Topic 4: Genetic Diversity Flashcards
define meiosis
A type of nuclear division that produces 4 daughter cells, each with half (haploid) the number of chromosomes of the parent cell
Purpose of meiosis
Produces genetically different daughter cells
Sexual reproduction results in genetic diversity in offspring
Survival advantage when environmental conditions change so individuals are more likely to survive
What are gametes?
They are haploid and 2 fuse together to produce an offspring with diploid number of chromosomes
Why are the chromosomes in homologous pairs similar?
One is from the male and one is from the female
They carry the same genes for the same polypeptides in the same positions (locus)
What are produced in meiosis?
4 daughter cells that aren’t identical, they are genetically different and form gametes
What is independent segregation?
It is a process of genetic variation
When homologous pairs line up on the equator , it is completely random as to which chromosomes from each pair end up together in daughter cells
what is the result of independent segregation?
New combinations of paternal and maternal chromosomes in gametes
When do independent segregation and crossing over occur?
During meiosis 1 when the homologous pairs line up
What is crossing over?
When homologous pairs line up and become twisted to form a bivalent. The bivalent causes sections of the chromatids to be swapped- so alleles were swapped.
Produces recombinant chromatids
define Chiasmata
Point where chromatids cross, break and rejoin
Calculation for the possible combinations of chromosomes following meiosis
2^n
2^n
How to calculate the possible combinations following meiosis
How to calculate the possible combinations after random fertilisation of 2 gametes?
(2^n)2
(2^n)2
How to calculate the possible combinations after random fertilisation of 2 gametes
Life cycle exam questions
- may get given the life cycle of an unknown organism
- meiosis will be where a diploid has divided and become haploid
Haploid to diploid is fertilisation
If it stays the same= mitosis
How many divisions are in meiosis?
2
Describe prophase 1
Chromosomes are visible because they have condensed
Describe metaphase 1
Homologous chromosomes line up at the equator, how they line up is random
Describe anaphase 1
Homologous chromosomes are pulled to opposite poles
Telophase 1
Cell prepares for first cell division
Metaphase 2
Individual chromosomes line up at the equator
Anaphase 2
Sister chromatids are separated to opposite poles
Telophase 2
Each daughter cell is haploid
What are chromosomes mutations?
Changes in structure or number of chromosomes
What do chromosome mutations lead to?
Inherited conditions because errors are present in the gametes
Polyploidy
Changes in whole sets of chromosomes- all homologous chromosomes fail to seperate properly
A diploid gamete is fertilised by a haploid gamete to give an organism 3 sets of chromosomes
May occur in plants
Non-disjunction
Changes in the number of individual chromosomes- sometimes individual homologous chromosomes fail to seperate during meiosis
What happens as a result of non-disjunction?
The resulting gamete has one more or one fewer chromosomes than it should
If a normal gamete fuses with this gamete, all resulting body cells will have the same number of chromosomes
This occurs in Down’s syndrome
When is a mutation inherited?
They occur randomly, they are inherited if they occur in gametes but not if in somatic cells
what are the causes of mutations?
They can occur randomly, but substances called mutagens can increase the natural mutation rate
Mutagens e.g ionising radiation, UV light, tobacco tar, mustard gas
Are all mutations harmful?
No
They can be helpful by producing new alleles, and increasing variation in a population
3 types of mutation
Addition
Deletion
Substitution
What are addition and deletion known as?why?
Frameshift mutations
Because they can significantly alter the resulting polypeptide chain
Addition
An extra nucleotide base is added
Deletion
A nucleotide is removed
Substitution
A nucleotide is replaced by one with a different base
Why aren’t all base substitutions bad?
The degenerate nature of the genetic code means that not all base substitutions cause a change in sequence of encoded amino acids
What is the effect of a mutation?
-The mutation causes a change in the sequence of bases in the dna
-this leads to a change in the mRNA
This leads to a change in the amino acid sequence in the polypeptide chain
This causes a change in the function of a protein because it has a different tertiary structure
Give one example of a substitution mutation
Sickle cell anemia
Caused by a substitution of one dna base which alters one amino acid and changes the structure & function of haemoglobin
What can mutations result in?
Differences in base sequences of alleles of a single gene may result in non-functional proteins, including non-functional enzymes
