Inheritance- Biology 3.5

DNA / Deoxyribonucleic acid

is a biomolecule [a chemical found in living organisms] – it is the genetic material in MOST living organisms.

What is DNA made out of?

DNA is made of very long strands, twisted to form a double helix, which contain four different compounds, called bases.

A sequence of three bases is the code for a particular amino acid. The order of bases controls the order in which amino acids are assembled to produce a particular protein. 

chromosome

is a thread-like structure made up of DNA. Chromosomes carry genetic information in the form of genes

In body cells the chromosomes are normally found in pairs - each pair has one maternal and one paternal chromosome.

gene

is a length of DNA that codes for a protein. Genes carry genetic information.

Asexual reproduction:

●​ only one individual is needed as the parent - no gametes required.

●​ no mixing of genetic information and so no genetic variation in the

offspring.

●​ the genetically identical offspring are known as clones.

●​ Involves mitosis.

Examples: binary fission in bacteria, Amoeba, starfish, aphids and plants

Sexual reproduction:

●​ Involves fertilisation (fusion) of male and female gametes produced by meiosis.

●​ The mixture of the genetic information from two parents leads to genetic

variation in the offspring

How fast can asexual and sexual reproduction occur, and why is this important?

• Asexual: Occurs quickly → allows fast colonisation.

• Sexual: Takes time → organisms may produce fewer offspring in a lifetime.

How many parents are needed for asexual vs sexual reproduction, and what are the effects?

• Asexual: Only one parent → no need to find a mate, less energy needed to make gametes.

• Sexual: Two parents → isolated individuals may not reproduce, more energy needed for gametes and mate attraction.

Is there genetic variation in offspring from asexual vs sexual reproduction? Why does this matter?

• Asexual: No variation (clones) → more disease risk, harmful genes passed on, no evolution.

• Sexual: Yes, variation → some offspring may survive environmental changes.

Mitosis

Body cells [somatic cells] are diploid [with TWO sets of chromosomes - one

paternal and one maternal]

●​ They divide by mitosis to produce two diploid cells which are genetically

identical.

●​ Mitosis is required for growth or to produce replacement cells for tissue

repair.

●​ Mitosis is also required for asexual reproduction.

Meiosis

Meiosis is a type of cell division that produces gametes (sex cells – sperm and eggs) with half the number of chromosomes (haploid, n) as the parent cell (diploid, 2n).

Purpose of Meiosis 

• To produce gametes for sexual reproduction.

• To ensure genetic variation in offspring.

• To maintain a constant chromosome number in a species across generations.

Mitosis vs Meiosis

Feature	Mitosis	Meiosis
Purpose	Growth, repair, and asexual reproduction	Production of gametes for sexual reproduction
Number of divisions	One	Two
Number of daughter cells	Two	Four
Chromosome number in daughter cells	Same as parent (diploid, 2n)	Half of parent (haploid, n)
Genetic variation	None – offspring are genetically identical (clones)	Yes – offspring genetically different due to crossing over and independent assortment Occurs in Somatic (body) cells Sex organs (ovaries and testes) Homologous chromosomes Do not pair up Pair up and may exchange segments (crossing over)

Diploid and Haploid 

Diploid (2n) Cells

• Definition: Cells that contain two sets of chromosomes — one set from the mother and one set from the father.

• Example (in humans):

  • Humans have 46 chromosomes in diploid cells (23 pairs).

  • These are found in body (somatic) cells — e.g. skin, muscle, and nerve cells.

Haploid (n) Cells

• Definition: Cells that contain only one set of chromosomes — half the normal number.

• Example (in humans):

  • Human gametes (sperm and egg) each have 23 chromosomes.

How does cell differentiation in animals differ from cell differentiation in plants?

Most types of animal cell differentiate at an early stage whereas many plant cells retain the ability to differentiate throughout life. In mature animals, cell division is mainly restricted to repair and replacement. 

Stem cells

​Undifferentiated cells that divide continually by mitosis

•​produce daughter cells that can differentiate and become specialised for

specific functions.

Where are stem cells found

Stem cells are found in:

•​Embryos

•​Umbilical cord blood.

•​In the adult human body – e.g. in bone marrow, skin, liver, etc.

What are stem cells used for?

Used in therapeutic cloning an embryo is produced with the same genes as the patient. Stem cells from the embryo will not be rejected  by the patient’s body so they may be used for medical treatment 

They also may be able to help conditions such as paralysis 

Social and ethical issues concerning the use of the stem cells? 

Source of Stem Cells

• Embryonic stem cells are obtained from early embryos.

• Ethical concern: Destroying embryos is seen by some as destroying potential human life.

2. Consent

• Stem cells can come from donated embryos or adult tissue.

• Ethical concern: Donors must give informed consent.

3. Religious and Cultural Beliefs

• Some religions oppose using embryos for research.

• Stem cell research may conflict with moral or cultural values.

4. Risk of Harm

• Experimental treatments may not be fully tested.

• Risk of tumors or immune rejection.

• Ethical concern: Patients may be exploited or harmed in trials.

5. Social Issues

• Cost: Stem cell therapies may be expensive, leading to inequality in access.

• Legal and regulatory concerns: Laws differ in different countries about what research is allowed.

Allele

a version of a gene

Each pair of chromosomes contains the same genes although each gene may have different forms called alleles.

Genotype:

genetic make-up of an organism in terms of the alleles present.

Phenotype:

observable features of an organism.

Homozygous:

having two identical alleles of a particular gene.

Heterozygous:

having two different alleles of a particular gene.

Dominant allele:

an allele that is expressed in homozygous and heterozygous

combinations. It masks the expression of the recessive allele.

Recessive allele

an allele that is only expressed when in homozygous combinations, when the dominant allele is not present.

inherited disorders caused by a faulty RECESSIVE allele

Cystic Fibrosis (lungs and digestive system gets clogged with mucus)

Inherited disorders caused by a faulty DOMINANT allele:

Polydactyly ( extra fingers )

Two faulty recessive alleles

Sickle cell anemia (abnormal haemoglobin and sickle-shaped red blood cells. These can get stuck in blood vessels)

change in the chromosome number.

Down syndrome (People with Down’s Syndrome have 47 chromosomes rather than 46.)

Tissue culture

Tissue culture is a technique used to grow new plants from small pieces of plant tissue under sterile and controlled conditions.

Steps in Tissue Culture Process:

1. Selection and Sterilization of Tissue Sample

   • A small piece of tissue (called an explant) is taken from the parent plant.

   • The explant is sterilized to remove microorganisms.

2. Growth on Nutrient Medium

   • The tissue is placed on a sterile agar gel containing essential nutrients and plant hormones (such as auxins and cytokinins).

   • This medium supports cell division and growth.

3. Formation of Callus

   • Cells divide and form a mass of undifferentiated cells called a callus.

4. Induction of Shoots and Roots

   • Hormones in the medium are adjusted to trigger shoot and root formation from the callus.

5. Development of Plantlets

   • The developing shoots and roots form tiny plantlets under controlled conditions.

6. Separation and Multiplication

   • The plantlets or callus can be divided and transferred to new growth media for further multiplication.

7. Acclimatization

   • Once plantlets are well developed, they are transferred to soil or natural conditions to grow into mature plants.

Advantages of tissue culture

. Production of mature plants is faster.

2. Multiple plants can be produced without seeds / pollinators.

3. Clones / identical copies produced – ensures desirable traits – ​

flowers / fruits etc.

4. Plants produced in sterile conditions – reduced chances of ​

transmitting diseases, pests and pathogens.

5. Mass propagation of plants for commercial use / Reforestation programs

6. Propagation of endangered plants.

disadvantages of tissue culture

Setting up of a plant tissue culture laboratory is very ​ expensive, including machines, reagents etc.

Processes must be handled by highly trained people.

Reduces genetic diversity.

Increases susceptibility of the propagated plants to diseases as they are all genetically identical.

If correct precautions are not taken, the whole stock may be contaminated or infected.

Embryo transplants

splitting cells from a developing animal embryo before they become specialized, then transplanting the identical embryos into host mothers

• Sperm Collection

  • Sperm is collected from a bull with desirable traits (e.g., high milk yield or strong genetics).

• Artificial Insemination

  • The sperm is used to artificially inseminate a selected cow (also of high genetic quality).

• Fertilization and Embryo Development

  • Fertilization occurs inside the cow’s uterus, forming zygotes.

  • These zygotes develop into early embryos (before they attach to the uterus wall).

• Embryo Removal

  • The developing embryos are carefully removed from the cow’s uterus after a few days.

• Embryo Splitting

  • Each embryo is divided into several smaller groups of cells, each capable of developing into a full calf.

  • These split embryos are genetically identical (clones).

• Implantation into Foster Mothers

  • The embryos are implanted into the uteruses of surrogate (foster) cows.

• Birth of Calves

  • Each surrogate cow gives birth to a genetically identical calf — clones of the original embryo.

Advantages of embryo transplant

Advantages

• Produces many high-quality offspring quickly.

• Clones inherit desirable traits (e.g., high milk production, disease resistance).

• Helps in preserving valuable or endangered breeds.

Disadvantages of embryo transplant

• Expensive and technically complex.

• Reduces genetic diversity.

• Ethical and welfare concerns about animal cloning.

Adult cell cloning

The nucleus is removed from an unfertilized egg cell and the nucleus from an adult

body cell, e.g. a skin cell, is inserted into the egg cell.

●​ An electric shock then acts as the catalyst for the egg cell to begin to divide to

form embryo cells.

●​ These embryo cells contain the same genetic information as the adult skin cell.

●​ When the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue its development.

Genetic engineering

– Modifying the genome of an organism by inserting a gene from a different organism for a desired characteristic.

How are genes transferred to other organisms in genetic engineering?

• Enzymes are used to isolate the required gene from chromosomes.

• The gene is inserted into a vector (usually a bacterial plasmid or virus).

• The vector carries the gene into the target cells, where it can function.

GM crops

includes crops resistant to insect attack or to herbicides

GM crops generally show increased yields.

Concerns of the GM crops 

effect on populations of wild flowers and insects 

uncertainty about the effects of eating GM crops on human health