Topic 3: Genetics Flashcards
Sexual reproduction
- Sexual reproduction involves the joining of two sex cells, or gametes during fertilisation.
- Organisms produced by sexual reproduction have two parents and are genetically similar to both but not identical to either.
The advantages of sexual reproduction:
- It produces variation in the offspring
- The species can adapt to new environments due to variation, which gives them a survival advantage
- A disease is less likely to affect all the individuals in a population
- Humans can speed up natural selection through selective breeding, which can be used, for example, to increase food production.
The disadvantages of sexual reproduction:
- Time and energy are needed to find a mate
- It is not possible for an isolated individual
Asexual reproduction
- Asexual reproduction only involves one parent so there is no joining of sex cells during fertilisation.
- Organisms produced by asexual reproduction are genetically identical to each other and their parent. -They are clones.
The advantages of asexual reproduction include:
- The population can increase rapidly when the conditions are favourable
- Only one parent is needed
- It is more time and energy efficient as you don’t need a mate
- It is faster than sexual reproduction.
The disadvantages of asexual reproduction include:
- It does not lead to variation in a population
- The species may only be suited to one habitat
- Disease may affect all the individuals in a population
What is meiosis used for?
It is used to produce cells for repair and asexual reproduction.
Meiosis
- Meiosis produces haploid non-identical sex cells, or gametes.
- These fuse to form a diploid fertilised egg cell during fertilisation.
- Meiosis produces sperm and egg cells in animals, and pollen and egg cells in plants.
Haploid
Only 1 set of chromosomes -23
Structure of DNA
- DNA is a polymer, made of many smaller units called nucleotides.
- The nucleotides join together, forming two strands; these, in turn, form a double helix structure.
- The double helix is held together by weak hydrogen bonding between complementary base pairs.
- Base A always pairs with T, and C always pairs with G forming a twisted ladder structure called a double helix
What is a nucleotide?
A nucleotide is made of a sugar and a phosphate group, with one of four different bases, A, C, T or G, attached.
Adenine
Pairs with Thymine
Cytosine
Pairs with Guanine
Genome
- The genome is one copy of all an organism’s DNA.
- In humans this is all the DNA that makes up the 23 pairs of chromosomes found in all diploid body cells.
- That is all the cells except sex cells or gametes, which only have half of a person’s genome.
Gene
- A gene is a small section of DNA in a chromosome.
- Each gene codes for a particular sequence of amino acids in order to make a specific protein.
- It is the unit of heredity, and may be copied and passed on to the next generation
Extracting DNA from fruit
Method
- Peel the skin from half a kiwi fruit and mash it up
- Mix a teaspoon of salt and small volume of washing up liquid into the fruit
- Gently heat this mixture at about 60°C for five minutes
- Filter the mixture and retain only the filtrate (the filtered liquid)
- Cool using an ice bath and gently pour chilled ethanol onto the top of the filtrate
Why is detergent used when extracting dna from fruit?
The detergent will break down the cell membranes to release the DNA
Why is salt used when extracting dna from fruit?
The salt will make the DNA stick together
Why is ice cold ethanol used when extracting dna from fruit?
The ethanol will cause the dna to come out of the solution as its not soluble in cold alcohol
How does the order of bases in a section of DNA decide the order of amino acids and therefore determine the shape of the protein?
- Genes are sections of the DNA.
- Each gene has the code for creating a specific protein.
- The sequence of bases in the gene controls which amino acids are joined in order to make a specific new protein (or enzyme) molecule.
- The proteins are then folded into their correct shape to make them functional.
Protein synthesis
Transcription
- The gene coding for the protein required untwists then unzips, the H-bonds between the strands break
- Free RNA nucleotides form complementary base pairs with one strand of DNA bases
- Weak hydrogen bonds form between base pairs and sugar phosphate bonds form between RNA nucleotides
- mRNA strand is synthesized
- mRNA peels off the DNA and moves out of the nucleus into the cytoplasm
Where does transcription occur?
In the nucleus
Difference between RNA nucleotides and DNA nuceotides
RNA nucleotides contain the same bases as DNA, except that T is replaced by U. U base pairs with A.
Protein synthesis
Translation
- The ribosomes are the sites of protein synthesis
- The mRNA strand attaches to a ribosome
- tRNA molecules transport specific amino acids to the ribosome
- Each mRNA codon codes for a specific amino acid
- The anti-codons and codons match up and form complementary base pairs
- Peptide bonds form between the adjacent amino acids to form the polypeptide (protein)
Where does translation occur?
Translation takes place on the ribosomes in the cytoplasm, or found on the rough Endoplasmic Reticulum (ER)
Mutations in non coding dna
- RNA polymerase attaches to dna bases in front of a gene.
- A mutation in this noncoding region may result in RNA polymerase not binding well, reducing transcription, and causing less protein to be produced.
- And, depending on the function of the protein, the phenotype of an organism may be affected by how much of it is made.
Mutation in coding dna
- If a mutation happens in a gene, it produces a genetic variant.
- The genetic variant may code for a different sequence of amino acids, which may change the shape of the final protein and so its activity
- The activity may increase, decrease or stop altogether
- This could end up changing the phenotype
What experiment did mendel do?
- Mendel crossed 2 pea plants of different heights - a tall plant and a dwarf one.
- All the offspring produced were tall pea plants
- He then crosses 2 of these tall offspring together
- It produced a 3:1 ratio of tall: dwarf plants
What did the experiment show?
- Mendel had shown that the height characteristics in pea plants was determine dby separately inherited “hereditary units” passed from each parent.
- The ratios of tall and dwarf plants in the offspring showed that the unit for all tall plants, T, was dominant over the unit for dwarf plants, t.
What 3 conclusions did Mendel reach?
- Characteristics in plants are determined by “hereditary units”
- Hereditary units are passed on unchanged from each parent, one unit from each parent
- Hereditary units can be dominant or recessive - if an individual has both dominant and recessive, dominant will be expressed
Why wasn’t the importance of Mendel’s work not realised straight away?
Scientists of the time didn’t have the background knowledge to properly understand mendel’s findings because they didn’t know about genes, chromosomes or dna
How are characteristic inherited affected by alleles
- The characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair
- The characteristic controlled by a recessive allele develops only if the allele is present on both chromosomes in a pair
Chromosome
The cell’s nucleus contains chromosomes. These are long, coiled molecules of DNA
Allele
Alleles are different forms of a gene. They can be dominant or recessive
Recessive
A recessive allele only shows if the individual has two copies of the recessive allele.
Dominant
A dominant allele always shows, even if the individual only has one copy of the allele.
Homozygous
individuals who are homozygous for a certain gene carry two copies of the same allele
Heterozygous
individuals who are heterozygous for a certain gene carry two different alleles
Genotype
Genotype describes the genetic make-up of an organism (the combination of alleles).
Phenotype
Phenotype describes the observable, physical characteristics that an organism has
Gamete
The cells from each parent that combine to form the zygote are called gametes. In humans, the male gamete is called sperm, and the female gamete is called an egg.
Zygote
The cell formed by the union of a male sex cell (a sperm) and a female sex cell (an ovum). The zygote develops into the embryo
Monohybrid inheritance
A cross between two parents who possess different forms (alleles) of a gene is called monohybrid inheritance
Gender determination
All eggs have one X chromosome, but a sperm can have either an X or a Y chromosome.
-So gender determination in humans depends on whether the sperm that fertilizes the egg carries X or Y
Codominance
Sometimes, neither of a pair of alleles is completely dominant or completely recessive. Instead of one of them completely hiding the effect of the other in a heterozygote, they both have an effect on the phenotype. This is called codominance
Inheritance of blood groups
- In humans, there are 4 blood types (phenotypes): A, B, AB, and O
- Blood type is controlled by 3 alleles: IA, IB, IO
- IO is recessive, two IO alleles must be present for the person to have type O blood
- IA and IB are codominant but both are dominant to Io. If a person receives an IA allele and a IB allele, their blood type is type AB, in which characteristics of both A and B antigens are expressed.
- Because IO is dominated by both IA and IB alleles, a person with blood group A could have the genotype IA IO or IA IA.
Sex linked genetic disorders
- Most genes on the sex chromosome s are carried on the X chromosome
- As men only have one X chromosome, they often only have one allele for sex linked genes
- Because they only have 1 allele, the characteristic for the allele is shown even if its recessive. This makes men more likely to show recessive characteristics for genes that are sex linked
How are human characteristics controlled?
Most human characteristics are controlled by many genes, not just one
What is genetic variation caused by?
- By organisms having different alleles which can lead to differences in phenotypes
- New alleles arising through mutation
- Sexual reproduction as it results in alleles being combined in lots of different ways in offspring
Types of variation
- Genetic
- Enviromental
Human genome project
Predict and prevent diseases
By knowing which genes predispose people to particular conditions, doctors will be able to predict which people are likely to suffer from a particular disease and offer a preventive course of action, which may involve medical treatment or lifestyle changes. Furthermore, cures could be found for genetic diseases like cystic fibrosis or sickle cell anaemia.
Human genome project
Improved medicine
Personalised medicines can be developed based on the way our individual bodies react to the disease and the treatments, which will be more effective because the medicines will be tailored for our specific medical needs.
Human genome project
Improve forensic science
Genetic fingerprinting helps to match a suspect to the biological material found at a crime scene. In the future it could be possible to figure out what a suspect looks like from DNA found at a crime scene e.g. their eye, hair and skin colour.
