Inheritance, Variation and Evolution 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 (sperm cells and egg cells in animals, pollen and egg cells in flowering plants).
Haploid (half the number of chromosomes)
What is meiosis?
Form of cell division involved in the formation of gametes (non-identical haploid cells) 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 fist 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 plled is random, creating variation)
Chromosome number is halved
What happens during the second stage of meiosis?
Chromosomes line 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? (2)
It increasaes genetic variation
It ensures that the zygote formed at fertilisation is diploid
Descrive 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 embtyo develops, cells differentiate
What is the advantage of secual 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 (2)
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 the advantages of asexual reproduction (3)
only one parent is required
Lots of offspring can be produced in a short period of time, enabling the rapid colonisation of an area and reducing competition from other species.
Requires less energy and time as do not need a mate
What is the disadvantage of asexual reproduction?
No genetic variation (except from spontaneous mutations) reducting the probability of a species being able to adapt to environmental change
Describe the circumastances 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 chromosomes (23 pairs)
How many chromosomes do human gametes have?
23 chromosomes
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 determines the structure (i.e. how it will fold) nad 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 is protein synthesis?
The formation of a protein from a gene
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 binds to a specific base sequence of non-coding DNA in front of a gene and moves along the DNA strand
- RNA polymerase joins free RNA nucleotides to 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 attaches 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 (carrier molecule)
- A polypeptide chain is formed from the sequence of amino acids which join together
Wha 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 variants 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 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 copes of the same allele (two recessive or two dominant)
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 punnett square?
They are represented using uppercase letters
How are recessive alleles represented in a punnett square?
They use the lowercase version of the same letter as the dominant allele
Draw a punnett square for a cross between a homozygous recessive blue eyed female (bb) with a heterozygous brown eyed male (Bb)
. B b
b Bb bb
b Bb bb
50% brown eyes Bb
50% blue eyes bb
PKU is a recessive condition. Two heterozygous parents (Pp) have offspring. Using a punnett square, predict the proportion of offspring that will have PKU
75% chance of normal phenotype
25% chance of PKU phenotype
What is the problem with single gene crosses?
Most characteristics are controlled by multiple alleles rather than just one
What is an inherited disorder?
A disorder caused by the inheritance of certain alleles
Give 2 examples of inherited disorders
polydactyly (having extra fingerse 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 (from an 8 cell embryo) and tested for disorder-causing alleles. If the celldoesn’t have any indicator alleles, then the originating embryo is implanted into the uterus