Structure of DNA - GCSE Biology
Introduction
- In our body, all the cells know what, when, and how to do.
- This is because there are instructions present inside the nucleus of each cell.
- It is called genetic information and is stored in the form of DNA.
- All forms of life having cells as their basic unit have DNA as their genetic material.
- However, certain viruses use RNA for the same purpose.
Real-life Examples:
What is DNA
- DNA is an abbreviation for Deoxyribonucleic Acid.
- It is the genetic material in most organisms.
- It is the carrier of the genetic information from one generation to another.
- In the nucleus, DNA is condensed with proteins and is present in the form of chromosomes.
Overall Structure of DNA
- A DNA molecule is a double-stranded structure.
- Both the strands spiral around each other, connected by base pairs in a double helix structure.
- The backbone of the strands is made of the sugars and phosphate groups.
- These two are attached to one of the four bases: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).
- Together, a base, a sugar, and a phosphate group form a nucleotide.
Double-Stranded Structure and Hydrogen Bonding
- The bases, A, T, C, and G, are slightly electrically charged.
- As opposite charges attract each other, they form weak attractive forces known as hydrogen bonds.
- Therefore, Cytosine forms three hydrogen bonds with Guanine, and Adenine forms two hydrogen bonds with Thymine.
- These weak hydrogen bonds hold two sugar and phosphate group backbones together, making DNA double-stranded.
Base Pairing & The DNA Code
- DNA has four bases in it.
- Bases form pairs, A with T, and G with C, known as complementary base pairs.
- These four bases come together in different sequences in a three-letter codon and encode for a protein.
- These coded instructions are present in genes.
- Difference in the order of the bases causes us all to have different genes.
- All the individuals have different DNA except for identical twins.
- Scientists can find an individual’s identity as parents transfer DNA to their offspring.
Organization of DNA
- Since DNA is a polymer (a long chain of nucleotides joined together), it cannot simply fit inside a cell.
- Therefore, a long molecule of DNA is wrapped around proteins, named as histones, to make chromatin.
- The chromatin coils numerous times to form a chromosome.
- In prokaryotes, for example, bacteria, DNA lies freely in the cytoplasm.
- However, in eukaryotes, like plants and animals, DNA is present enclosed in the nucleus.
Organization of DNA in Prokaryotic Cells
- In prokaryotic cells, DNA is found free in the cytoplasm.
- It is in the form of one circular DNA molecule, chromosomal DNA.
- There is no association of DNA with histone proteins.
- Other than this, plasmids are also found in these cells.
- These are small, circular DNAs that carry extra genes.
Organization of DNA in Eukaryotic Cells
- In eukaryotic cells, DNA is present inside the nucleus.
- Unlike prokaryotes, its shape is linear.
- It is condensed with histone proteins to form chromatin.
- Other than nuclear DNA, mitochondria and chloroplasts have their own DNA.
Frequently Asked Questions
Solution:
DNA (Deoxyribonucleic Acid) is the genetic material found in cells.
Solution:
The shape of DNA is a double helix, which looks like a twisted ladder.
Solution:
A nucleotide is the basic building block of DNA. Each nucleotide has: a deoxyribose sugar, one nitrogen base, and a phosphate group.
Solution:
Weak hydrogen bonds hold two DNA strands together.
Solution:
Four bases found in DNA are Adenine (A), Thymine (T), Guanine (G), Cytosine (C).
Solution:
The specific base pairing between Adenine and Thymine, and Guanine and Cytosine is known as complementary base pairing.
Solution:
A gene is a short section of DNA that codes for a specific protein.
Solution:
Human cells have 23 pairs of chromosomes, that is, 46 in total.
Eye Diagram - GCSE Biology
Introduction
- Our body has five basic sense organs, and the eye is one of them.
- It has a layer known as the retina that contains receptor cells.
- The receptor cells in the retina are of two types: Cones and Rods.
- It is with the help of the eye that we can see various objects, colors, and lights.
Real-life uses:
Diagram of the Eye
Eye Parts and Functions
Adaptation in the Eye
- The process in which the retina and pupil adjust according to varying light intensities that fall on the eye is called Adaptation.
- In Dim Light, the pupils dilate to allow more light to enter, and the rod cells in the retina become more active.
- These adjustments enable correct vision in low light.
- In Bright Light, the retina interprets high-intensity light and sends signals to the brain. The brain, in response, constricts the pupils.
- With this, potential damage to the retina is prohibited.
Accommodation in the Eye
- The natural ability of the eye to change its lens’s shape to adjust its focus so that objects at different distances can be seen clearly is called Accommodation.
How Does The Eye Focus on a Near Object?
- To see a near-placed object clearly, the eye must increase its focusing power.
- For this, the ciliary muscles contract, loosening up the suspensory ligaments.
- This makes the lens fat and more curved.
- Therefore, light rays refract more and focus on the retina.
How Does The Eye Focus on a Distant Object?
- To see a distant object clearly, the eye must reduce its focusing power.
- For this, the ciliary muscles relax, tightening the suspensory ligaments.
- This makes the lens thinner and less curved.
- Thus, light rays refract less and focus on the retina.
Defects of the Eye
Myopia (Short-Sightedness)
- Myopia (Short-sightedness) is the condition in which objects placed at a distance appear faded.
- This happens because the rays of light focus before the retina.
Causes:
- Lengthy eyeball or Extra-curved cornea.
- The patients suffering from this condition are short-sighted or myopic.
Hyperopia/Hypermetropia (Long-Sightedness)
- Hyperopia/Hypermetropia (Long-sightedness) is the condition in which objects placed nearby appear blurred.
- This happens when the rays of light focus behind the retina.
Causes:
- Small eyeball or less curved cornea.
- The patients suffering from this condition are long-sighted or hyperopic or hypermetropic.
Treating Eye Defects
- Correction of Myopia is done with a diverging lens.
- The rays of light are diverged so that after refraction from the eye lens, they focus on the retina, not in front of it.
- Correction of Hyperopia is done with a converging lens.
- The rays of light are converged so that after refraction from the eye lens, they focus on the retina, not behind it.
Frequently Asked Questions
Solution:
The main parts include: cornea, iris, pupil, lens, ciliary muscles, retina, and optic nerve.
Solution:
Adaptation is the adjustment in the eye as per varying light intensities.
Solution:
In dim light, the pupil widens so that more light can enter the eye.
Solution:
Accommodation is the process of changing the shape of the lens to focus on near or distant objects.
Solution:
Myopia is when distant objects appear blurry. It happens when the eyeball is too long or the lens is too curved.
Solution:
Hyperopia is when nearby objects appear faded. It happens when eyeball is too short or the lens is too flat.
Solution:
Muscles in the iris control and change the size of the pupil. It can make it smaller or bigger.