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L2 - Chromosomal Elements &
Euchromatin vs
Heterochromatin
Specialized elements of chromosomes
We’re now going to discuss some specialised elements of chromosomes. These
special parts of chromosomes are there to ensure the accurate replication and
segregation of chromosomes.
There are three specialised elements:
1. Origins of replication
2. Telomeres
3. Centromeres.
The cell performs different activities and different functions at separate times in
the cell cycle. Broadly speaking, the cell cycle can be divided into dividing
segments called mitosis, and the segments of time between mitosis is called
interphase.
So a cell has two broad parts of a cell cycle, cell division, and interphase.
The interphase part of the cell cycle is further divided into gap, G1 and G2 gap
phases, and the DNA synthesis stage.
So if you can divide a cell into two daughter cells, you have to double the DNA
before you halve it yet again into the two daughter cells, so that each daughter
cell receives an exact copy of the initial cell.
Let’s start at cell division. During mitosis, the double DNA is divided across two
daughter cells. The cells now can continue their cellular activities, including
perhaps at this stage metabolism, and doubling components of the cell that’s not
the DNA, in anticipation for the S phase when the S phase, the chromosomes,
each of the 46 chromosomes are duplicated in the cell.
L2 - Chromosomal Elements & Euchromatin vs Heterochromatin 1
, However, the DNA replication stage can have some errors, so those errors are
checked or double-checked during the gap or G2 phase, where the duplicated
doubled chromosomes are checked for error and any repairs are made.
Once all the quality control has been done, the cell can now enter mitosis. And
what’s important to remember is that you do not have DNA replication or the
synthesis happening during mitosis when you’re dividing the cell, and when the
cells are dividing, you can’t have metabolism and metabolic processes
happening.
So, separate functions occur at separate times of the cell cycle, and that’s very
important to remember.
Origins of replication
What we’re going to talk about next is the first specialised element of the
chromosomes, and that is called the origins of replication. As the word implies,
this is where replication starts along the chromosome.
These are areas of chromatin that are accessible to the replication machinery,
and as a result, these regions of accessible DNA are often devoid of any
nucleosomes and is generally more loosely compacted.
The area where replication occurs is called the replicon, or the replication unit.
And here, DNA can go both ways in terms of DNA replication. Another feature of
replicons, other than not having any nucleosomes, is that is has sequences,
repeat sequences and sequences that are incredibly rich in the bases A and T.
A-T bonds have two H bonds, hydrogen bonds, as opposed to three H bonds
you have between a G-C base pairing, which makes it easier to access and
separate the double-stranded DNA between a string of A-Ts than it would be for
G-Cs.
The next thing to remember is that there are many origins of replication present
along a chromosome, and many origins of replication are active at the same
time.
These origins of replication are scattered throughout the chromatin along a
particular chromosome.
So, how many origins of replication do we have in the cell? We have 10,000
origins of replication.
L2 - Chromosomal Elements & Euchromatin vs Heterochromatin 2
L2 - Chromosomal Elements &
Euchromatin vs
Heterochromatin
Specialized elements of chromosomes
We’re now going to discuss some specialised elements of chromosomes. These
special parts of chromosomes are there to ensure the accurate replication and
segregation of chromosomes.
There are three specialised elements:
1. Origins of replication
2. Telomeres
3. Centromeres.
The cell performs different activities and different functions at separate times in
the cell cycle. Broadly speaking, the cell cycle can be divided into dividing
segments called mitosis, and the segments of time between mitosis is called
interphase.
So a cell has two broad parts of a cell cycle, cell division, and interphase.
The interphase part of the cell cycle is further divided into gap, G1 and G2 gap
phases, and the DNA synthesis stage.
So if you can divide a cell into two daughter cells, you have to double the DNA
before you halve it yet again into the two daughter cells, so that each daughter
cell receives an exact copy of the initial cell.
Let’s start at cell division. During mitosis, the double DNA is divided across two
daughter cells. The cells now can continue their cellular activities, including
perhaps at this stage metabolism, and doubling components of the cell that’s not
the DNA, in anticipation for the S phase when the S phase, the chromosomes,
each of the 46 chromosomes are duplicated in the cell.
L2 - Chromosomal Elements & Euchromatin vs Heterochromatin 1
, However, the DNA replication stage can have some errors, so those errors are
checked or double-checked during the gap or G2 phase, where the duplicated
doubled chromosomes are checked for error and any repairs are made.
Once all the quality control has been done, the cell can now enter mitosis. And
what’s important to remember is that you do not have DNA replication or the
synthesis happening during mitosis when you’re dividing the cell, and when the
cells are dividing, you can’t have metabolism and metabolic processes
happening.
So, separate functions occur at separate times of the cell cycle, and that’s very
important to remember.
Origins of replication
What we’re going to talk about next is the first specialised element of the
chromosomes, and that is called the origins of replication. As the word implies,
this is where replication starts along the chromosome.
These are areas of chromatin that are accessible to the replication machinery,
and as a result, these regions of accessible DNA are often devoid of any
nucleosomes and is generally more loosely compacted.
The area where replication occurs is called the replicon, or the replication unit.
And here, DNA can go both ways in terms of DNA replication. Another feature of
replicons, other than not having any nucleosomes, is that is has sequences,
repeat sequences and sequences that are incredibly rich in the bases A and T.
A-T bonds have two H bonds, hydrogen bonds, as opposed to three H bonds
you have between a G-C base pairing, which makes it easier to access and
separate the double-stranded DNA between a string of A-Ts than it would be for
G-Cs.
The next thing to remember is that there are many origins of replication present
along a chromosome, and many origins of replication are active at the same
time.
These origins of replication are scattered throughout the chromatin along a
particular chromosome.
So, how many origins of replication do we have in the cell? We have 10,000
origins of replication.
L2 - Chromosomal Elements & Euchromatin vs Heterochromatin 2
