Genetics is a field of biology that studies how traits are passed from parents to their offspring. The passing of traits from parents to offspring is known as heredity, therefore, genetics is the study of heredity. This introduction to genetics takes you through the basic components of genetics such as DNA, genes, chromosomes and genetic inheritance.
Genetics is built around molecules called DNA. DNA molecules hold all the genetic information for an organism. It provides cells with the information they need to perform tasks that allow an organism to grow, survive and reproduce. A gene is one particular section of a DNA molecule that tells a cell to perform one specific task.
Heredity is what makes children look like their parents. During reproduction, DNA is replicated and passed from a parent to their offspring. This inheritance of genetic material by offspring influences the appearance and behavior of the offspring. The environment that an organism lives in can also influence how genes are expressed.
DNA
DNA is the cornerstone of genetics and is the perfect place to start for an introduction to genetics. DNA stands for deoxyribonucleic acid and it is the molecule that holds the genetic information for a cell and an organism.
A DNA molecule contains a code that can be used by a cell to express certain genes. Specific sections of a DNA molecule provides the information to build specific proteins which can then be used by a cell to express the desired gene.
A DNA molecule is a nucleic acid, one of the four molecules of life. It comes in the form of a long, linear molecule referred to as a strand. Each strand of DNA is bonded to a second strand of DNA to form a DNA double helix. In eukaryotic cells, DNA is found in the nucleus as a tightly coiled double helix.
DNA molecules are replicated during cell division. When a cell divides, the two new cells contain all the same DNA that the original cell had.
In sexual reproduction with two parents, half of the DNA of the offspring is provided by each of the parents. The genetic material of a child is made from 50% of their mother’s DNA and 50% their father’s DNA.
GENES
A gene is a specific segment of a DNA molecule that holds the information for one specific protein. DNA molecules have a unique code for each gene which codes for their specific protein. Some organisms can have more than 100,000 different genes so they will have 100,000 unique sequences of DNA ‘code’.
Genes are the basic unit of heredity. The genes of an individual are determined by their parent or parents. A bacteria that is born by one parent cell splitting into two cells and has the exact same genes as their one parent cell.
A human, on the other hand, has two copies of each gene – one set from their mother and a second set from their father. Different forms of the same gene are called alleles. For each gene, a human can have two different alleles or two of the same alleles – one from each parent.
Physical traits such as eye color or height are often determined by the combination of multiple genes. The environment an individual lives in also impacts how genes are expressed.
CHROMOSOMES
A chromosome is a structure made from tightly packed strands of DNA and proteins called histones. Strands of DNA are tightly wrapped around the histone proteins and form into long worm-shaped structures called ‘chromatids’. Two chromatids join together to form a chromosome.
Chromosomes are formed in the nucleus of a cell when a cell is dividing. It is possible to see chromosomes under an ordinary light microscope if the cell is in the right stage of cell division.
The number of chromosomes varies between species. Humans have 46 chromosomes. Some species can have many more than 100 chromosomes while others can have as little as two.
GENETIC INHERITANCE
Inheritance is the backbone of genetics and is an important topic to cover in an introduction to genetics. Long before DNA had been discovered and the word ‘genetics’ had been invented, people were studying the inheritance of traits from one generation to the next.
Genetic inheritance occurs both in sexual reproduction and asexual reproduction. In sexual reproduction, two organisms contribute DNA to produce a new organism. In asexual reproduction, one organism provides all the DNA and produces a clone of themselves. In either, genetic material is passed from one generation to the next.
Experiments performed by a monk named Gregor Mendel provided the foundations of our current understanding of how genetic material is passed from parents to their offspring.
Plants make their own food by photosynthesis. Carbon dioxide and water react together in the presence of light and chlorophyll to make glucose and oxygen. The glucose is converted into starch, fats and oils for storage. It is used to make cellulose for cell walls, and proteins for growth and repair. It is also used by the plant to release energy by respiration.
Photosynthesis and respiration
Photosynthesis
Photosynthesis is a chemical reaction that happens in the chloroplasts of plant cells. It produces glucose for use by the plant, and oxygen as a waste product. Here are the equations for photosynthesis:
Carbon dioxide + waterglucose + oxygen
6CO2 + 6H2OC6H12O6 + 6O2
Light energy is absorbed by chlorophyll in the chloroplasts for photosynthesis to happen.
Respiration
It is not just animals that respire – plants carry out respiration as well. Plants respire all the time because their cells need energy to stay alive, but plants can only photosynthesise when they are in the light.
Time of day
Photosynthesis
Respiration
Day
Active
Active
Night
None
Active
The diagrams summarise what this means for the overall release of carbon dioxide or oxygen from plants. Remember that respiration uses oxygen and produces carbon dioxide.
How plants affect the atmosphere: day
Understanding photosynthesis
You should be able to describe how the understanding of the process of photosynthesis has developed.
Ancient Greek scientists
Scientists in ancient Greece believed that plants gained mass only by taking in minerals from the soil. They would not have tested this idea.
Jan Baptist van Helmont (1580-1644)
Van Helmont carried out an experiment to see if the idea from ancient Greece was correct. He grew a willow tree in a weighed amount of soil. After five years, he discovered that the willow tree weighed about 74 kg more than it did at the start. As the weight of the soil had hardly changed, van Helmont concluded that plant growth cannot only be due to minerals from the soil. He thought that the extra plant material had come from the water alone.
Joseph Priestley (1733 - 1804)
Joseph Priestley carried out an experiment that showed that plants produce oxygen. He put a mint plant in a closed container with a burning candle. The candle flame used up the oxygen and went out. After 27 days, Priestley was able to re-light the candle. This showed that plants produce a gas that allows fuels to burn. This gas is oxygen.
Using glucose
The glucose made in photosynthesis is transported around the plant as soluble sugars. Glucose is used in respiration to release energy for use by the plant's cells. However, glucose is converted into insoluble substances for storage. These insoluble storage substances include:
Oils
Fats
Starch
The advantages of using insoluble substances such as starch for storage, rather than soluble substances such as glucose, include:
They do not affect the water concentration inside cells
They do not move away from the storage areas in the plant
Glucose and starch can be converted into other substances in plants. For example:
Cellulose for cell walls
Proteins for growth and repair
Limiting factors
Three factors can limit the speed of photosynthesis - light intensity, carbon dioxide concentration and temperature.
Light intensity
Without enough light, a plant cannot photosynthesise very quickly, even if there is plenty of water and carbon dioxide. Increasing the light intensity will boost the speed of photosynthesis.
Carbon dioxide concentration
Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air. Even if there is plenty of light, a plant cannot photosynthesise if there is insufficient carbon dioxide.
Temperature
If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesise if it gets too hot.
If you plot the rate of photosynthesis against the levels of these three limiting factors, you get graphs like the ones above.
In practice, any one of these factors could limit the rate of photosynthesis.
Read on if you're taking the higher paper.
Understanding photosynthesis - Higher tier
A two-stage process
Photosynthesis is a process with two main stages:
Light energy is used to split water, releasing oxygen gas and hydrogen ions
Carbon dioxide gas combines with the hydrogen to make glucose
You do not need to know equations for the two separate stages, only for the overall process.
6CO2 + 6H2OC6H12O6 + 6O2
Evidence for the two stages
Experiments using isotopes of carbon, hydrogen and oxygen have increased our understanding of photosynthesis. One of these experiments involves an isotope of oxygen, 18O.
The more common isotope of oxygen in water or carbon dioxide, 16O, can be replaced by 18O. The transfer of these 18O atoms into other substances can be traced. The results were:
Plants watered with water containing 18O atoms release oxygen gas containing 18O atoms
Plants supplied with carbon dioxide containing 18O atoms do not release oxygen gas containing 18O atoms
This shows that the oxygen gas produced by photosynthesis comes from water and not carbon dioxide.
Animal cells are typical of the eukaryotic cell, enclosed by a plasma membrane and containing a membrane-bound nucleus and organelles. Unlike the eukaryotic cells of plants and fungi, animal cells do not have a cell wall. This feature was lost in the distant past by the single-celled organisms that gave rise to the kingdom Animalia. Most cells, both animal and plant, range in size between 1 and 100 micrometers and are thus visible only with the aid of a microscope.
The lack of a rigid cell wall allowed animals to develop a greater diversity of cell types, tissues, and organs. Specialized cells that formed nerves and muscles—tissues impossible for plants to evolve—gave these organisms mobility. The ability to move about by the use of specialized muscle tissues is a hallmark of the animal world, though a few animals, primarily sponges, do not possess differentiated tissues. Notably, protozoans locomote, but it is only via nonmuscular means, in effect, using cilia, flagella, and pseudopodia.
The animal kingdom is unique among eukaryotic organisms because most animal tissues are bound together in an extracellular matrix by a triple helix of protein known as collagen. Plant and fungal cells are bound together in tissues or aggregations by other molecules, such as pectin. The fact that no other organisms utilize collagen in this manner is one of the indications that all animals arose from a common unicellular ancestor. Bones, shells, spicules, and other hardened structures are formed when the collagen-containing extracellular matrix between animal cells becomes calcified.
Animals are a large and incredibly diverse group of organisms. Making up about three-quarters of the species on Earth, they run the gamut from corals and jellyfish to ants, whales, elephants, and, of course, humans. Being mobile has given animals, which are capable of sensing and responding to their environment, the flexibility to adopt many different modes of feeding, defense, and reproduction. Unlike plants, however, animals are unable to manufacture their own food, and therefore, are always directly or indirectly dependent on plant life.
Most animal cells are diploid, meaning that their chromosomes exist in homologous pairs. Different chromosomal ploidies are also, however, known to occasionally occur. The proliferation of animal cells occurs in a variety of ways. In instances of sexual reproduction, the cellular process of meiosis is first necessary so that haploid daughter cells, or gametes, can be produced. Two haploid cells then fuse to form a diploid zygote, which develops into a new organism as its cells divide and multiply.
The earliest fossil evidence of animals dates from the Vendian Period (650 to 544 million years ago), with coelenterate-type creatures that left traces of their soft bodies in shallow-water sediments. The first mass extinction ended that period, but during the Cambrian Period which followed, an explosion of new forms began the evolutionary radiation that produced most of the major groups, or phyla, known today. Vertebrates (animals with backbones) are not known to have occurred until the early Ordovician Period (505 to 438 million years ago).
Cells were discovered in 1665 by British scientist Robert Hooke who first observed them in his crude (by today's standards) seventeenth century optical microscope. In fact, Hooke coined the term "cell", in a biological context, when he described the microscopic structure of cork like a tiny, bare room or monk's cell. Illustrated in Figure 2 are a pair of fibroblast deer skin cells that have been labeled with fluorescent probes and photographed in the microscope to reveal their internal structure. The nuclei are stained with a red probe, while the Golgi apparatus and microfilament actin network are stained green and blue, respectively. The microscope has been a fundamental tool in the field of cell biology and is often used to observe living cells in culture. Use the links below to obtain more detailed information about the various components that are found in animal cells.
Centrioles - Centrioles are self-replicating organelles made up of nine bundles of microtubules and are found only in animal cells. They appear to help in organizing cell division, but aren't essential to the process.
Cilia and Flagella - For single-celled eukaryotes, cilia and flagella are essential for the locomotion of individual organisms. In multicellular organisms, cilia function to move fluid or materials past an immobile cell as well as moving a cell or group of cells.
Endoplasmic Reticulum - The endoplasmic reticulum is a network of sacs that manufactures, processes, and transports chemical compounds for use inside and outside of the cell. It is connected to the double-layered nuclear envelope, providing a pipeline between the nucleus and the cytoplasm.
Endosomes and Endocytosis - Endosomes are membrane-bound vesicles, formed via a complex family of processes collectively known as endocytosis, and found in the cytoplasm of virtually every animal cell. The basic mechanism of endocytosis is the reverse of what occurs during exocytosis or cellular secretion. It involves the invagination (folding inward) of a cell's plasma membrane to surround macromolecules or other matter diffusing through the extracellular fluid.
Golgi Apparatus - The Golgi apparatus is the distribution and shipping department for the cell's chemical products. It modifies proteins and fats built in the endoplasmic reticulum and prepares them for export to the outside of the cell.
Intermediate Filaments - Intermediate filaments are a very broad class of fibrous proteins that play an important role as both structural and functional elements of the cytoskeleton. Ranging in size from 8 to 12 nanometers, intermediate filaments function as tension-bearing elements to help maintain cell shape and rigidity.
Lysosomes - The main function of these microbodies is digestion. Lysosomes break down cellular waste products and debris from outside the cell into simple compounds, which are transferred to the cytoplasm as new cell-building materials.
Microfilaments - Microfilaments are solid rods made of globular proteins called actin. These filaments are primarily structural in function and are an important component of the cytoskeleton.
Microtubules - These straight, hollow cylinders are found throughout the cytoplasm of all eukaryotic cells (prokaryotes don't have them) and carry out a variety of functions, ranging from transport to structural support.
Mitochondria - Mitochondria are oblong shaped organelles that are found in the cytoplasm of every eukaryotic cell. In the animal cell, they are the main power generators, converting oxygen and nutrients into energy.
Nucleus - The nucleus is a highly specialized organelle that serves as the information processing and administrative center of the cell. This organelle has two major functions: it stores the cell's hereditary material, or DNA, and it coordinates the cell's activities, which include growth, intermediary metabolism, protein synthesis, and reproduction (cell division).
Peroxisomes - Microbodies are a diverse group of organelles that are found in the cytoplasm, roughly spherical and bound by a single membrane. There are several types of microbodies but peroxisomes are the most common.
Plasma Membrane - All living cells have a plasma membrane that encloses their contents. In prokaryotes, the membrane is the inner layer of protection surrounded by a rigid cell wall. Eukaryotic animal cells have only the membrane to contain and protect their contents. These membranes also regulate the passage of molecules in and out of the cells.
Ribosomes - All living cells contain ribosomes, tiny organelles composed of approximately 60 percent RNA and 40 percent protein. In eukaryotes, ribosomes are made of four strands of RNA. In prokaryotes, they consist of three strands of RNA.
In addition the optical and electron microscope, scientists are able to use a number of other techniques to probe the mysteries of the animal cell. Cells can be disassembled by chemical methods and their individual organelles and macromolecules isolated for study. The process of cell fractionation enables the scientist to prepare specific components, the mitochondria for example, in large quantities for investigations of their composition and functions. Using this approach, cell biologists have been able to assign various functions to specific locations within the cell. However, the era of fluorescent proteins has brought microscopy to the forefront of biology by enabling scientists to target living cells with highly localized probes for studies that don't interfere with the delicate balance of life processes.
Plant cells are eukaryotic cells or cells
with membrane bound nucleus. Generally, plant cells are larger than animal cells and
are mostly similar in size and are rectangular or cube shaped. Plant cells are similar to animal cells in
being eukaryotic and they have similar cell organelles.
Plant cells are eukaryotic cells i.e., the DNA in aplant cellis enclosed within the nucleus. The most important
distinctivestructureofplant cellis the presence of the cell wall outside the cell membrane.
It forms the outer lining of the cell. The cell wall mostly constitutes of
cellulose and its main function is providing support
and rigidity. Plants cells also contain many membrane bound cellularstructures. These organelles carry out specific functions necessary
for survival and normal operation of the cells. There are a wide range of
operations like producing hormones, enzymes, and all metabolic activities of
the cell.
Your body includes organ systems,
such as the digestive system, made of individual organs, such as the stomach,
liver, and pancreas, which work together to carry out a certain function (in
this case, breaking down and absorbing food). These organs, in turn, are made
of different kinds of tissues, which are groups of cells which work together to
perform a specific job. For example, your stomach is made of muscle tissue to
facilitate movement and glandular tissue to secreteenzymesfor chemical breakdown of food molecules. These tissues, in turn,
are made of cells specialized in shape, size, and componentorganelles,
such as mitochondria forenergyand microtubules for movement.
Plants, too, are made of organs,
which in turn are made of tissues.Plant tissues,
like ours, are constructed of specialized cells, which in turn contain specificorganelles.
It is these cells, tissues, and organs that carry out the dramatic lives of
plants.
The features that are distinctive inplant cells are as follows:
Plant cells contain cell structures like cell wall, plastids, and
large vacuoles.
·Cell wall provide plant cells rigidity and
structural support and cell to cell interaction.
·Plastidshelp in storage of plant products.
·Chloroplasts aid in carrying out the
process of photosynthesis to produce food for the plants.
·Vacuoles are water-filled, membrane bound
organelles which stores useful materials.
Plants have specialized cells in
order to perform certain functions for the survival of plants. Some cells
manufacture and store organic molecules, others transport nutrients throughout
the plant. Some specialized plant cells include: parenchyma cells,
collenchyma cells, sclerenchyma cells, water conducting cells and food
conducting cells.
Plants cell
constitute of membrane bound nucleus and many cellular structures. These organelles carry out
functions that are necessary for the proper functioning and survival of the
cell. The cell organelles of the plant are enclosed by a cell wall and cell
membrane. The constituents of the cell are suspended in the cytoplasm or cytosol.
Structures found in plant cells but
not animal cells include a largecentral
vacuole,cell wall, and plastids such aschloroplasts.
·The largecentral vacuoleis surrounded by its own membrane and
containswaterand
dissolvedsubstances.
Its primary role is to maintain pressure against the inside of the cell wall,
giving the cell shape and helping to support the plant.
·Thecell wallis located outside thecell membrane.
It consists mainly ofcelluloseand
may also containlignin, which makes it more rigid. The cell wall
shapes, supports, and protects the cell. It prevents the cell from absorbing
too muchwaterand
bursting. It also keeps large, damaging molecules out of the cell.
·Plastidsare
membrane-bound organelles with their ownDNA. Examples arechloroplastsand
chromoplasts.Chloroplastscontain the green pigmentchlorophylland carry outphotosynthesis.
Chromoplasts make and store other pigments. They give flower petals their
bright colors.
All parts of the plant play a significant role in the proper
functioning of the cell. Unlike animals,plant cells are surrounded by a rigid cell wall.
·Cell wall: The cell wall is a rigid layer
that surrounds the plant cells. It is made up of cellulose.
Cell wall is a characteristic feature to cells of plants. Plant cell walls are primarily made up of
cellulose. Plant cell wall consists of three layers:
the primary cell wall, secondary cell wall and the middle lamella. It is
located outside the cell membrane whose main function is to provide rigidity,
strength, protection against mechanical stress and infection. Cell wall is made
up of cellulose, pectins,glycoproteins, hemicellulose and lignin.
·Cell membrane: It is the outer boundary of
the cell, it encloses the cytoplasm and the organelles of the cells. In plants
cells it is inside the cell wall. The cell membrane is semi permeable, allowing
only specific substances to pass through and blocking others.
·Chloroplasts: It is an elongated or
disc-shaped organelle containing chlorophyll. They have two membranes and
have structures that look like stack of coins.
They are flattened structures which contain chemical
chlorophyll. The process of photosynthesis occurs in this region of the plant cell. The chlorophyll is
a green pigment that absorbs energy from sunlight to make food
for the plants by converting light energy into chemical energy.
·Cytoskeleton:It
is a network of fibers made up of micro-tubule and micro-filament. They
maintain the shape and gives support to the cell.
·Microtubules:They
are hollowcylinder like structures found in the
cytoplasm of the cells. Its function is transport and structural support.
·Microfilaments:Microfialments
are solid rod likestructures whose primary
function is structural support.
·Plasmodesmata:They
are microscopic channels which traverse the cell walls ofplant cells
and enables transport and communication between them.
·Vacuole:Vacuoles
are known as cells storage center.Plant cells
have large membrane bound chamber called vacuole. Its main function is storage.
Vacuoles are found in the cytoplasm of mostplant cells.
They are membrane bound organelles, they perform functions of secretion,
excretion and storage.
·Tonoplast:A
vacuole that is surrounded by a membrane is called tonoplast.
·Plastids:Plastids
are storage organelles. They store products like starch for synthesis of fatty
acids and terpenes.
·Leucoplast:They
are a type of plastid which are non-pigmented.
·Chromoplast:They
are plastids responsible for pigment synthesis and storage. They are found in
photosynthetic eukaryotic species. They are found in colored organs of
plants like fruits and flowers.
·Golgi complex:The
Golgi bodies look like the endoplasmic reticulum and are situated near the
nucleus. They are found in almost all eukaryotic cells. Their main function is
to process and package macromolecules synthesized from other parts of the cell.
The Golgi apparatus is referred to as the cell's packaging center.
·Ribosomes:Ribosomes
are smallest and the most abundant cell organelle. It comprises of RNA and
protein. Ribosomes are sites for protein synthesis. They are found in all cells
because protein are necessary for the survival of the cell. The ribososomes are
known as the protein factories of the cell.
·Endoplasmic reticulum:Endoplasmic
reticulum is a membrane bound compartment, which look like flattened sacs lined
side by side. It is a large network of interconnecting membrane
tunnels. It is composed of both rough endoplasmic reticulum and smooth
endoplasmic reticulum.
They are responsible for protein
translation, and protein transport to be used in the cell membrane. They also
aid in sequestration of calcium, and production and storage of glycogen and
other macromolecules.
·Mitochondria:Mitochondria
are surrounded by two membranes. They are described as the 'power plants' of
the cell as they convert glucose to energy molecules (ATP). They possess their
own hereditary material which help in self duplication and
multiplication.
·Lysosome:Lysosome
contain digestive enzymes. They digest excess or worn out organelles, food
particles and any foreign bodies.
·Microbody:It
is a single membrane bound organelle that comprises of degradative enzymes
·Cytoplasm:It
is a gel-like matrix inside enclosed by the cell membrane. The cytoplasm
supports cell organelles and also prevents the cell from bursting or shrinking.
·Nucleus:It
is the control center of the cell. It is bound by a double membrane known as
the nuclear envelope. It is a porous membrane, it allows passage of
substances and is a distinctive characteristic of the eukaryotic cell. Most of
the genetic material is organized as multiple long linear DNA molecules. The
nucleus directs all the activities of the cell and also help in protein formation.
ØTypes of Plant Cells
There are three basic types of cells
in most plants. These cells make up ground tissue, which will be discussed in
another concept. The three types of cells are described inTablebelow. The different types of plant cells have
different structures and functions.
Plastids are cell organelles that
store specific things found only inplant cellbut
absent in animal cells.Inplant cellthey
are found in the cytoplasm. Plastids are spherical or ovoid in shape. They
are involved in manufacture and storage of certain important chemical
compounds.The term plastids was coined bySchimper
in 1885 and was derived from aGreekword'plastikas'which meansformedormoulded.
Plastids in plants include chloroplasts, chromoplasts, leucoplasts, amyloplast,
elaioplast and proteinoplast/aleuronoplast depending on the function they play.
·Chloroplasts:
The word chloroplast is
derived from theGreekwordchlorosmeaning green andplastmeaning form or entity. It is the
most important plastid as they are involved in photosynthesis. The
chloroplasts are situated near the surface of the cell and in parts where there
is sufficient reception of sunlight. The shape of the cholorplast varies,
it may be spheroid or ovoid or discoid.For a given cell type the size of plastid is constant but it differs
from species to species.It is about 4-5 microns in length and 1-3 microns in
thickness. The number of chloroplast may be 20 to 40 per cell may be upto
1000, the number varies from species to species but is constant for a plant.
Chloroplasts are disc-shaped and are
enclosed by a double membrane. Within the inner membrane is a protein-rich
substance known as stroma, it is embedded in a membrane system. This membrane
system forms membrane bound vesicles called thylakoids. The thylakoids lie in
stacks called grana. This contains the photosynthetic pigments - chlorophyll a
and b and carotenoids. Lamellae are tubular membranes which interconnect the
grana.
glucose + oxygen
