DNA, Inheritance and Variation

Created by

Amelie Viney

Cards (65)

What is sexual reproduction?
Involves the production do gametes by meiosis
A gamete from each parent fuses to form a zygote
What are gametes?
> Sex cells
-> sperm and egg in animals, pollen and egg is flowering plants
> Haploid: half the number or chromosomes
What is meiosis?
> Form of cell division involved in the formation of gametes in reproductive gametes
> Chromosome number is halved
> Involves 2 divisions
What must occur prior to meiosis?
Interphase: copies of genetic info are made during the process
Process of meiosis:
> Chromosome pairs line up along equator
> Then are separated and moved to opposite poles of the cell (random)
> Chromosome number is halved
> Cell membrane splits
> Chromosomes line up along equator again
> Chromatids are separated and moved to opposite poles of the cell
> Four unique haploid gametes are produced
Describe fertilisation and its resulting outcome:
> Gametes join together to restore the normal number of chromosomes
> The new cell then divides by mitosis
> As the embryo develops, cells differentiate
What is the advantage of sexual reproduction?
Increases genetic variation in offspring
-> increases probability of of a species adapting and survival environmental changes
-> means not all individuals are susceptible to the same diseases
What are the disadvantages of sexual reproduction?
> Two parents required
-> makes production difficult in endangered populations or species which exhibit solitary lifestyles
> More time and energy is required
-> fewer offspring are produced
What is asexual reproduction?
> Involves mitosis only
> Produces genetically identical offspring known as daughter cells
What are the advantages of asexual reproduction?
> Only one parent required
> Lots of offspring can be produced quickly
-> allows rapid colonisation of an area
> Requires less time and energy because a mate is not required
What are the disadvantages of asexual reproduction?
No genetic variation (except for spontaneous mutations)
-> reduces probability of special being able to adapt to environmental change
What is DNA?
A double stranded polymer of nucleotides, wound to form a double helix
The genetic material found in the nucleus
Define genome:
The entire genetic material of an organism
Why is understanding the human genome important?
Searching for genes linked to different types of disease
Treating genetic disorders
What is a chromosome?
A long coiled molecule of DNA that carries genetic info in the form of genes
How many chromosomes do human body cells have?
46 (23 pairs)
How many chromosomes do human gametes have?
23
Define gene:
A small section of DNA that does 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?
Sugar
Phosphate
Base (A,T,C or G)
What are the names of the 4 bases found in nucleotides?
Adenine
Thymine
Cytosine
Guanine
How do 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 and T, C and G) joined by weak hydrogen bonds
How does a gene code for a protein?
> A sequence of 3 bases in a gene forms a triplet
> Each triplet codes for an amino acid
> The order of amino acids determines the structure 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 specific to the shape of its substrate
What is protein synthesis?
The formation of a protein from a gene
Stages of protein synthesis:
Transcription: the formation of mRNA from a DNA template
Translation: a rinosome joins amino acids in a specific order dictated by mRNA to form a protein
Outline transcription
(the first step)
> DNA double helix unwinds
> RNA polymerase binds to specific base sequence of non coding DNA in front of a gene and moves along DNA strand
> RNA polymerase joins the free RNA nucleotides to the complementary bases on the coding DNA strand
> mRNA formation complete: mRNA detaches and leaves the nucleus
Outline translation:
(2nd Step)
> mRNA attaches to a ribosome
> Ribosome reads the mRNA bases in triplets: each triplet codes for 1 amino acid which is brought to the rinosome by a carrier 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 can result in either mostly no change or genetic variants of the protein
> If a mutation changes the amino acid sequence, protein structure and function may change
-> an enzyme may no longer fit in its substrate binding site
What is non-coding DNA?
DNA which does not code for a protein but instead controls gene expression
Define alleles:
difference versions of the same gene
What is a dominant allele? 
A version of s 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 (both recessive or both dominant)
What is meant when an organism is heterozygous?
When an organism has two different versions of the same gene
Define genotype:
The genes present for a trait
Define phenotype:
The visible characteristic
Examples of inherited disorders:
Polydactyly: having extra fingers of toes (caused by a dominant allele)
Cystic fibrosis: a disorder of cell membrane (caused y a recessive allele)
How are embryos screened for inherited disorders?
During IVF, one cell is removed and 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 sex chromosomes?
Males have XY chromosomes
Females have XX chromosomes