B6 - Inheritance, variation and evolution Flashcards
What is A-sexual reproduction?
- Asexual reproduction does not involve sex cells or fertilisation
- Only one parent is required so there is no fusion of gametes and no mixing of genetic information.& offspring are genetically identical to the parent and to each other (clones)
- Only mitosis is involved in asexual reproduction
What is Mitosis?
What is it used for?
Mitosis is part of the cell cycle, which involves:
- cell growth, and the increase of the number of structures in the cell (mitochondria, ribosomes)
- genetic material is copied
- mitosis occurs leading to chromosomes separation and cell division
Mitosis is also used for:
- growth
- repair to damaged tissue
- replacement of worn-out cells
Examples of organism that use asexual reproduction
Only one parent is needed in asexual reproduction. There is no fusion of gametes so genetic material does not mix, which means that the offspring produced through this process are genetically identical clones to the parent.
Examples of organisms that use asexual reproduction include:
- bacteria
- production of spores by fungi
What is sexual reproduction?
What are the differnt gametes
In sexual reproduction there is mixing of genetic information which leads to variety in the offspring. The formation of gametes involves meiosis
Sexual reproduction uses the process of meiosis, which creates gametes. The process of meiosis happens in the male and female reproductive organs. As a cell divides to form gametes:
- copies of the genetic information is made
- the cell divides twice to form four gametes, each with a single set of chromosomes (haploid)
- all gametes are genetically different from each other
What is Fertilisation?
Fertilisation is the fusion of the nucleus of a male gamete with the nucleus of a female gamete. In humans, each gamete has half the number of the total 46 chromosomes that the body requires. 23 chromosomes within a gamete are referred to as a haploid. When the two gametes combine, they merge the two sets of chromosome to have 46, which are referred to as diploid.
This produces a new cell called a zygote, which will mature into an embryo. The number of cells increase by mitosis, and as the embryo develops, the cells begin to differentiate (or specialise).
- Fertilisation is defined as the fusion of gamete nuclei, and as each gamete comes from a different parent, there is variation in the offspring
- The formation of gametes involves meiosis
What is Mitosis and Meiosis?
- Mitosis is a type of nuclear division that gives rise to cells that are genetically identical
- It is used for growth, repair of damaged tissues, replacement of cells and asexual reproduction
- Meiosis is a type of nuclear division that gives rise to cells that are genetically different
- It is used to produce gametes (sex cells)
What is fertilisation and what is its importance?
Gametes join at fertilisation to restore the normal number of chromosomes. The new cell divides by mitosis. The number of cells increases. As the embryo develops cells differentiate
Importance
- Increases genetic variation of offspring
- 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 is a Genome?
- The entire set of the genetic material of an organism is known as its genome
- Biologists now know the entire human genome. (they have worked out all the genes that are found in humans) This means that:
- We can find genes linked to increased chance of disease e.g. cancer or Alzheimer’s
- It will help us understand and treat inherited disorders like cystic fibrosis
- We can trace human migration patterns which helps people find out their ancestry.
What is the structure of DNA
- The genetic material in the nucleus of a cell is composed of a chemical called DNA.
DNA is a polymer made up of two strands forming a double helix. The DNA is contained in structures called chromosomes. - DNA, or deoxyribonucleic acid, is the molecule that contains the instructions for growth and development of all organisms
What is a gene?
- A gene is a small section of DNA found on a chromosome. Each gene codes for a particular sequence of amino acids. These sequences of amino acids form different types of proteins
- There are many different types of proteins but some example of these could be:
- structural proteins such as collagen found in skin cells
- enzymes
- hormones
- Genes control our characteristics as they code for proteins that play important roles in what our cells do
What is a Nucleotide?
- DNA is a polymer.made from individual subunits of DNA are called nucleotides
- Each nucleotide consists of a common sugar and phosphate group with one of four different bases ATTACHED TO THE SUGAR
How are bases paired?
Base Pairing: Basics
- There are four different nucleotides
- These four nucleotides contain the same phosphate and deoxyribose sugar, but differ from each other in the base attached
- There are four different bases: Adenine (A), Cytosine (C), Thymine (T) and Guanine (G)
Base Pairing
- The bases on each strand pair up with each other, holding the two strands of DNA in the double helix
- The bases always pair up in the same way:
- Adenine always pairs with Thymine (A-T)
- Cytosine always pairs with Guanine (C-G)
- This is known as ‘complementary base pairing’
What is needed for coding an Amino Acid?
What is the double helix?
Coding for Amino Acids
- A sequence of three bases is the code for a particular amino acid
- The order of bases controls the order in which amino acids are assembled to produce a particular protein.
Double Helix
- 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
- It is this sequence of bases that holds the code for the formation of proteins
What is protein synthesis?
Proteins are made in the cell cytoplasm on structures called ribosomes
- Ribosomes use the sequence of bases contained within DNA to make proteins
- DNA cannot travel out of the nucleus to the ribosomes (it is far too big to pass through a nuclear pore) so the base sequence of each gene is transcribed onto an RNA molecule called messenger RNA
- 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 by carrier molecules(tRNA)
- The ribosome then reads the triples of bases on the mRNA and uses this to join correct amino acids in the correct order.
- Once the protein chain is complete it then folds into its unique shape and forms a protein
What happens if there is a change in DNA?
What is the function of Ribosomes?
Changes to Proteins
- 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)
What are the different types of proteins?
- The protein’s unique shape enables the proteins to fulfil a specific function.
- Enzymes – proteins that act as biological catalysts to speed up chemical reactions occurring in the body
- Hormones – proteins that carry messages around the body
- Structural proteins – proteins that provide structure and are physically strong (eg. collagen )
What are Mutations?
- Mutations are random changes that occur in the sequence of DNA bases in a gene or a chromosome. Mutations occur continuously
- 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
- Most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed
- There are different ways that a mutation in the DNA base sequence can occur:
- Insertion
- Deletion
- Substitution
What are Insertions, Deletions and Substitution?
Insertions
- A new base is randomly inserted into the DNA sequence
- An insertion mutation changes the amino acid that would have been coded for by the group of three bases in which the mutation occurs
- Remember – every group of three bases in a DNA sequence codes for an amino acid
- An insertion mutation also has a knock-on effect by changing the groups of three bases further on in the DNA sequence
Deletions
- A base is randomly deleted from the DNA sequence
- 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 are substitutions?
- A base in the DNA sequence is randomly swapped for a different base
- 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
- The height of pea plants is controlled by a single gene that has two alleles: tall and short
- The tall allele is dominant and is shown as T
- The small allele is recessive and is shown as t
‘Show the possible allele combinations of the offspring produced when a pure breeding short plant is bred with a pure breeding tall plant’
- The height of pea plants is controlled by a single gene that has two alleles: tall and short
- The tall allele is dominant and is shown as T
- The small allele is recessive and is shown as t
‘Show the possible allele combinations of the offspring produced when two of the offspring from the first cross are bred together’
- All of the offspring of the first cross have the same genotype, Tt (heterozygous), so the possible combinations of offspring bred from these are: TT (tall), Tt (tall), tt (short)
- There is more variation in this cross, with a 3:1 ratio of tall : short
- The F2 generation is produced when the offspring of the F1 generation (pure-breeding parents) are allowed to interbreed
- The height of pea plants is controlled by a single gene that has two alleles: tall and short
- The tall allele is dominant and is shown as T
- The small allele is recessive and is shown as t
‘Show the results of crossing a heterozygous plant with a short plant’
What do Family trees tell us?
- 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
Males are indicated by the square shape and females are represented by circles
- Affected individuals are red and unaffected are blue
- Horizontal lines between males and females show that they have produced children (which are shown underneath each couple)
- The family pedigree above shows:
- Both males and females are affected
- Every generation has affected individuals
- There is one family group that has no affected parents or children
- The other two families have one affected parent and affected children as well
What is cystic fibrosis?
- 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
- Cystic fibrosis is caused by a recessive allele (f)
- This means:
- 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
- If both parents are carriers, the chance of them producing a child with cystic fibrosis is 1 in 4, or 25%
- If only one of the parents is a carrier (with the other parent being homozygous dominant), there is no chance of producing a child with cystic fibrosis
What is Polydactyly?
- Polydactyly is a genetic disorder that causes someone to be born with extra fingers or toes
- Polydactyly is caused by a dominant allele (D)
- This means:
- Even if only one parent is a carrier, the disorder can be inherited by offspring
Pros and cons of embryo screening?
Pros:
- Can avoid suffering
- Can prevent spending lots of money on treatments
cons:
- Process is expensive
- Life may appear cheap
- Unethical
- Assunes disabled people cannot live happy lives
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)
- It is a developing technology and is not always successful
- 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 disorder
Gene therapy involves these basic steps:
- identify the gene involved in the genetic disorder
- restriction enzymes cut out the normal allele
- many copies of the allele are made
- copies of the normal working allele are put into the cells of a person who has the genetic disorder due to a mutated or faulty copy of an allele
The problems involved in the process:
- the alleles may not go into every target cell, which are cells that need the new non-faulty cell
- the alleles may be inserted into the chromosomes in random places, rather than in the required position, so they do not work properly
- some treated cells may be replaced naturally by the patient’s own untreated cells, as cells are frequently replaced through the process of mitosis during growth and repair
