Chromosome Structure and the Eukaryotic Cell Cycle Flashcards
What do chromosomes contain?
Chromosomes contain the genetic material of an organism
Where are genes located within chromosomes?
Genes are physically located within the chromosomes
What are chromosomes composed of?
Chromosomes are composed of DNA and proteins
What does the DNA-protein complex make in chromosomes?
Chromatin
What is the primary function of genetic material?
The primary function of genetic material is to store needed information
What is chromosome compaction?
Chromosome compaction is the process by which DNA is tightly packed into the nucleus of a cell
What is a nucleosome?
A nucleosome is a repeating unit of DNA wrapped around an octamer of histone proteins
How does DNA wrap around histone proteins in a nucleosome?
DNA wraps itself around histone proteins in a nucleosome through the attraction between the negative charges of DNA and the positive charges of histones
How much does the length of DNA shorten when it wraps around histone proteins?
The length of DNA shortens about sevenfold when it wraps around histone proteins
What is the structure of the nucleosome units when they are further compacted?
The nucleosome units are organized into a more compact structure that is 30 nm in diameter
What is the effect of further nucleosome compaction on DNA length?
Further nucleosome compaction shortens the nucleosome structure another sevenfold
Why is the structure of the more compact nucleosome units difficult to determine?
Because the conformation of the DNA may be substantially altered when extracted from living cells
What are radial loop domains?
Loops of DNA that extend outwards from a central protein scaffold, forming a hierarchical structure of chromatin
What is the interaction between 30-nm fibers and nuclear matrix proteins?
The 30-nm fibers of chromatin are thought to interact with nuclear matrix proteins, providing additional stability and organisation to chromosomes.
How are chromosomes separated?
Each chromosome is located in a discrete, non-overlapping chromosome territory within the nucleus
Do different chromosomes become intertwined when they are not compacted?
No, different chromosomes are not substantially intertwined even when they are not compacted
Why is the level of compaction of chromosomes not uniform?
Different regions of chromosomes have different functions and therefore require different levels of compaction
What are the 2 forms of chromatin that exist?
- heterochromatin
- euchromatin
What is heterochromatin?
- a highly compacted form of chromatin
- transcriptionally inactive, meaning that genes are not read to produce RNA
What is euchromatin?
- a less condensed form of chromatin
- capable of gene transcription
Where is most chromosomal regions found in non-dividing cells?
Most chromosomal regions are found in non-dividing cells in a partially compacted form
When do chromosomes compact further?
Chromosomes compact further when the cell is preparing to divide (cell division)
What happens to all euchromatin during cell division?
All euchromatin is converted to heterochromatin during cell division
What happens to most transcriptional activities during cell division?
Most transcriptional activities cease during cell division
What happens to radial loops in metaphase chromosomes?
In metaphase chromosomes, highly compacted radial loops remain anchored to a scaffold formed from proteins
Do eukaryotic chromosomes occur in sets?
Yes
How many different chromosomes do humans have?
23 different chromosomes
What are sex chromosomes in humans?
X and Y chromosomes
How many chromosomes do humans have in most cell types?
Two sets of 23 chromosomes so 46 in total in most cells
What does diploid mean?
Diploid describes cells with a pair of each chromosome (2n, 2 sets of chromosomes)
What does haploid mean?
Haploid describes cells such as gametes which have only 1 member of each pair of chromosomes (1n)
What are homologous chromosomes (homologues)?
Members of a pair of chromosomes that are similar and nearly identical in genetic composition
Where do homologues come from in diploid organisms?
- one pair of homologous chromosomes from the mother
- the other pair from the father
What do slight differences in homologous chromosomes provide?
Provide variation in gene function
What are differences in genetic composition between homologues called?
Differences in genetic composition between homologues are called single nucleotide polymorphisms (SNPs)
Do sex chromosomes differ in size and genetic composition?
Yes
What are sister chromatids?
Sister chromatids are the individual, replicated copies of each chromosome that are still attached to each other
When do sister chromatids form?
Sister chromatids form when a cell replicates its chromosomes before cell division
Why do chromosomes often appear as X’s?
Because they consist of two identical sister chromatids that are joined together by a structure called the centromere
What is karyotype analysis?
Karyotype analysis is a field of genetics that involves microscopic examination of chromosomes
What can be seen through light microscopy in karyotype analysis?
- tightly compacted chromosomes that have distinctive shapes
- the number of chromosomes
What does the banding pattern in karyotype analysis enable?
The banding pattern, created by chemical dyes, enables the identification of similar chromosomes
How are chromosomes identified by structure?
- size
- location of the centromere
- presence of bands
after staining
What is the naming convention for the short and long arms of a chromosome?
- short arm = p
- long arm = q
Draw a pair of chromosomes and label it with p, q and the centromere
slide 17
What are the four types of chromosome structure based on centromere location?
- metacentric (middle)
- submetacentric (off-center)
- acrocentric (near end)
- telocentric (at the end)
(slide 17)
Draw a pair of chromosomes for each of the 4 types of location of centromere and label it
slide 17
What is Giemsa staining used for in chromosome analysis?
To create banding patterns on chromosomes, which can be used for identification and analysis
- this technique is known as G-banding
What is the cell cycle?
A series of events that occur in a cell leading to cell growth and division into two daughter cells
What are the phases of the cell cycle?
- G1 (first gap)
- S (DNA synthesis)
- G2 (second gap)
- M (mitosis)
What are checkpoint proteins?
- act as sensors to determine if the cell is in proper condition to divide
- can delay cell division until problems are fixed
- or prevent it entirely if necessary
What can happen if there is a loss of checkpoint function in the cell cycle?
Can lead to mutation - e.g. cancer as cells gain the ability to divide continuously
What is the role of cyclin and cyclin-dependent kinase in the cell cycle?
(cdks) are proteins responsible for advancing a cell through the phases of the cell cycle
- amount of cyclin varies throughout the cell cycle
- kinases controlling the cell cycle (e.g. cdk) must bind to a cyclin to be active
How many critical regulatory points or checkpoints exist in eukaryotic cells?
3
What are the 3 critical regulatory points or checkpoints in eukaryotic cells?
- G1 checkpoint (restriction point)
- G2 checkpoint
- Metaphase checkpoint
What happens during the Synthesis phase (S-phase) of the cell cycle?
- DNA replication occurs resulting in the production of duplicated strands of DNA called sister chromatids
- sister chromatids are tightly associated at the centromere until they separate during mitosis
What is mitotic cell division?
Mitotis is when a cell divides to produce 2 new cells that are genetically identical to the original.
What is the original cell called in mitotic cell division?
The original cell is called the mother cell
What are the new cells called in mitotic cell division?
The new cells are called daughter cells
What is cytokinesis?
Process of cell division that begins during anaphase and is completed after telophase
What is the purpose of mitotic cell division?
- asexual reproduction
- for production and maintenance of multicellularity
What are the 4 phases of mitosis?
(Interphase)
- prophase
(pro-metaphase)
- metaphase
- anaphase
- telophase
(Cytokinesis)
What is the mitotic spindle?
Responsible for organising and sorting the chromosomes during mitosis
What is the mitotic spindle composed of?
The mitotic spindle is composed of microtubules
- microtubules formed from tubulin proteins
What are centrosomes and their role in mitosis?
Centrosomes are microtubule organizing centers (MTOCs)
-single centrosome duplicates at the beginning of M phase and define a cellular pole
What is the role of centrosomes?
Responsible for organising & sorting chromosomes during mitosis
What are the 3 types of microtubules in the spindle apparatus?
- astral
- polar
- kinetochore
What is the role of astral microtubules in mitosis?
Position the spindle apparatus within the cell
What is the role of polar microtubules in mitosis?
Separate the two poles of the cell
What is the role of kinetochore microtubules in mitosis?
Attached to the kinetochore of a chromosome at one end, and to the centrosome at its other end
What happens during interphase?
- chromosomes are maximally extended
- found in the nucleus
- cell not in mitosis
i.e. G1, S, and G2 phases of the cell cycle
What happens during prophase?
- chromosomes have already replicated to produce sister chromatids
- nuclear membrane dissociates into small vesicles
- chromatids condense into compacted structures (readily visible by light microscopy)
What happens during prometaphase?
- mitotic spindle is completely formed
- centrosomes move apart to separate the two poles
- spindle fibers interact with sister chromatids
- two kinetochores on each pair of sister chromatids are attached to kinetochore microtubules from opposite poles
What happens during metaphase?
- pairs of sister chromatids are aligned along a plane halfway between the poles called the metaphase plate
- they are organised into a single row
- when this alignment is complete, the cell is in metaphase
What happens during anaphase?
- connections between the pairs of sister chromatids are broken
- each chromatid, now an individual chromosome, is linked to only one of the two poles by one or more kinetochore microtubules
- kinetochore microtubules shorten, pulling the chromosomes toward the pole to which they are attached
- two poles move farther away from each other as overlapping polar microtubules lengthen and push against each other
What happens during telophase?
- chromosomes have reached their respective poles and decondense
- nuclear membranes now re-form to produce two separate nuclei
What happens during cytokinesis in mitosis?
During cytokinesis in mitosis, the two nuclei are segregated into separate daughter cells.
How does the process of cytokinesis differ between animals and plants?
- animals = a cleavage furrow constricts like a drawstring to separate the cells
- plants = a cell plate forms a cell wall between the two daughter cells
(slide 34)
What is meiosis?
Meiosis is a type of cell division that results in the formation of four haploid cells, each with half the number of chromosomes as the original diploid cell.
What is required for sexual reproduction?
Sexual reproduction requires a fertilisation event in which two haploid gametes unite to create a diploid cell called a zygote.
What are the haploid cells produced in meiosis called?
Called gametes, which are the sperm and egg cells
What are the stages of meiosis I?
- Prophase I
- Prometaphase I
- Metaphase I
- Anaphase I
- Telophase & cytokinesis I
What are the stages of meiosis II?
- Prophase II
- Prometaphase II
- Metaphase II
- Anaphase II
- Telophase and cytokinesis II
How does meiosis differ from mitosis?
2 key differences:
1) Homologous pairs form a bivalent or tetrad at Prophase I
2) Crossing over occurs between homologous chromosome pairs to generate novel gene combinations
When does meiosis occur in the cell cycle?
Like mitosis, meiosis begins after a cell has progressed through the G1, S, and G2 phases of the cell cycle
When does crossing over occur in Meiosis?
Crossing over occurs during prophase I of Meiosis
How is a bivalent or tetrad?
When homologous pairs of sister chromatids associate with each other, lying side by side
What is synapsis?
Process by which homologous pairs of sister chromatids associate with each other
- forming a structure called the synaptonemal complex during prophase I of Meiosis
What happens during crossing over?
Physical exchange of chromosome pieces between the homologous chromosomes of a bivalent during prophase I of Meiosis.
What is a chiasma?
A site where the arms of the homologous chromosomes tend to separate but remain temporarily adhered at a crossover site
What happens during Prophase I of Meiosis I?
- replicated chromosomes condense
- bivalents form as the nuclear membrane breaks down
(slide 44)
What happens during Prometaphase I of Meiosis I?
- spindle apparatus complete
- chromatids are attached to kinetochore microtubules
(slide 44)
What happens during Metaphase I of Meiosis I?
Bivalents organised along metaphase plate as double row
(slide 44)
What happens during Anaphase I of Meiosis I?
Segregation of homologues occurs.
- connections between bivalents break, but not the connections that hold sister chromatids together
- a joined pair of chromatids migrates to one pole, while the homologous pair of chromatids moves to the opposite pole
(slide 44)
What happens during Telophase I of Meiosis I?
- sister chromatids reach their respective poles and decondense
- nuclear membranes reform
(slide 44)
What is the result of Meiosis I?
The original diploid cell had its chromosomes in homologous pairs, while the two cells produced at the end of Meiosis I are haploid - they only have a single copy of each chromosome.
Is there an S phase between Meiosis I and Meiosis II?
No, there is no S phase between Meiosis I and Meiosis II
Are the sorting events of Meiosis II similar to those of Mitosis?
Yes, the sorting events of Meiosis II are similar to those of Mitosis
What happens during Anaphase II of Meiosis II?
Sister chromatids are separated
How does mitosis differ to meiosis?
- mitosis produces two diploid daughter cells that are genetically identical
- meiosis produces four haploid daughter cells
What is a life cycle?
A life cycle is a sequence of events that produces another generation of an organism.
What is the alternation that occurs in the life cycle of sexually reproducing organisms?
For sexually reproducing organisms, the alternation involves haploid cells or organisms and diploid cells or organisms.
What is a diploid-dominant species?
An organism where the majority of cells are have 2 sets of chromosomes
- most animal species are diploid
What is a haploid-dominant species?
An organism where the majority of cells are have 1 set of chromosomes
- many fungi and some protists
What is the alternation of generations?
A life cycle found in plants and some algae where there is an intermediate dominance between generations.
What is a sporophyte?
The multicellular diploid organism (in alternation of generations life cycle)
What is a gametophyte?
The multicellular haploid organism (in the alternation of generations life cycle)
Does the relative size of sporophyte and gametophyte vary?
Yes it varies between species
List processes of chromosomal structural mutations
- deletions
- duplications
- inversions
- translocations
What are deletions in chromosomal structural mutations?
Deletions refer to a chromosomal segment that is missing
What are duplications in chromosomal structural mutations?
Duplications refer to a chromosomal segment that occurs 2 or more times in a row
What are inversions in chromosomal structural mutations?
Inversions refer to a chromosomal segment that has changed direction along a chromosome
What are translocations in chromosomal structural mutations?
Translocations refer to one chromosomal segment becoming attached to another chromosome
- can be simple or reciprocal
What is euploid in changes in chromosome number?
Euploid refers to the chromosome number that is viewed as the normal number
- in diploid organism, 2 sets of chromosomes is normal
- a euploid human has 46 chromosomes
What is polyploid in changes in chromosome number?
Polyploid refers to exactly 3 or more sets of chromosomes. Examples include triploid (3n), tetraploid (4n), and hexaploid (6n)
What is aneuploidy in changes in chromosome number?
Aneuploidy refers to an alteration in the number of chromosomes that is not an exact multiple of a set but is any random number.
- monosomy & trisomy
What is monosomy and trisomy?
Special cases of aneuploidy in which there is only one fewer (monosomy) or one more (trisomy) chromosome than the expected euploid number of chromosomes.
What causes changes in chromosome numbers?
Non-disjunction
What is non-disjunction in changes in chromosome number?
Non-disjunction occurs when chromosomes do not sort properly during mitosis or meiosis.
- during meiosis, this can produce gametes with too many or too few chromosomes
What is interspecies breeding?
Interspecies breeding refers to the crossing of two different species to produce offspring.
What are the 3 types of interspecies breeding?
- alloploid
- allopolyploidy
- allotetraploid
What are alloploid organisms?
Alloploid organisms have at least one set of chromosomes from two or more different species.
What is allopolyploidy?
Allopolyploidy is the occurrence of two or more complete sets of chromosomes from two or more different species.
What is an allotetraploid?
An allotetraploid has two complete sets of chromosomes from two different species, for a total of four sets.
Do animals tolerate deviations from diploidy well?
No, they usually do not tolerate deviations from diploidy well and it is usually lethal.
Do plants commonly exhibit polyploidy?
Yes, plants commonly exhibit polyploidy.
What is Trisomy 21?
Down’s syndrome - relatively low in severity
