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NAME: EMMANUELLA OKYEREGROUP 2
BIOLOGY
UNIT 11 B
CELL DIVISON IN EUKARYOTIC CELLS
INTRODUCTION:
A cell, which uses the capacity of self-replication, is among the smallest units of
life. In this assignment, I'll talk about how cells divide, including mitosis and
meiosis, as well as how human chromosomes form, work, and are organised.
For organisms to maintain themselves and engage in sexual or asexual
reproduction, cells divide to replace damaged or worn-out cells. The process of
nourishing and repairing is referred to as mitosis, whereas the act of
reproducing to create a new creature is referred to as meiosis and comprises
egg cells and sperm cells.
CHROMOSOMES
,https://i.pinimg.com/originals/ca/30/55/
ca3055607e79f4be981e02e5286df48a.jpg
Chromosomes are the genetic components found in every cell. In a eukaryotic
cell, they can be found in the nucleus. They have a structure akin to thread. A
eukaryotic cell's chromosomes are made up of DNA and proteins called
histones that are connected to it. These traits are passed on from parents to
children and from one generation to the next; they are genetically determined.
The variances among the various kinds of organisms are caused by variations in
the proteins that DNA codes for.
STRUCTURE OF CHROMOSOMES
As said earlier chromosomes are thread like structure that consist of long
strands of DNA that are tightly coiled and condensed. They have a
characteristic X-shape when the cell is about to divide, consisting of two
identical sister chromatids held together at a region called the centromere.
Each chromatid is a complete copy of the DNA molecule. The ends of
chromosomes are protected by structures called telomeres.
1. DNA Packaging: Chromosomes consist of long strands of DNA that are
tightly coiled and condensed. DNA is a double-stranded molecule
composed of nucleotide building blocks, each consisting of a sugar, a
phosphate group, and one of four nitrogenous bases: adenine (A),
thymine (T), cytosine (C), and guanine (G). To fit within the cell nucleus,
which is relatively small compared to the length of DNA molecules, DNA
is highly compacted and organized. The primary level of compaction
involves wrapping DNA around proteins called histones. DNA and
histones together form structures known as nucleosomes. Nucleosomes,
in turn, coil and stack to form a more condensed structure called
chromatin. DNA is wrapped around proteins called histones to form
structures known as nucleosomes. The primary function of histones is to
facilitate DNA compaction and organization. Histones are small,
positively charged proteins that interact with the negatively charged DNA
molecules. There are five main types of histones: H1, H2A, H2B, H3, and
H4. They form an octamer structure composed of two copies of each
histone type.
,2. Chromatids and Centromeres: During the cell cycle, chromosomes
undergo replication to prepare for cell division. Before replication, each
chromosome consists of a single DNA molecule. However, after
replication, each chromosome consists of two identical DNA molecules,
known as sister chromatids. Sister chromatids are held together at a
specific region called the centromere. The centromere plays a crucial role
in ensuring accurate separation of sister chromatids during cell division.
It also serves as an attachment site for proteins involved in chromosome
movement. When a cell is about to divide, chromosomes adopt a
characteristic X-shape, where the two sister chromatids are visually
apparent. This X-shaped structure facilitates the precise distribution of
genetic material to daughter cells during cell division.
3. Telomeres: Telomeres are structures located at the ends of
chromosomes. They consist of repeated DNA sequences that do not
encode specific genes but play essential roles in preserving chromosome
stability and integrity. Telomeres act as protective caps, preventing
degradation and fusion of chromosome ends. During DNA replication,
the enzymes responsible for DNA synthesis cannot fully replicate the
ends of linear chromosomes. As a result, telomeres shorten with each
cell division. This phenomenon is associated with cellular aging and
limited replicative capacity. To counteract telomere shortening, the
enzyme telomerase adds back specific DNA sequences to telomeres,
thereby maintaining their length. Telomerase activity is higher in stem
cells and certain types of cells with high proliferative potential.
4. Higher Levels of Chromosome Organization: In addition to the
compaction at the level of nucleosomes and chromatids, chromosomes
undergo further organization to achieve higher levels of compaction and
condensation. This organization is crucial for efficient packaging and
regulation of gene expression. Chromosomes are divided into discrete
regions called chromosomal bands based on staining patterns observed
under a microscope. Each band contains specific genes and regulatory
elements. The banding pattern allows for the identification and mapping
of genes on chromosomes. At the highest level of compaction,
chromosomes condense further during cell division to form distinct
structures known as mitotic chromosomes. Mitotic chromosomes are
highly condensed and visible under a light microscope. They are
, organized and aligned along the metaphase plate, ensuring the accurate
segregation of genetic material to daughter cells during cell division.
In conclusion, the structure of chromosomes involves the tight coiling and
condensation of DNA molecules. Chromosomes consist of sister chromatids
held together at a centromere. Telomeres protect the ends of chromosomes,
and higher levels of organization contribute to efficient packaging and
regulation of gene expression.
https://alevelbiology.co.uk/wp-content/uploads/2017/11/Chromosome-
Structure.jpg
FUNCTIONS OF CHROMOSOMES
The primary task of chromosomes is to transmit genetic material from one
generation to the next. Chromosomes also play a crucial role in the processes
of development, reproduction, repair, and regeneration that are necessary for
their existence.
The DNA is shielded by chromosomes from tangling and damage.
The regulation of gene expression is aided by histone and non-histone proteins.
During cell division, spindle fibres connected to the centromere aid in the
movement of the chromosome.
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