Course title
Molecular biology
Course cod
1012
Student name
Saud Rabea alatwi
Student ID
411001975
1) What is a nucleic acid and explain the structure of DNA and RNA?
Nucleic acids
Nucleic acids are biomolecules that store, transmit, and express genetic information in living
organisms. They are composed of repeating units called nucleotides, which are made up of a
sugar molecule (either ribose in RNA or deoxyribose in DNA), a phosphate group, and a
nitrogenous base. The nitrogenous bases are adenine (A), guanine (G), cytosine (C), thymine (T)
(in DNA), and uracil (U) (in RNA). The sequence of these nitrogenous bases in DNA and RNA
determines the genetic information that is stored and transmitted. Nucleic acids play a
fundamental role in a wide range of biological processes, including DNA replication,
transcription, and translation, as well as protein synthesis and regulation of gene expression.
DNA
The structure of DNA is a double helix, consisting of two strands of nucleotides that are twisted
around each other. Each nucleotide is made up of a sugar molecule (deoxyribose), a phosphate
group, and a nitrogenous base (adenine, guanine, cytosine, or thymine). The nitrogenous bases
are held together by hydrogen bonds, with A always pairing with T and G always pairing with C.
This base pairing is essential for DNA replication. The double helix structure is stabilized by
several types of interactions, including hydrogen bonds, van der Waals interactions, and
hydrophobic interactions. The uniform diameter of the helix allows for efficient packing of DNA
in the cell. The double helix model proposed by Watson and Crick in 1953 is still widely
accepted and forms the basis for our understanding of DNA biology.
RNA
RNA, or ribonucleic acid, is a single-stranded molecule that is structurally similar to DNA. Like
DNA, RNA is composed of repeating nucleotide units, but it contains the sugar ribose rather than
deoxyribose. RNA nucleotides also contain the nitrogenous bases adenine, guanine, cytosine,
and uracil instead of thymine. The nucleotides in RNA are linked together through a
phosphodiester bond between the 3' carbon of one sugar and the 5' carbon of the next sugar,
forming a sugar-phosphate backbone. The nitrogenous bases project from the sugar-phosphate
backbone, and RNA folds into complex secondary and tertiary structures due to complementary
base pairing and other interactions between nucleotides. RNA plays a crucial role in gene
expression, including transcription and translation.
structure of DNA and RNA
2) What is cell cycle in biology and what are the differences between meiosis and mitosis?
The cell cycle is the process by which a cell grows, replicates its DNA, and divides into two
daughter cells. The cell cycle is divided into two main stages: interphase and mitosis. Each of
these stages consists of several distinct phases, which are explained below:
1. Interphase: This is the longest phase of the cell cycle, and it is divided into three subphases:

G1 phase: During this phase, the cell grows and synthesizes new proteins and organelles.

S phase: During this phase, DNA replication occurs, and the cell synthesizes a copy of its
genetic material.

G2 phase: During this phase, the cell continues to grow and prepares for cell division.
2. Mitosis: This is the stage of the cell cycle where the cell divides into two identical daughter
cells. It is divided into four sub-phases:

Prophase: During this phase, the chromatin condenses into chromosomes, and the nuclear
envelope breaks down.

Metaphase: During this phase, the chromosomes align in the middle of the cell, and the
spindle fibers attach to the centromeres of each chromosome.

Anaphase: During this phase, the spindle fibers pull the sister chromatids apart, and the
chromosomes move towards the opposite poles of the cell.

Telophase: During this phase, the nuclear envelope reforms around each set of
chromosomes, and the chromosomes begin to decondense.
3. Cytokinesis: This is the final stage of the cell cycle, during which the cytoplasm divides and
the cell separates into two daughter cells.
the differences between meiosis and mitosis
Feature
Meiosis
Mitosis
Purpose
To produce gametes (sex cells) for
reproduction
To produce identical daughter cells
for growth and repair
Number of divisions
Two (meiosis I and meiosis II)
One
Number of daughter
cells produced
Four
Two
Genetic variation
Daughter cells are genetically unique
due to crossing over and random
assortment of chromosomes
Daughter cells are genetically
identical to parent cell
Chromosome
number of daughter
cells
Half the number of chromosomes as
the parent cell
The same number of chromosomes
as the parent cell
Occurs in
Diploid (2n) cells
Both diploid (2n) and haploid (n)
cells
Stages
Prophase I, Metaphase I, Anaphase I,
Telophase I, Prophase II, Metaphase II,
Anaphase II, Telophase II
Prophase, Metaphase, Anaphase,
Telophase
Significance
Essential for sexual reproduction and
genetic diversity
Essential for growth, repair, and
asexual reproduction
the references:
1.