4.6.1 Reproduction

4.6.1.1 Sexual and asexual reproduction

Reproduction

• Reproduction is one of the essential processes of life.
  • It occurs not only between human and animals, but also in every single one of our cells.
  • There are two different types of reproduction and they are present for different purposes.
  • First we will look at mitosis in detail.

Mitosis

• Mitosis leads to the formation of two daughter cells that are identical to the original cell.
  • They are known as diploid cells.
  • It works via asexual reproduction.
  • It is paramount in growth and repair, as you require many of the same types of cells.
  • Mitosis is very prevalent in the skin and muscle cells.

Asexual Reproduction

• Asexual reproduction takes place only via mitosis.
  • It only requires one parent cell, therefore has no mixing of genetic material.
  • As there is no mixing of genetic material, the daughter cells formed are clones – genetically identical copies of the original, or parent, cell.

Stages of Mitosis

1. First, the chromatids line up on the equator of the cells.

2. Then, they are separated and sent to the poles of the cell.

3. The cell then splits into two, to form two new chromosomes.

4. A cell membrane forms between these two and two new cells are formed.

Sexual Reproduction

• Sexual reproduction is different to asexual reproduction as it does not form genetically identical clones.
  • Instead, it forms daughter cells with a mix of the characteristics of the parent cells.
  • This occurs are there is a mixing of genetic information.
  • As we saw, in mitosis there is no mixing of genetic information.

Meiosis

• Therefore, another process must take place.
  • This is the process of meiosis.
  • We will go on to discuss meiosis in more detail in the next tutorial, however it is important to remember a few things.

Meiosis

• Gametes are the sex cells of organisms.
  • In animals, they are the sperm and egg cells, however in plants, they are pollen and egg cells.
  • Gametes are an important product of meiosis.
  • Whilst gametes are being formed, the genetic material from the parent cells are mixed to form a daughter cell.
  • We will go into this in more detail in the next tutorial.

4.6.1.2 Meiosis

Meiosis

• In meiosis, the original cell divides twice, forming four daughter cells.
  • Only one set of chromosomes is given to each daughter cell.
  • This means that the cell is haploid.
  • These cells formed are the gamete cells.
  • As they only contain one set of chromosomes each, they will be genetically different from one another.

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Meiosis Products and Fertilisation

• Meiosis produces 4 non-identical haploid daughter cells.
  • Therefore during fertilisation, when the egg and sperm fuse, two haploid cells join to form one diploid cell.
  • In the final diploid cell, half the chromosomes come from the egg (mother) and half from the sperm (father).

Gametes

• The gametes each have half of the original numbers of chromosomes.
  • This means that to form an embryo, the number of chromosome has to double and be back to the original number.
  • This requires two gametes to fuse together.

Human Gametes

• The sperm and egg cells are the human gametes.
  • Each contains 23 chromosomes.
  • Therefore, they must fuse together to form a cell with 46 chromosomes.
  • This cell is known as a zygote.

Embryo

• Over time, the zygote turns into an embryo.
  • The embryo’s cells then divide by mitosis.
  • Once the embryo is large enough, chemical signals trigger the specialisation of cells by differentiation.
  • Examples of these signals include the Sonic Hedgehog signal.

4.6.1.3 Advantages and disadvantages of sexual and asexual reproduction (biology only)

Sexual Reproduction

• Sexual reproduction and asexual reproduction both have their pros and cons.
• First, we can discuss the positives of sexual reproduction.
  • Sexual reproduction leads to variation in offspring
  • It can also lead to survival advantages through natural selection.

Variation

• The greatest positive of sexual reproduction is the variety in offspring.
  • As genetic material mixes, the daughter cells are not identical.
  • This means that after fertilisation, offspring produced will mostly be genetically different to one another.

Variation and Environmental Change

• Variation may not initially seem to make a big difference initially, however it can prove a great advantage when the environment changes.
  • This could aid the survival of the organism.

Variation and Natural Selection

• Natural selection is a principle stating that the fittest survive.
  • Therefore, due to sexual reproduction, individuals are different and so some have advantages over others.

Disadvantages of Sexual Reproduction

• As meiosis is a long process and a mate is required, it is a very time and energy costly process.
  • Moreover, it cannot be done alone.

Asexual Reproduction

• Asexual reproduction also has many advantages over sexual reproduction.
  • It only requires one parent.
  • Asexual reproduction is quicker than sexual reproduction.
  • It produces clones.

One Parent

• Asexual reproduction is interesting as it only requires one parent.
  • This means that the cells involved do not need to worry about finding a mate.
  • This saves a lot of time and is far more energy efficient than sexual reproduction.

Speed

• Asexual reproduction is quicker than sexual reproduction, as only mitosis is required.
  • Meiosis is in general, a longer process than mitosis.

Genetically Identical

• Asexual reproduction leads to the production of genetically identical offspring.
  • When the parent cell has a favourable characteristic that allow it to survive in the current environment, it will then pass it on to the offspring through mitosis.
  • The process is very quick and so many advantaged offspring can be made rapidly.

Disadvantages of Asexual Reproduction

• It cannot lead to variation, as there is no transfer of genetic material.
  • This means that the offspring may not suit the environment.
  • This could lead to easy wipe out through disease.

Organisms which Reproduce Sexually and Asexually

• Some organisms are lucky enough to be capable of both asexual and sexual reproduction.
• The AQA specification requires you to know a few examples of such organisms:
  • Malaria Parasites
  • Fungi
  • Some plants

Malaria Parasites

• Malaria is transmitted by a parasite known as Plasmodium.
  • This parasite is taken up in both humans and mosquitos.
  • When the parasite is in humans, it reproduces asexually, however it reproduces sexually in mosquitos.

Fungi

• Many types of fungi have more than one method of reproduction.
  • They can reproduce asexually through budding, which is the process of forming spores.
  • However, they can also reproduce sexually.

• Some plants can reproduce both sexually and asexually.
  • Strawberry plants are capable of reproducing both sexually and asexually.
  • They produce seeds for sexual reproduction and runners for asexual reproduction.
  • Daffodils on the other hand use seeds for sexual reproduction and bulb division for asexual reproduction.

4.6.1.4 DNA and the genome

DNA and the Genome

• DNA is the genetic material that provides the blueprint for building organisms.
  • They follow patterns, however no two people, except from identical twins, have the same exact DNA.
  • In order to understand DNA, we must first consider what a genome is.

The Genome

• All of the DNA and genetic material in the whole body of an organism is its genome.
  • This can teach us many secrets about an organism.
  • Recently, the whole of the human genome was mapped out.

Mapping the Genome and Healthcare

• By mapping the genome, doctors have been able to work out what genes make patients predestined for diseases, such as the breast cancer, which is influenced by the BRCA gene.
  • Moreover, they have been able to identify the migration patterns of humans and use DNA to see how different inherited diseases have manifested.

DNA Structure

• DNA, or Deoxyribonucleic acid, is the chemical that makes up the genetic material of humans.
  • It is a polymer made up of many individual units.
  • These units then form two strands that are attracted to one another to form a double helix.

DNA Structure

• It is not feasible for the DNA to simply be wrapped into the nucleus of a cell.
  • Instead, it is packaged into chromosomes and the chromosomes reside in the nucleus.

4.6.1.5 DNA structure (biology only)

The Genome

• As we now know, DNA is a very important molecule in the body. It is also very complex.
  • We need to have a simplified understanding for the GCSE syllabus.
  • DNA is made from four different nucleotides.
  • Nucleotides are made of different structures
  • There are four nitrogenous bases.
  • Purines must bind with pyrimidines.

DNA and Nitrogenous Bases

• There are four nitrogenous bases which are:
  • Adenine (A)
  • Thymine (T)
  • Cytosine (C)
  • Guanine (G).

Purines and Pyrimidines

• The nitrogenous bases bind with one another to join nucleotides together.
  • This is a stronger bond than the bond with the phosphate groups and the nucleotides.
  • Adenine and guanine are purines, whereas thymine and cytosine are pyrimidines.

Purines and Pyrimidines

• Purines bind with pyrimidines, therefore Adenine must bind with Thymine and Cytosine must bind with Guanine.

Purines and Pyrimidines

• Each amino acid is coded for by a sequence of 3 bases.
  • The code that defines which sequences of bases for which amino acids is known as the triplet code.

Triplet Code Features

• This code has three important features.
• It is universal – all organisms use the same codons (sequence of three bases) to form proteins
• It is degenerate – this means that several codons can code for the same protein.
  • This shields organisms from mutation to a certain extent.
• It is non-overlapping – this means that the code is read as a sequence of three bases at a  time and then you move on to the next three bases.

Introduction to Protein Synthesis

• DNA’s biggest process is protein synthesis.
  • This is how the growth and repair of our body occurs.
  • DNA provides a template upon which proteins are coded for.
  • There is some slightly confusing terminology in this tutorial.

Transcription and Translation

• Protein synthesis is the process by which the base sequence found in genes on DNA is used to make polypeptides.
• It occurs in two major phases:

1. Transcription
2. Translation

RNA

• RNA is ribonucleic acid.
  • An important part of protein synthesis is that the DNA stays in the nucleus.
  • A copy of the DNA, known as mRNA (messenger RNA) leaves the nucleus and moves to the ribosome.
  • The ribosome is the organelle responsible for protein synthesis.
  • The copying of the DNA is known as transcription.

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tRNA

• When the mRNA moves to the ribosome, it recruits tRNA (transfer RNA structures) that are complementary to the base sequence of the mRNA.
  • Remember we discussed earlier what complementary bases are – A’s complementary base is T and C’s complementary base is G.
  • The tRNA molecule carry amino acids with it.
  • The amino acids then bond with another and polypeptides are formed.
  • This is translation.

DNA Base Order and Protein Structure

• This therefore means that the order of the bases in the DNA determines what amino acids are recruited and what polypeptides are made.
  • Therefore, the structure of the DNA influences what protein is made.

Protein Shape and Enzymes

• Once these protein chains are formed, they are folded in a few ways, so each protein has its own unique shape.
  • This is imperative for enzymes, as they have their own individual active sites.

Gene Expression and Proteins

• The expression of proteins can be altered by switching genes on and off.
• DNA is separated into coding and non-coding DNA.
  • The coding DNA is what directly determines the proteins formed as a copy of it is made.
  • Non-coding DNA on the other hand has a different function as it works to turn on and off genes.

Gene Expression and Cell Differentiation

• This can lead to differences in the appearance of the expression of the gene.
  • As every cell contains all of the DNA in the body, genes must be switched on and off in order to stop every cell producing every protein.
  • Therefore, different cells have different genes turned on and off.

Mutations

• Mutations are random changes in the DNA sequence.
  • They can lead to alterations in the polypeptide chain and so can lead to non-functional proteins.
  • The majority of mutations don’t cause profound changes in proteins.
• The triplet code is degenerate.
  • This means that in many circumstances, one base may change, but the amino acid coded
  • for by the codon will not.
  • This means that the right protein will still form.
  • The protein may only change slightly, therefore the functionality may not actually be altered.

Mutations

• However, in cases such as enzymes, even small mutations can lead to changes in the active site.
  • This could easily make them non-functional.
• This could also be problematic in DNA that codes for structural proteins.
  • If a sequence for a key structural protein is altered, it may lead to big problems in the body.

Types of Mutation

• Mutations can occur in three ways:
  • Deletion.

• Mutations can occur in three ways:
  • Insertion.

• Mutations can occur in three ways:
  • Substitution.

4.6.1.6 Genetic inheritance

Gametes and Chromosomes

• A chromosome is a structure in the nucleus of a cell that is made up of one condensed molecule of DNA.

• A gamete is a sex cell.
  • These cells contain half the normal number of chromosomes.
  • A normal cell contains 46 chromosomes. These are diploid cells.
  • Gametes (sex cells), contain 23 chromosomes. This means that they are haploid cells.

Genes and Alleles

• A gene is a section of DNA that codes for a certain protein or characteristic.
• An allele is a certain version of a gene.

Genes and Alleles

• A person will have two copies of an allele for a given gene.

Dominant and Recessive Alleles

• When a dominant allele is present, it is always expressed in an organism.
• The recessive allele is only expressed in the absence of a dominant allele.

Genotype and Phenotype

• The genotype is the genetic sequence of an organism.
• The phenotype is the physical characteristics expressed by an organism based on the environment and genotype.

Red-Green Colourblindness

• The majority of characteristics require a few different genes in order to be expressed.
• Some specific characteristics on the other hand however are controlled by only one gene.
  • An example of this is the red-green colourblindness gene in humans and fur colour in mice.
• Red-green colour blindness is more complicated, due to other circumstances, but it is important to remember these two examples.

Dominant Alleles

• A good example to understand the terminology used above is hair colour.
• The allele for dark hair is dominant, whereas the allele for light hair is recessive.
• If both alleles provided by the parents (the genotype) are dominant, the person will have dark hair (the phenotype).
  • This is regarded to be homozygous dominant.

Recessive Alleles and Heterozygous Genotypes

• If both alleles are recessive, the person will have light hair.
  • This is considered to be homozygous recessive.

Recessive Alleles and Heterozygous Genotypes

• If one dominant allele and one recessive allele is present, the person will have dark hair.
  • This is considered to be heterozygous.
• This works as the dark hair is the dominant allele.
  • As this allele is present, it must be expressed.

Genotype and Phenotype

• With some genes, you can predict the phenotype that is expressed, dependent on the alleles of the parents.
  • This can be done with single gene crosses.
  • Therefore, these phenotypes must only be influenced by one set of alleles.
  • When more than one set of alleles influences a phenotype, it is much harder to predict the probability of the characteristics shown.
• The best way to explain this is through a worked example.
  • Let us pretend that the father has dark hair and the mother has blonde hair.

• The best way to explain this is through a worked example.
  • The alleles for the father could be DD (homozygous dominant) or Dd (heterozygous).

• The best way to explain this is through a worked example.
  • The alleles for the mother must be dd (homozygous recessive).

• The best way to explain this is through a worked example.
  • If the father is heterozygous dominant (Dd), we can use a Punnett square to predict the possible genotypes for the offspring of the mum and dad in the example.

• You can use Punnett Squares with any combination of alleles as long as they are single gene crosses.

• First, find the alleles of the parents.
• Then place the alleles of one parent at the top of the punnet square and one set of alleles on the side.

• Then write one allele from each parent per box.

• Do this for four boxes which will allow you to predict the alleles.

• As you can see, two Dd (heterozygous) and two dd (homozygous recessive) are formed which means that the probability of having dark hair is 2/4 – therefore 0.5.
• The probability of having blonde hair is then also 2/4 – therefore 0.5.

• The ratio therefore would be 1:1.
• This technique can be used to work out the proportion of having certain characteristics.
• Using direct proportion, 2 out of 4 offspring are Dd and 2 are dd.

4.6.1.7 Inherited disorders

How Disorders are Inherited

• Some disorders can be inherited genetically.
  • They can be transmitted from the parents to the offspring via the inheritance of different alleles.
  • There are two main examples that are required in this syllabus.
  • They are polydactyly and cystic fibrosis.

Polydactyly

• Polydactyly is the presence of extra fingers or toes.
  • This is caused by a dominant allele, therefore, if any of the offspring inherit the allele for polydactyly.
  • They will most definitely suffer from the condition, unless there is any mutation.

Cystic Fibrosis

• However, sufferers of Cystic Fibrosis have a mutation in their code for certain chloride channels in their cell membranes.
  • This leads to a secretion of extra thick mucus.
  • This can cause huge breathing difficulties to patients.
  • It is caused by a recessive allele.
  • Therefore, it can be passed on by unaffected heterozygous parents.

• Cystic Fibrosis is another important genetic condition.
  • Some cells produce a secretion known as mucus.
  • However, sufferers of Cystic Fibrosis have a mutation in their code for certain chloride channels in their cell membranes.
  • It is caused by a recessive allele.
• Therefore, it can be passed on by unaffected heterozygous parents.

Types of Screening

• Embryonic screening can take place in order to test for alleles.
• This process can take place in many ways:
  • Amniocentesis when a sample of amniotic fluid from the area surrounding the foetus can be tested.
  • Chorionic Villus Sampling when a sample from the placenta is taken and checked.

Ethics in Non-IVF Pregnancy

• Once the parent has their results, there is an ethical quagmire.
  • They will be in the knowledge of a portion of the health of the child.
  • This can lead to an element of choice in some cases.
  • Parents may get the option of terminating the pregnancy.

Ethics in IVF

• During IVF, if an embryo is found to have a genetic disorder, it is not implanted.
  • There are questions regarding the ethics of this, as the embryo is then destroyed.

Cost

• One thing to remember is that all of this testing is very expensive, however the cost of the healthcare for the child who suffers from a disorder may be much greater.

False Results

• If the results are a false positive or a false negative, they could lead to great trauma to a family.

4.6.1.8 Sex determination

Sex Chromosomes Explanation

• As we discussed earlier, normal human body cells contain 23 pairs of chromosomes, so 46 overall.
  • 22 of these control characteristics and are known as autosomes.
  • 1 pair is the sex chromosomes and these chromosomes control gender.
  • The sex chromosomes differ in males and females.
  • Males – The male sex chromosomes are X and Y.
  • Females – The female sex chromosomes are X and X.

Sex-Linked Conditions

• As some characteristics are dependent on sex, you must be able to work out the probabilities of sex linked genetic diseases

• The biggest difference here is that the top column and left most row are no longer just dependent on the alleles, instead they are dependent on gender.