B13. Reproduction Flashcards
What is sexual reproduction?
Type of reproduction
Involves the production of gametes by meiosis
A gamete from each parent fuses to form a zygote
Genetic information from each gamete is mixed so the resulting zygote is unique
What are gametes
Sex cells
What is meiosis?
Form of cell division involved in the formation of gametes in reproductive organs
Chromosome number is halved
Involves two divisions
What must occur prior to meiosis?
Interphase - copies of genetic information are made during this process
What happens during the first stage of meiosis?
Chromosome pairs line up along the cell equator
The pair of chromosomes are separated and move to opposite poles of the cell (the side to which each chromosome is pulled is random, creating variation)
Chromosome number is halved
What happens during the second stage of meiosis?
Chromosomes ling up along the cell equator
The chromatids are separated and move to opposite poles of the cell
Four unique haploid gametes are produced
Why is meiosis important for sexual reproduction?
It increases genetic variation
It ensures that the zygote formed at fertilisation is diploid
Describe fertilisation and its resulting outcome
Gametes join together to restore the normal number of chromosomes and the new cell then divides by mitosis (which increases the number of cells)
As the embryo develops, cells differentiate
What is the advantage of sexual reproduction?
It creates genetic variation in offspring, increasing the probability of a species adapting to and surviving environmental changes
Natural selection can be speeded up by humans in selective breeding to increase food production
Describe the disadvantages of sexual reproduction
Two parents are required. This makes reproduction difficult in endangered populations or in species which exhibit solitary lifestyles.
More time and energy is required so fewer offspring are produced
What is asexual reproduction?
Type of reproduction
Involves mitosis only
Produces genetically identical offspring known as daughter cells
Describe 3 advantages of asexual reproduction
Only one parent is required
Lots of offspring can be produced in a short period of time
Requires less energy and time as it does not need a mate
What is the disadvantage of asexual reproduction?
No genetic variation reducing the probability of a species being able to adapt to environmental change
Describe the circumstances in which malarial parasites reproduce sexually and asexually
Sexual reproduction in the mosquito
Asexual reproduction in the human host
Describe the circumstances in which fungi reproduce sexually and asexually?
Asexual reproduction by spores
Sexual reproduction to give variation
Describe the circumstances in which plants reproduce sexually and asexually
Sexual reproduction to produce seeds
Asexual reproduction by runners (e.g. strawberry plants) or bulb division (e.g. daffodils)
What is DNA?
A double-stranded polymer of nucleotides, wound to form a double helix
The genetic material of the cell found in its nucleus
Define genome
The entire genetic material of an organism
Why is understanding the human genome important?
The whole human genome has been studied and is important for the development of medicine in the future
Searching for genes linked to different types of disease
Understanding and treating inherited disorders
Tracing human migration patterns from the past
What is a chromosome?
A long, coiled molecule of DNA that carries genetic information in the form of genes
How many chromosomes do human body cells have?
46
How many chromosomes do human gametes have?
23
Define gene
A small section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein
What are the monomers of DNA
Nucleotides
What are DNA nucleotides made up of?
Common sugar
Phosphate group
One of four bases: A,T,C or G
State the full names of the four bases found in nucleotides
Adenine
Thymine
Cytosine
Guanine
Describe how nucleotides interact to form a molecule of DNA
Sugar and phosphate molecules join to form a sugar-phosphate backbone in each DNA strand
Base connected to each sugar
Complementary base pairs (A pairs with T, C pairs with G) joined by weak hydrogen bonds
Explain how a gene codes for a protein?
A sequence of three bases in a gene forms a triplet
Each triplet codes for an amino acid
The order of amino acids determine the structure (i.e. how it will fold) and function of protein formed
Why is the ‘folding’ of amino acids important in proteins such as enzymes
The folding of amino acids determines the shape of the active site which must be highly specific to the shape of its substrate
What are the two stages of protein synthesis.
- Transcription
- Translation
What does transcription involve?
The formation of mRNA from a DNA template
Outline transcription
- DNA double helix unwinds
- RNA polymerase bids to a specific base sequence of non-coding DNA in from of a gene and moves along the DNA strand
- RNA polymerase joins free RNA nucleotides complementary bases on the coding DNA strand
- mRNA formation complete, mRNA detaches and leaves the nucleus
What does translation involve?
A ribosome joins amino acids in a specific order dictated by mRNA to form a protein
Outline translation
- mRNA attached to a ribosome
- Ribosome reads the mRNA bases in triplets. Each triplet codes for one amino acid which is brought to the ribosome by a tRNA molecule
- A polypeptide chain is formed from the sequence of amino acids which join together
What is a mutation?
A random change in the base sequence of DNA which results mostly in no change to the protein coded for, or genetic variations of the protein (slight alteration but appearance and function remain).
Mutations occur continuously
Describe the effect of a gene mutation in coding DNA
If a mutation changes the amino acid sequence, protein structure and function may change (an enzyme may no longer fit its substrate binding site or a structural protein may lose its strength)
If a mutation does not change amino acid sequence, there is no effect on protein structure or function
What is the non-coding DNA?
DNA which does not code for a protein but instead controls gene expression
Describe the effect of a gene mutation in non-coding DNA
Gene expression may be altered, affecting protein production and the resulting phenotype
What are alleles?
Different versions of the same gene
What is a dominant allele?
A version of a gene where only one copy is needed for it to be expressed
What is a recessive allele?
A version of a gene where two copies are needed for it to be expressed
What is meant when an organism is homozygous?
When an organism has two copies of the same allele
What is meant when an organism is heterozygous?
When an organism has two different versions of the same gene (one dominant and one recessive)
What is the genotype?
The genes present for a trait
What is the phenotype?
The visible characteristic
How are dominant alleles represented in a punnet square?
They are represented using uppercase letters
How are recessive alleles represented in a punnet square
They use the lowercase version of the same letter as the dominant allele
What is an inherited disorder?
A disorder caused by the inheritance of certain alleles
Give two examples of inherited disorders
Polydactyly (having extra fingers or toes) - caused by a dominant allele
Cystic fibrosis (a disorder of cell membranes) - caused by a recessive allele
How are embryos screened for inherited disorders?
During IVF, one cell is removed an tested for disorder-causing alleles. If the cell doesn’t have any indicator alleles, then the originating embryo is implanted into the uterus
What are the ethical issues concerning embryo screening?
It could lead to beliefs in society that being disable or having a disorder is less human or associated with inferiority
The destruction of embryos with inherited disorders is seen by some as murder as these would go on to become human beings
It could be viewed as part of the concept of designer babies as it may be for the parents convenience or wishes rather than the child’s wellbeing
What are the economic issues concerning embryo screening?
Costs of hospital treatment and medication will need to be considered if it is known that a child will have an inherited disorder and financial support explored if necessary
What are the social issues concerning embryo screening?
Social care for children with inherited disorders may need to be considered if parents are unable to provide care
If an embryo is found to have an inherited disorder and is terminated, this can prevent a child and its parents from potential suffering in the future due to the disorder
What is gene therapy?
The insertion of a normal allele into the cells of a person with an inherited disorder to functionally replace the faulty allele
What are the ethical issues concerning gene therapy?
Some people believe that it is going against and ‘playing God’
The introduced genes could enter sex cells and so be passed to future generations
What are sex chromosomes?
A pair of chromosomes that determine sex:
Males have an X and a Y chromosome
Females have two X chromosomes
What does the inheritance of a Y chromosome mean that an embryo develops into a male?
Testes development in an embryo is stimulated by a gene present on the Y chromosome
What is a sex-linked characteristic?
A characteristic that is coded for by an allele found on a sex chromosome
Why are the majority of genes found on the X chromosome rather than the Y chromosome?
The X chromosome is bigger than the Y chromosome so more genes are carried on it
Why are men more likely to show the phenotype for a recessive sex-linked trait than women?
Many genes are found on the X chromosome that have no counterpart on the Y chromosome
Women (XX) have two alleles for each sex-linked gene whereas men (XY) often only have one allele, only one recessive allele is required to produce the recessive phenotype in males
