proteins
•Two major types of proteins (histone and nonhistone proteins)
HISTONES
•Small, positively-charged (basic) proteins complexed with DNA in eukaryotic chromosomes
•Five types of histones exist in most eukaryotic cells: H1, H2A, H2B, H3, and H4
•Bind to DNA by an attraction to negatively charged phosphate groups of DNA
•Compacts DNA in nuclei, and regulates DNA activity
METAPHASE CHROMOSOME
•During mitosis and meiosis, chromatin fibers fold and pack into thick, rodlike chromosomes
Euchromatin and Heterochromatin
In interphase, chromatin fibers are loosely packed in some regions (euchromatin) and densely packed in others (heterochromatin)
•Some regions of heterochromatin include genes that have been turned off and placed in a compact storage form
•Example: X-chromosome inactivation in mammalian females
•Early in development, one of the two X chromosomes is packed into a block of heterochromatin (Barr body) and inactivated
Karyotype preparation
•Giemsa reagent stains DNA and produces characteristic G-light or G-dark patterns for each chromosome
•G-dark regions have a higher DNA density
•The density of active genes is higher in G-light bands
Anatomy of a chromosome
Telomeres Centromere Non/Sister Chromatids Non/Homologous chromosomes Metacentric and acrocentric
Where and when does DNA replication occur? (Euk)
RNA primer in DNA replication causes a problem for replicating linear chromosomes of eukaryotes
- When the primer is removed, it leaves a gap at the 5′ end of the new DNA strand that DNA polymerase can’t fill – causing the chromosome to shorten with each replication
- The ends of most eukaryotic chromosomes are protected by a buffer of noncoding DNA (the telomere) consisting of short , repeating sequences (the telomere repeat)
Telomerase
With each replication, some telomere repeats are lost, but genes are unaffected
•Buffering fails when the entire telomere is lost
- Telomerase stops the shortening of telomeres by adding telomere repeats to chromosome ends:
- An RNA section binds to DNA and is the template for addition of telomere repeats
- Active ONLY in rapidly dividing embryonic cells, in germ cells – and in cancerous somatic cells
Mitosis
Interphase: G1 - Chromosomes unreplicated, extend throughout nucleus G2 - after replication during S phase, each chromosome is double Mitosis: Prophase Prometaphase Metaphase Anaphase Telophase
G1 of following Interphase
Mitotic spindle
In animal cells
•Centrosome divides, the two parts move apart to form spindle poles
•Microtubules form early spindle
In plant cells with no centrosome
•Spindle forms from microtubules
•Assemble in all directions surrounding nucleus
Centromere
•The region of the chromosome that binds the kinetochore and attaches to the chromosome to the spindle fibres
Kinetochore microtubules
•Connect chromosomes to spindle poles
•Nonkinetochore microtubules
•Extend between spindle poles without connecting to chromosomes
•At spindle midpoint, microtubules from one pole overlap with those from opposite pole
Cytokinesis
After telophase
Cell Cycle Control
Before mitosis - chromsomes completely duplicated?
After Metaphase - proper alignment?
Interphase - cell sufficient size? proper signals from environment was received?
Cells Cannot Divide Indefinitely
Cellular senescence: Loss of proliferative ability over time
•Two candidates for responsibility of cellular senescence
•DNA damage
•Telomere shortening
Cancer
Control of cell division is lost
•Cells divide continuously and uncontrollably
•Form rapidly growing mass of cells that interferes with body functions
- Cancer cells break loose from their original tumor in a process called metastasis
- Form additional tumors in other parts of the body
