Topic 6a - Reproduction Flashcards
What is mitosis?
Mitosis is a type of nuclear division that gives rise to cells that are genetically identical.
What is mitosis used for? (4)
- growth
- repair of damaged tissues
- replacement of cells
- asexual reproduction
What is meiosis?
Meiosis is a type of nuclear division that gives rise to cells that are genetically different.
What is meiosis used for?
It is used to produce gametes (sex cells).
What is 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.
What are the gametes of animals?
sperm cells and egg cells
What are the gametes of flowering plants?
pollen cells and egg cells
What is fertilisation?
Fertilisation is defined as the fusion of gamete nuclei.
Why is there variation in offspring from sexual reproduction?
In sexual reproduction there is mixing of genetic information (from gametes) which leads to variety in the offspring
What does the formation of gametes involve?
The formation of gametes involves meiosis.
What is asexual reproduction?
Asexual reproduction is defined as a process resulting in genetically identical offspring from one parent.
What does asexual reproduction not involve?
Asexual reproduction does not involve sex cells or fertilisation.
How many parents are required for asexual reproduction? What does this mean?
Only one parent is required so there is no fusion of gametes and no mixing of genetic information.
Are offspring the same or different in asexual reproduction?
The offspring are genetically identical to the parent and to each other (clones)
What process does asexual reproduction only involve?
Mitosis
How do cells in reproductive organs divide? What do they form?
Cells in reproductive organs divide by meiosis to form gametes (sex cells).
What must be halved when forming a gamete?
The number of chromosomes must be halved when the gametes are formed.
Why must the number of chromosomes be halved when gametes are formed?
Otherwise, there would be double the number of chromosomes after they join at fertilisation in the zygote.
What are gametes described as?
Haploid cells
What is a haploid cell?
A cell that has half the normal number of chromosomes (1 set).
What is a diploid cell?
A cell that contains 2 sets of chromosomes - a normal body cell.
What is the 4 step process of cell division by meiosis to produce haploid gamete cells? - best to use a whiteboard / blurting
- 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.
What is important about the process of cell division by meiosis to produce haploid gamete cells?
- Produces gametes
- Increases genetic variation of offspring
How does meiosis produce variation when forming gametes?
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.
What happens to gametes at fertilisation?
Gametes join at fertilisation to restore the normal number of chromosomes.
What do male and female gametes become when they fuse?
A zygote
What does a zygote contain?
The full number of chromosomes, half of which came from the male gamete and half from the female gamete.
What happens to the zygote to form an embryo?
The zygote divides by mitosis to form two new cells, which then continue to divide and after a few days form an embryo.
What happens to the embryonic cells with time?
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.
What are the advantages of sexual reproduction? (4)
- 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)
- The ability to use it and control it for our own needs, i.e. with selective breeding
What are the disadvantages of sexual reproduction? (2)
- Takes time and energy to find mates
- Difficult for isolated members of the species to reproduce
What are the advantages of asexual reproduction? (4)
- The population can be increased rapidly when conditions are right
- Can exploit suitable environments quickly
- More time and energy-efficient
- Reproduction is completed much faster than sexual reproduction
What are the disadvantages of asexual reproduction? (3)
- Limited genetic variation in population - offspring 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
How is malaria caused and spread to humans?
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.
How do malaria parasites reproduce?
These malarial parasites reproduce asexually in the human host, but sexually in the mosquito.
How do some fungi reproduce sexually and asexually? (3)
- 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.
How do plants produce seeds?
Many plants produce seeds via sexual reproduction but are also able to reproduce asexually.
How do plants produce seeds via asexual reproduction in two 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).
What is a genome?
The entire set of the genetic material of an organism is known as its genome.
Has the whole human genome now been studied?
Yes
What is the genetic material of the nucleus composed of?
DNA
What does DNA contain?
DNA, or deoxyribonucleic acid, is the molecule that contains the instructions for growth and development of all organisms.
What is DNA - structurally?
DNA is a polymer made up of two strands forming a double helix.
How is DNA contained?
DNA is contained in structures called chromosomes, which are located in the nucleus.
What is a gene?
A gene is a short length of DNA found on a chromosome.
What does each gene code for? What do these form?
Each gene codes for a particular sequence of amino acids, which form different types of proteins.
What are 3 examples of different types of proteins?
- structural proteins, such as collagen found in skin cells
- enzymes
- hormones
What do genes control? Why?
Genes control our characteristics as they code for proteins that play important roles in what our cells do.
What was 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.
Why is the Human Genome Project an important breakthrough?
- 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.
What are the individual subunits of DNA - the polymer?
Individual subunits of DNA are called nucleotides.
What does a nucleotide contain?
Deoxyribose sugar, phosphate, and a base.
What do the deoxyribose sugar and phosphate in a nucleotide form?
They form the backbone of a starnd of DNA.
How many different types of nucleotides are there?
4
What do the different nucleotides contain?
These four nucleotides contain the same phosphate and deoxyribose sugar, but differ from each other in the base attached.
What are the different bases?
A C T G
What do the bases on each strand of DNA do? What does this do?
The bases on each strand pair up with each other, holding the two strands of DNA in the double helix.
What are the base pairings?
- A and T
- C and G
What are the unchanging base pairings known as?
Complementary base pairing
How do the base pairings join/bond?
Via hydrogen bonds
What do bases code for?
A sequence of three bases is the code for a particular amino acid.
What does the order of the bases in an amino acid control?
The order of bases controls the order and different types of amino acids that are joined together.
What do the amino acid sequences (made from the three bases) form?
These amino acid sequences then form a particular type of protein.
What does the order of the bases in DNA eventually determine?
The order of bases in the DNA eventually determines which proteins are produced/formed.
How is the double helix structure of DNA formed?
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.
Where are proteins made?
Proteins are made in the cell cytoplasm on structures called ribosomes.
How do ribosomes make proteins in summary?
Ribosomes use the sequence of bases contained within DNA to make proteins.
What is the 4 step process of protein synthesis? - blurting/ whiteboard
- 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.
What can mRNA also be referred to as?
A template
How can a protein be changed? What does this mean?
- 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).
How does the ribosome form an amino acid chain? (4)
- 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.
How does the amino acid chain turn into 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 ( a unique shape).
What does the unique shape of a protein allow the protein to do?
Perform a specific function.
What are 3 main things proteins can be?
- Enzymes
- Hormones
- Structural proteins
What are enzymes/what do they do?
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).
What are hormones/what do they do?
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).
What are structural proteins/what do they do?
Proteins that provide structure and are physically strong (eg. collagen is a structural protein that strengthens connective tissues such as ligaments and cartilage).
What are mutations?
Mutations are random changes that occur in the sequence of DNA bases in a gene or a chromosome.
How often do mutations occur?
Mutations occur continuously
How do mutations in DNA impact proteins?
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.
Is it usual fro mutations to impact proteins?
Most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed.
What are the three ways that a mutation in the DNA base sequence can occur?
- Insertions
- Deletions
- Substitutions
What is an insertion?
A new base is randomly inserted into the DNA sequence.
How does an insertion impact the DNA/proteins? (2)
- An insertion mutation changes the amino acid that would have been coded for by the group of three bases in which the mutation occurs.
- An insertion mutation also has a knock-on effect by changing the groups of three bases further on in the DNA sequence.
What is deletion?
A base is randomly deleted from the DNA sequence.
How does a deletion impact the DNA/proteins? (2)
- 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.
What is substitution?
A base in the DNA sequence is randomly swapped for a different base.
How does a substitution impact the DNA/proteins?
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.
How can mutations impact proteins? (2)
- Although mutations altering the protein are uncommon, 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.
What are 2 examples where a mutation can impact a proteins ability to function?
- 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.
Do all parts of DNA code for proteins? What parts of DNA are there? What impact does variations have?
Not all parts of the DNA code for proteins, there is a coding and non-coding part of DNA, which can switch genes on and off, so variations in these areas may affect gene expression, and if the correct protein is synthesised or not.
How does genetic variation impact coding DNA?
Coding DNA codes for proteins, so variations in this part may alter the activity of a protein.
What is a Gamete?
Gametes are sex cells (in animals: sperm and ovum: in plants pollen nucleus and ovum).
What is a chromosome?
Chromosomes are thread-like structures of DNA, carrying genetic information in the form of genes. They are located in the nucleus of cells.
What is a gene?
Genes are short lengths of DNA found on chromosomes. They code for specific proteins.
What is an allele?
Alleles are different versions of a particular gene.
What is a dominant allele?
A dominant allele is always expressed, even if only one copy is present.
What is a recessive allele?
A recessive allele is only expressed if two copies are present (therefore no dominant allele present).
What does homozygous mean?
If the two alleles of a gene are the same, we describe the individual as being homozygous (homo = same).
What does heterozygous mean?
If the two alleles of a gene are different, we describe the individual as being heterozygous (hetero = different).
What is a genotype?
The combination of alleles that control each characteristic is called the genotype.
What is a phenotype?
The observable characteristics of an organism (seen just by looking - Like eye colour, or found through testing - like blood type) is called the phenotype.
What are some characteristics controlled by?
Some characteristics are controlled by a single gene, such as fur colour in mice; and red-green colour blindness in humans.
What is the inheritance of single genes that control a characteristic called?
The inheritance of these single genes is called monohybrid inheritance (mono = one).
How many alleles do we have? Where do they come from? (3)
- 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 ‘say’ the same thing.
What can alleles be?
Dominant or recessive
What inheritance does a dominant allele need?
A dominant allele only needs to be inherited from one parent in order for the characteristic to show up in the phenotype.
What inheritance does a recessive allele need?
A recessive allele needs to be inherited from both parents in order for the characteristic to show up in the phenotype..
What happens if there is one recessive and one dominant allele?
If there is only one recessive allele, it will remain hidden and the dominant characteristic will show.
What are the types of homozygous?
An individual could be homozygous dominant (having two copies of the dominant allele), or homozygous recessive (having two copies of the recessive allele).
How do you express alleles in a diagram for monohybrid inheritance? (2)
- When completing genetic diagrams, alleles are abbreviated to single letters.
- The dominant allele is given a capital letter and the recessive allele is given the same letter, but lower case.
What are most characteristics a result of?
Most characteristics are a result of multiple genes interacting, rather than a single gene.
What are characteristics controlled by more than one gene described as? What is the inheritance of them called?
Polygenic => and the inheritance of these characteristics is called polygenic inheritance
Why is polygenic inheritance difficult to show on a diagram?
Polygenic inheritance is difficult to show using genetic diagrams because of the wide range of combinations.
What is an example of polygenic inheritance? (2 bullet points)
- 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.
- This means that there are several different phenotypes beyond brown and blue; green and hazel being two examples.
What are most phenotypes the result of?
Most phenotype features are the result of multiple genes (polygenic inheritance) rather than single gene inheritance (monohybrid inheritance).
What is the genetic diagram used to express monohybrid inheritance?
A Punnett square
What does a Punnett square show? What can be calculated from this?
- 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.
What is a family tree used for? (2)
- 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.
How can you make predictions of an offsprings’ characteristics?
By calculating the probabilities of the different phenotypes that could occur using a Punnet square.
What are inherited diseases?
Some disorders are inherited (passed from parents to offspring). These disorders are caused by the inheritance of certain alleles.
What are examples of 2 inherited diseases?
- Cystic fibrosis
- Polydactyly
What is cystic fibrosis? (3)
- 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.
What is cystic fibrosis caused by?
A recessive allele
What does it mean that cystic fibrosis is caused by a recessive allele? (2)
- 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.
What is polydactyly?
Polydactyly is a genetic disorder that causes someone to be born with extra fingers or toes.
What is polydactyly caused by?
A dominant allele
What does it mean that cystic fibrosis is caused by a dominant allele?
Even if only one parent is a carrier, the disorder can be inherited by offspring.
What is embryo screening? (3)
- 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 (eg. cystic fibrosis) can be detected during this analysis.
What are the arguments for embryo screening? (3)
- Can avoid suffering by stopping children from being born with genetic disorders.
- Treatment for disorders costs governments and taxpayers a lot of money.
- There are laws in place to stop embryo screening being abused.
What is an example of a law in place to stop embryo screening being abused?
Parents are not allowed to choose the sex of the baby unless they are trying to prevent certain genetic disorders which are sex-linked.
What are the arguments against embryo screening? (3)
- This process could imply that people with genetic disorders are
“undesirable’, which is not fair or true. - Embryo screening is a very expensive process and therefore is not available to all potential parents - only those who can pay for it.
- This could be a ‘slippery slope’— if the process becomes more affordable and more people want to screen their embryos, we may end up in a situation where the process of embryo screening is abused and used to produce
‘desirable’ offspring.
What is gene therapy?
Gene therapy is the process by which normal alleles are inserted into the chromosomes of an individual who carries defective alleles (eg. those that cause a genetic disorder).
Is gene therapy successful?
It is a developing technology and is not always successful.
What debates does gene therapy raise? (3)
- 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.
What do the 23 chromosome pairs in humans control?
22 pairs control characteristics only, but one of the pairs carries the genes that determine sex.
What is the sex chromosome for females?
XX
What is the sex chromosome for males?
XY
How can the inheritance of sex be shown?
The inheritance of sex can be shown using a genetic diagram (known as a Punnett square), with the X and Y chromosomes taking the place of the alleles usually written in the boxes.
