Meiosis/mutation Flashcards
Codon def
a sequence of three nucleotides which together form a unit of genetic code
What do gene mutations involves?
-a change in the base sequence of chromosomes
when can gene mutations arise?
-spontaneously during DNA replication
Describe base substitution
-Base substitution
> a nucleotide in a DNA molecule is replaced by another nucleotide which has a different base
what are two possible consequences of a base substitution?
> the a/a coded for may be changed which could affect a protein ( see protein topic )
however, new triplet of bases may still code for same a/a due to degenerate nature of genetic code, in which most a/a have more than one codon
Describe base deletion + consequence
-A nucleotide is lost from the normal DNA sequence
-a/a sequence of polypeptide completely different as one deleted nucleotide causes all triplets in a sequence to be read differently as they are shifted to the left
What can increase the rate of mutation
mutagenic agents
When can mutations in the number of chromosomes arise
spontaneously by chromosome non-disjunction during meiosis
How to calculate the number of different diiferent combinations of chromosomes following meiosis, without crossing over?
2 to the power of n, where n=number of homologous chromosome pairs
What does meiosis produce?
-daughter cells that are genetically different from each other
What is the first stage of meiosis ?
What are three differences between meiosis and mitosis
-Mitosis produces cells with the same number of chromosomes as the parent cell, whereas meiosis produces cells with half the number of chromosomes as the parent cell
-Mitosis produces daughter cells that are genetically identical, meisosis produces daughter cells that are genetically different from each other and parent cell
-mitosis produces 2 daughter cells, meiosis produces four
When would meiosis occur in an organisms life cycle?
-before sexual reproduction, as meiosis produces gametes which have half the number of chromosomes of the parent cell
How does random fertilisation of haploid gametes further increase genetic variation within a species?
-as it creates genetic variation between zygotes (cell formed by the fusion of male and female gametes) as each will have a unique combination of alleles
haploid def
A cell which has half of the usual complete set of chromosomes
diploid def
A cell with a complete set of chromosomes
What are the two stages prior to meiosis
-before meiosis, DNA unravels and replicates so there are two copies of each chromosome, chromatids
-The DNA condenses to form double armed chromosomes, called centromeres, each made of 2 sister chromatids, joined together by a centromere
Describe meiosis 1 and the stage immediately after in terms of the chromosome content of cells
-Meiosis 1(first division), the chromosomes arrange themselves at random into homologous pairs
-these homologous pairs are then seperated, halving the chromosome number
Describe meiosis 2, and the step immediately after in terms of the chromosome content of the cells
-Meiosis 2 (second division), the pairs of sister chromatids that make up each chromosome are seperated ( the centromere is divided)
-four haploid cells (gametes) that are genetically different from each other are produced
gene
a length of DNA that codes for a polypeptide
locus
the posistion of a gene on a chromosome or DNA molecule
allele
one of the different forms of a particular gene
homologous chromosome
a pair of chromosomes, one of maternal and one of paternal, that have the same gene loci
Describe the independent segregation of of homologous chromosomes
-In meiosis 1 (first division) the chromosomes arrange themselves at random into homologous pairs
-one of each of these pairs will go to each daughter cell (depending on how the chromosomes are lined up)
-seeing as the pairs are lined up at random, the combination of chromosomes of maternal and paternal origin that goes into a daughter cell at meiosis 1 is also a matter of chance
-this is called independent segregation
How does the independent segregation of homologous chromosomes result in genetic variation?
-each member of a homologous pair having the same genes and therefore determining the same characteristics
-but the alleles of these genes may differ
-the independent segregation of these chromosomes therefore results in new genetic combinations
Describe the process of crossing over between homologous chromosomes and how it leads to further genetic variation among daughter cells.
-Meiosis 1(first division), the chromosomes arrange themselves at random into homologous pairs then:
>The chromatids of each pair become twisted around each other
> During this, tensions are created and portions of the chromatids break off
>these broken portions might rejoin with the chromatids of their homologous partner
> Usually it is the equivalent portions of homologous chromosomes that are exchanged
-in this way, genetic combinations of maternal and paternal alleles are produces
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