Chap9: Meiosis & Genetic Diversity Flashcards
Pls look at the textbook for diagrams and memorise them.
Define mutation
Any change to the quantity or the base sequence of the DNA of an organism is known as a mutation.
Define Gene Mutation
Any change to 1/more nucleotide bases, or a change in the sequence of the bases, in DNA is known as a gene mutation.
If a mutation occurs during the formation of gametes, what does this mean for the offspring?
This may be inherited,
often producing sudden and distinct differences between individuals
How can gene mutations arrive?
(And by what 2 ways?)
Gene mutations can arise spontaneously during DNA replication
+ includes base substitution and base deletion.
Define chromosome mutations
Changes in the structure or number of whole chromosomes
What is polyploidy?
In what type of organisms does this mainly occur?
Changes in whole sets of chromosomes that occur when organisms have three or more sets of chromosomes rather than the usual two.
Occurs mostly in PLANTS.
What is non-disjunction?
What does this usually result in?
Sometimes individual homologous pairs of chromosomes fail to separate during meiosis.
This usually results in a gamete having either one more or one fewer chromosome.
What 2 ways can chromosome mutations occur?
- Polyploidy (changes in WHOLE sets of chromosomes)
- Non-disjunction (changes in the number of INDIVIDUAL chromosomes)
Give an example of non-disjunction in humans.
Down syndrome
(additional chromosome 21)
If non-disjunction occurs, how many chromosomes will the off-spring have?
On fertilisation with a gamete that has the normal complement of chromosomes,
the resultant offspring have more or fewer chromosomes than normal in all their body cells.
What is hybridisation?
Hybridisation is combining the genes of different varieties or species of organisms to produce a hybrid.
Sometimes this is followed by organisms that have additional complete sets of chromosomes (polyploidy)
What are the 2 ways that cell division occurs?
Mitosis and Meiosis
Define Mitosis and Meiosis
- Mitosis produces two daughter cells with the same number of chromosomes as the parent cell and as each other.
- Meiosis usually produces four daughter cells, each with half the number of chromosomes as the parent cell.
Importance of meiosis
In humans, what is the diploid no. of chromosomes?
How many chromosomes does a cell have?
In humans, the diploid number of chromosomes is 46,
which means that this cell would have 92 chromosomes
What is known as the haploid number of chromosomes?
Every diploid cell of an organism has 2 complete sets of chromosomes: 1 set provided by each parent.
During meiosis, homologous pairs of chromosomes separate, so that only one chromosome from each pair enters a daughter cell.
This is known as the haploid number of chromosomes which, in humans, is 23.
When two haploid gametes fuse at fertilisation, the diploid number of chromosomes is restored.
Explain the process of meiosis
- 2 nuclear divisions, occurs immediately one after the other
-In the 1st division (meiosis 1) homologous chromosomes pair up and their chromatids wrap around each other.
Equivalent portions of these chromatids may be exchanged in a process called crossing over. By the end of this division the homologous pairs have separated, with 1 chromosome from each pair going into 1 of the 2 daughter cells.
- In the 2nd meiotic division (meiosis 2) the chromatids move apart. At the end of meiosis 2, 4 cells have usually been formed.
In humans, each of these cells contains 23 chromosomes.
In addition to halving the number of chromosomes,
What does meiosis also produce, and what does this lead to?
meiosis also produces genetic variation among the offspring,
which may lead to adaptations that improve survival chances
How does meiosis produce genetic variation?
- INDEPENDANT SEGREGATION of homologous chromosomes
- New combinations of maternal and paternal alleles by CROSSING OVER
Define Gene
a length of DNA that codes for a polypeptide
Define Locus
the position of a gene on a chromosome or DNA molecule
Define Allele
one of the different forms of a particular gene.
Define what homologous chromosomes are.
a pair of chromosomes, one maternal and one paternal, that have the same gene loci.
At which division of meiosis does Independent segregation occur?
During meiosis 1
Explain the process of independent segregation
During meiosis 1, each chromosome lines up alongside its homologous partner.
Homologous pairs arrange themselves in this line at random.
One of each pair will pass to each daughter cell.
Which one of the pair goes into the daughter cell, and with which one of any of the other pairs, depends on how the pairs are lined up in the parent cell.
Since the pairs are lined up at random, the combination of chromosomes of maternal and paternal origin that go into the daughter cell at meiosis 1 is also a matter of chance.
What can you say is the same about the chromosomes in a homologous pair?
What is different in them?
Each member of a homologous pair of chromosomes = exactly the same genes = determines the same characteristics.
The ALLELES of these genes may differ.
The independent assortment, of these chromosomes therefore produces new genetic combinations.
Explain the process of Crossing Over
- Chromatids of each pair become twisted around one another.
- During this twisting process tensions are created and portions of the chromatids break off.
- These broken portions might then rejoin with the chromatids of its homologous partner.
- Usually it is the equivalent portions of homologous chromosomes that are exchanged.
- In this way new genetic combinations of maternal and paternal alleles are produced
The chromatids cross over one another many times and so the process is known as crossing over.
Describe the process of recombination.
The broken-off portions of chromatid recombine with another chromatid
What happens if recombination does or does not occur?
no recombination by crossing over = only two different types of cell are produced.
recombination occurs = 4 different cell types are produced.
Crossing over therefore increases genetic variety even further.
Formula for no. of chromosomes in an organism to determine the number of possible combinations of chromosomes for each daughter cell:
2” where n = the number of pairs of homologous chromosomes
