25 Questions
During interphase of the cell-division cycle, what happens to the two daughter DNA molecules produced by DNA replication?
They are separately folded to produce two sister chromosomes, or sister chromatids, held together at their centromeres
What is the structure at which sister chromatids are held together?
Centromere
When are metaphase chromosomes formed?
After DNA replication is complete
What is the final level in the hierarchy of chromosome packaging?
Mitotic chromosome condensation
What is the purpose of chromosome compaction during mitosis?
To protect the DNA molecules from being broken
What drives the coiling of each interphase chromosome to produce a mitotic chromosome?
The energy of ATP hydrolysis
What are condensins?
Large protein complexes that contain SMC proteins
When does the condensation of interphase chromosomes into mitotic chromosomes occur?
During M phase
What is the function of condensins in DNA?
To make large right-handed loops in DNA using ATP hydrolysis
What is the function of telomeres?
To stabilize the chromosome structure
What is the function of the kinetochore?
To assemble on the centromere and link the chromosome to microtubules
What is the characteristic of centromeres?
They are required for chromatid separation late in mitosis
What type of DNA sequences are found in human centromeres?
Alpha satellite DNA
What is the function of heterochromatin?
It silences the expression of genes that are experimentally placed into it
What is the characteristic of telomeres?
They are found at the ends of chromatids
What is the repeating sequence found in telomeres?
-TTAGGG-
What is the primary function of telomeres in chromosomes?
To allow cells to divide without losing genes
Why do cells become senescent or die after dividing around 50 to 70 times?
Because telomeres get progressively shorter until the cells become senescent or die
What is the role of telomerase in cells?
To lengthen telomeres during DNA replication
In which cells is telomerase activity present?
Germline and certain hematopoietic cells
What happens if telomeres are not present in chromosomes?
The main part of the chromosome would get shorter each time a cell divides
Why do chromosomes have telomeres?
To allow cells to divide without losing genes
What is the consequence if reproductive cells did not have telomerase to maintain the length of their telomeres?
The organism would soon go extinct
What happens to telomeres as cells age?
They shorten with age
In which cells is telomerase reactivated?
Most cancers and immortalized cells
Study Notes
Chromosome Structure
- Chromosomes are formed by folding two sister DNA molecules produced during interphase
- Sister chromatids are held together at their centromeres
- Chromosomes are duplicated structures formed after DNA replication is complete
Chromosome Condensation
- Mitotic chromosome condensation is the final level of chromosome packaging
- It is a highly organized and dynamic process that serves two important purposes:
- Disentangling and alignment of sister chromatids
- Protection of DNA molecules from breakage during mitosis
- Condensation occurs in M phase and requires condensins, a class of proteins that use ATP hydrolysis to coil interphase chromosomes
Centromere
- Centromeres are required for chromatid separation during mitosis
- They are packaged into heterochromatin, which silences gene expression
- Human centromeres extend over hundreds of thousands of nucleotide pairs and consist of short, repeated DNA sequences (alpha satellite DNA)
Kinetochore
- The kinetochore is a protein structure on chromatids where spindle fibers attach during cell division
- It assembles on the centromere and links the chromosome to microtubules during mitosis and meiosis
Telomere
- Telomeres are DNA-protein complexes at chromosome ends that stabilize chromosome structure
- They are characterized by many repeats of the sequence -TTAGGG-
- Telomeres shorten with each cell division, leading to cellular senescence or death
- Telomerase, an enzyme, synthesizes telomere sequences to maintain their length during DNA replication
- Telomerase is active in sperm and eggs, but not in somatic cells
Telomere Function
- Telomeres prevent chromosome shortening and fusion during DNA replication
- They allow cells to divide without losing genes
- Without telomeres, chromosome ends could fuse together, causing cellular malfunction or death
This quiz covers the structure of chromosomes, centromeres, and telomeres, as well as the process of DNA replication during interphase of the cell division cycle. It explores how sister chromatids are formed and held together at their centromeres.
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