6.1 Reproduction Flashcards
Mitosis and meiosis
Mitosis is a type of nuclear division that gives rise to cells that are genetically identical.
It is used for growth, repair of damaged tissues, replacement of cells and asexual reproduction.
Meiosis is a type of nuclear division that gives rise to cells that are genetically different.
It is used to produce gametes (sex cells).
Sexual reproduction
Sexual reproduction is a process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote (fertilised egg cell) and the production of offspring that are genetically different from each other.
The gametes of animals are the sperm cells and egg cells.
The gametes of flowering plants are the pollen cells and egg cells.
Fertilisation is defined as the fusion of gamete nuclei, and as each gamete comes from a different parent, there is variation in the offspring.
The formation of gametes involves meiosis.
Asexual reproduction
Asexual reproduction does not involve sex cells or fertilisation.
Only one parent is required so there is no fusion of gametes and no mixing of genetic information.
As a result, the offspring are genetically identical to the parent and to each other (clones).
Asexual reproduction is defined as a process resulting in genetically identical offspring from one parent.
Only mitosis is involved in asexual reproduction.
Meiosis
Cells in reproductive organs divide by meiosis to form gametes (sex cells).
The number of chromosomes must be halved when the gametes are formed.
Otherwise, there would be double the number of chromosomes after they join at fertilisation in the zygote (fertilized egg).
This halving occurs during meiosis, and so it is described as a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells.
It starts with chromosomes doubling themselves as in mitosis and lining up in the centre of the cell.
After this has happened the cells divide twice so that only one copy of each chromosome passes to each gamete.
We describe gametes as being haploid - having half the normal number of chromosomes.
Because of this double division, meiosis produces four haploid cells.
Process of meiosis
Each chromosome is duplicated (makes identical copies of itself), forming X-shaped chromosomes.
First division: the chromosome pairs line up along the centre of the cell and are then pulled apart so that each new cell only has one copy of each chromosome.
Second division: the chromosomes line up along the centre of the cell and the arms of the chromosomes are pulled apart.
A total of four haploid daughter cells will be produced.
Importance of meiosis
Produces gametes eg. sperm cells and egg cells in animals, pollen grains and ovum cells in plants.
Increases genetic variation of offspring.
Meiosis produces variation by forming new combinations of maternal and paternal chromosomes every time a gamete is made, meaning that when gametes fuse randomly at fertilisation, each offspring will be different from any others.
Fertilisation
Gametes join at fertilisation to restore the normal number of chromosomes.
When the male and female gametes fuse, they become a zygote (fertilised egg cell).
This contains the full number of chromosomes, half of which came from the male gamete and half from the female gamete.
The zygote divides by mitosis to form two new cells, which then continue to divide and after a few days form an embryo.
Cell division continues and eventually many of the new cells produced become specialised (the cells differentiate) to perform particular functions and form all the body tissues of the offspring.
The process of cells becoming specialised is known as cell differentiation.
Advantages and disadvantages of sexual reproduction
Advantages - Increases genetic variation.
The species can adapt to new environments due to variation, giving them a survival advantage.
Disease is less likely to affect the population due to variation.
Natural selection can be sped up by humans in selective breeding to increase food production.
We have controlled sexual reproduction in cows and selectively bred them to produce offspring that produce more milk and more meat than they would have under natural conditions.
Disadvantages - Takes time and energy to find mates.
Difficult for isolated members of the species to reproduce.
Advantages and disadvantages of asexual reproduction
Advantages - The population can be increased rapidly when the conditions are right.
Can explore suitable environments quickly.
More time and energy efficient.
Reproduction is completed much faster than sexual reproduction.
Disadvantages - Limited genetic variation in population - offspring’s are genetically identical to their parents.
The population is vulnerable to changes in conditions and may only be suited for one habitat.
Disease is more likely to affect the whole population as there is no genetic variation.
Malarial parasites reproduction
Malaria is caused by parasites that are carried by mosquitoes.
The parasites are transferred to a human when the mosquito feeds on the human’s blood.
These malarial parasites reproduce asexually in the human host, but sexually in the mosquito.
Fungi reproduction
Many fungi reproduce both asexually and sexually.
These species of fungi release spores, which develop into new fungi.
These spores can be produced via asexual or sexual reproduction.
Spores that are produced via sexual reproduction show variation (they are genetically different from each other).
Plants reproduction
Many plants produce seeds via sexual reproduction but are also able to reproduce asexually.
They reproduce asexually in different ways:
Some plants (e.g. strawberry plants) produce ‘runners’ (stems that grow horizontally away from the parent plant, at the end of which new identical offspring plants form).
Some plants (e.g. daffodils)
reproduce via bulb division (new bulbs form from the main bulb underground and then grow into new identical offspring plants).
Genome
The entire set of the genetic material of an organism is known as its genome.
Biologists now know the entire human genome (they have worked out all the genes that are found in humans).
DNA
The genetic material in the nucleus of a cell is composed of a chemical called DNA.
DNA, or deoxyribonucleic acid, is the molecule that contains the instructions for growth and development of all organisms.
DNA is a polymer made up of two strands forming a double helix.
DNA is contained in structures called chromosomes.
Chromosomes are located in the nucleus of cells.
Genes
A gene is a short length of DNA found on a chromosome.
Each gene codes for a particular sequence of amino acids.
These sequences of amino acids form different types of proteins.
There are many different types of proteins but some example of these could be:
structural proteins such as collagen found in skin cells
enzymes
hormones
Genes control our characteristics as they code for proteins that play important roles in what our cells do.
The human genome project
The Human Genome Project (completed in 2003) was the name of the international, collaborative research effort to determine the DNA sequence of the entire human genome and record every gene in human beings.
This was a very important breakthrough for several reasons:
From a medical perspective, as it has already and will continue to improve our understanding of the genes linked with different types of disease and inherited genetic disorders, as well as the help us in finding treatments.
The human genome has also made it possible to study human migration patterns from the past, as different populations of humans living in different parts of the world have developed very small differences in their genomes.
Nucleotides
DNA is a polymer (a molecule made from many repeating subunits).
These individual subunits of DNA are called nucleotides.
Each nucleotide consists of a common sugar and phosphate group with one of four different bases attached to the sugar.
Base pairing
There are four different nucleotides.
These four nucleotides contain the same phosphate and deoxyribose sugar, but differ from each other in the base attached.
There are four different bases: Adenine (A), Cytosine (C), Thymine (T) and Guanine (G).
The bases on each strand pair up with each other, holding the two strands of DNA in the double helix.
The bases always pair up in the same way:
Adenine always pairs with Thymine (A-T)
Cytosine always pairs with Guanine (C-G)
This is known as ‘complementary base pairing’.
Coding for amino acids
A sequence of three bases is the code for a particular amino acid.
The order of bases controls the order and different types of amino acids that are joined together.
These amino acid sequences then form a particular type of protein.
In this way, it is the order of bases in the DNA which eventually determines which proteins are produced.
Double helix
The phosphate and sugar section of the nucleotides form the ‘backbone’ of the DNA strand (like the sides of a ladder) and the base pairs of each strand connect to form the rungs of the ladder.
It is this sequence of bases that holds the code for the formation of proteins.
The DNA helix is made from two strands of DNA held together by hydrogen bonds.
Protein synthesis
Proteins are made in the cell cytoplasm on structures called ribosomes.
Ribosomes use the sequence of bases contained within DNA to make proteins.
DNA cannot travel out of the nucleus to the ribosomes (it is far too big to pass through a nuclear pore) so the base code of each gene is transcribed onto an RNA molecule called messenger RNA (mRNA).
mRNA can move out of the nucleus and attaches to a ribosome (the mRNA acts as a messenger between DNA and the ribosome).
The correct sequence of amino acids are then brought to the ribosome and joined together.
This amino acid sequence then forms into a protein.
Changes to protein
A change in DNA structure may result in a change in the protein synthesised by a gene.
If there is a change in the order of the bases in a section of DNA (eg. in a gene), then a different protein may be produced.
This protein may not function in the same way as the original protein would have (before the change occurred in the DNA).
Function of ribosomes
The ribosome ‘reads’ the code on the mRNA in groups of three.
Each triplet of bases codes for a specific amino acid.
Carrier molecules bring specific amino acids to add to the growing protein chain in the correct order.
In this way, the ribosome translates the sequence of bases into a sequence of amino acids that make up a protein.
Once the amino acid chain has been assembled, it is released from the ribosome so it can fold and form the final structure of the protein.
The triplet code of DNA (carried by mRNA) is read by the ribosome and amino acids are attached together in a specific sequence to form the protein.
Protein structure
When the protein chain is complete it folds up to form a unique shape.
This unique shape enables the proteins to fulfil a specific function. For example, proteins can be:.
Enzymes – proteins that act as biological catalysts to speed up chemical reactions occurring in the body (eg. maltase is an enzyme that breaks down maltose into glucose).
Hormones – proteins that carry messages around the body (eg. testosterone is a hormone that plays an important role in the development of the male reproductive system and development of male secondary sexual characteristics, such as increased muscle mass and growth of body hair).
Structural proteins – proteins that provide structure and are physically strong (eg. collagen is a structural protein that strengthens connective tissues such as ligaments and cartilage).
Mutations
Mutations are random changes that occur in the sequence of DNA bases in a gene or a chromosome.
Mutations occur continuously.
As the DNA base sequence determines the sequence of amino acids that make up a protein, mutations in a gene can sometimes lead to a change in the protein that the gene codes for.
Most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed.
Insertion
A new base is randomly inserted into the DNA sequence.
An insertion mutation changes the amino acid that would have been coded for by the group of three bases in which the mutation occurs.
Remember – every group of three bases in a DNA sequence codes for an amino acid.
An insertion mutation also has a knock-on effect by changing the groups of three bases further on in the DNA sequence.
Deletions
A base is randomly deleted from the DNA sequence.
Like an insertion mutation, a deletion mutation changes the amino acid that would have been coded for by the group of three bases in which the mutation occurs.
Like an insertion mutation, a deletion mutation also has a knock-on effect by changing the groups of three bases further on in the DNA sequence.
Substitutions
A base in the DNA sequence is randomly swapped for a different base.
Unlike an insertion or deletion mutation, a substitution mutation will only change the amino acid for the group of three bases in which the mutation occurs; it will not have a knock-on effect.
Effects of mutation
Most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed.
However, a small number of mutations code for a significantly altered protein with a different shape.
This may affect the ability of the protein to perform its function. For example:
If the shape of the active site on an enzyme changes, the substrate may no longer be able to bind to the active site.
A structural protein (like collagen) may lose its strength if its shape changes.
Gene switching
Not all parts of DNA code for proteins.
Some non-coding parts of DNA can switch genes on and off.
This means they can control whether or not a gene is expressed.
Variations in these areas of DNA may affect how genes are expressed.
if a mutation occurs in a section of non-coding DNA that controls gene expression, the expression of these genes may be altered or in some cases, the mutation may cause them not to be expressed at all.
Gametes
Gametes are sex cels - (in animals sperm and ovum) (in plants pollen nucleus and ovum).
Alleles
Alleles are different versions of a particular gene.
A dominant allele is always expressed, even if only one copy is present.
A recessive allele is only expressed if two copies are present (therefore no dominant allele present).
The combination of alleles that control each characteristic is a genotype.
Monohybrid inheritance
Some characteristics are controlled by a single gene.
The inheritance of these single genes is called monohybrid inheritance.
As we have two copies of each chromosome, we have two copies of each gene and therefore two alleles for each gene.
One of the alleles is inherited from the mother and the other from the father.
This means that the alleles do not have to contain the same genetic code.
The combination of alleles that control each characteristic is called the genotype.
Alleles can be dominant or recessive.
A dominant allele only needs to be inherited from one parent in order for the characteristic to show up in the phenotype.
A recessive allele needs to be inherited from both parents in order for the characteristic to show up in the phenotype.
If there is only one recessive allele, it will remain hidden and the dominant characteristic will show.
If the two alleles of a gene are the same, we describe the individual as being homozygous.
An individual could be homozygous dominant (having two copies of the dominant allele), or homozygous recessive (having two copies of the recessive allele).
If the two alleles of a gene are different, we describe the individual as being heterozygous.
Multiple gene inheritance
Most characteristics are a result of multiple genes interacting, rather than a single gene.
Characteristics that are controlled by more than one gene are described as being polygenic.
Polygenic characteristics have phenotypes that can show a wide range of combinations in features.
The inheritance of these polygenic characteristics is called polygenic inheritance (poly = many/more than one).
Polygenic inheritance is difficult to show using genetic diagrams because of the wide range of combinations.
An example of polygenic inheritance is eye colour – while it is true that brown eyes are dominant to blue eyes, it is not as simple as this as eye colour is controlled by several genes.
Predicting inheritance
Monohybrid inheritance is the inheritance of characteristics controlled by a single gene.
This can be determined using a genetic diagram known as a Punnett square.
A Punnett square diagram shows the possible combinations of alleles that could be produced in the offspring.
From this, the ratio of these combinations can be worked out.
Remember the dominant allele is shown using a capital letter and the recessive allele is shown using the same letter but lower case.
Family trees
Family tree diagrams are usually used to trace the pattern of inheritance of a specific characteristic (usually a disease) through generations of a family.
This can be used to work out the probability that someone in the family will inherit the genetic disorder.
Predicting probability
A Punnett square diagram shows the possible combinations of alleles that could be produced in the offspring.
From this, the ratio of these combinations can be worked out.
However, you can also make predictions of the offspring characteristics by calculating the probabilities of the different phenotypes that could occur.
Inherited diseases
Some disorders are inherited (passed from parents to offspring).
These disorders are caused by the inheritance of certain alleles.
For example, cystic fibrosis and polydactyly are two genetic disorders that can be inherited.
Cystic fibrosis
Cystic fibrosis is a genetic disorder of cell membranes.
It results in the body producing large amounts of thick, sticky mucus in the air passages.
Over time, this may damage the lungs and stop them from working properly.
Cystic fibrosis is caused by a recessive allele (f).
This means:
People who are heterozygous (only carry one copy of the recessive allele) won’t be affected by the disorder but are ‘carriers’.
People must be homozygous recessive (carry two copies of the recessive allele) in order to have the disorder.
If both parents are carriers, the chance of them producing a child with cystic fibrosis is 1 in 4, or 25%.
If only one of the parents is a carrier (with the other parent being homozygous dominant), there is no chance of producing a child with cystic fibrosis.
Polydactyly
Polydactyly is a genetic disorder that causes someone to be born with extra fingers or toes.
Polydactyly is caused by a dominant allele (D).
This means:
Even if only one parent is a carrier, the disorder can be inherited by offspring.
Embryo screening
In vitro fertilisation (IVF) is the process by which embryos are fertilised in a laboratory and then implanted into the mother’s womb.
A cell can be taken from the embryo before being implanted and its genes can be analysed.
It is also possible to get DNA from the cell of an embryo that’s already in the womb and analyse its genes in the same way.
Genetic disorders (e.g. cystic fibrosis) can be detected during this analysis.
Gene therapy
Gene therapy is the process by which normal alleles are inserted into the chromosomes of an individual who carries defective alleles (e.g. those that cause a genetic disorder).
It is a developing technology and is not always successful.
The process raises similar economic, social and ethical concerns to embryo screening:
Many people believe that gene alteration is unnatural.
Many believe it is a good idea as it can help to alleviate suffering in people with genetic disorders.
Advantages and disadvantages of embryo screening
Advantages - Can avoid suffering by stopping children being born with genetic disorder
Disadvantages - This process could imply that people with genetic disorders are undesirable.
Embryo screening is very expensive and therefore not available to everyone.
It could be abused to change the gender of the baby to produce a desirable offspring.
Human chromosomes
Chromosomes are thread like structures of DNA, carrying genetic information in the form of genes. They are located in the nucleus of the cell.
Ordinary human body cells contain 23 pairs of chromosomes
22 pairs control characteristics only, but one of the pairs carries the genes that determine sex
In females, the sex chromosomes are the same (XX)
In males, the sex chromosomes are different (XY)
