Unit 4 - Meiosis + Mutation Flashcards
What is a chromosome?
An independent DNA molecule.
What is a gene?
A base sequence of DNA that codes for a polypeptide or functional RNA.
What is an allele?
Different forms of the same gene.
What is a locus?
The fixed position of a gene on a particular chromosome.
Definition of homologous chromosomes?
A pair of chromosomes which have the same genes at the same gene loci but may have different alleles.
What is a haploid cell?
A cell containing one of each type of chromosome. (n)
What is a diploid cell?
A cell containing 2 of each type of chromosome as pairs (2n)
What is a gamete?
A sex cell (n) containing one of each type of chromosome, in humans, the egg and sperm.
What is sexual reproduction?
When 2 gametes (n) join to make a zygote (the first cell (2n) of a new organism). The joining of the gametes is called fertilisation.
What does meiosis result in?
4 haploid daughter cells that are genetically different from each other from a single diploid parent cell.
What are the key points of meiosis?
-Homologous chromosomes pair, forming a bivalent, and line up along the equator of the cell, through independent segregation - different combinations of maternal and paternal chromosomes results.
-Crossing over occurs, resulting in different combinations of alleles.
-During the first division, homologous chromosomes separate and are pulled apart by their centromeres to opposite poles of the cell.
-During the 2nd division of meiosis, sister chromatids separate and are pulled apart by their centromeres to opposite poles of the cell.
How does crossing over of homologous chromosomes occur?
The homologous chromosomes come together as a pair, forming a bivalent. Chromatids on different chromosomes, so non-sister chromatids, cross over. The point of cross over is called a chiasma. Equal lengths of non-sister chromatids are exchanged. The resulting chromatids contain the same genes but different combinations of alleles.
How does independent segregation of chromosomes occur?
When homologous chromosomes line up in the equator they do so completely randomly. Once they separate during the first meiotic division, it is completely random which chromosome from each pair ends up in which daughter cell. So the 4 daughter cells produced by meiosis from the original cells have different combinations of those parental chromosomes (but still 1 of each type).
How does random fertilisation of haploid gametes occur?
During fertilisation, any sperm can fertilise any egg. This results in the potential for many different combinations of pairs of chromosomes in the resulting zygote (but still 2 of each type of chromosome in each pair).
Differences between meiosis and mitosis?
Meiosis: -Diploid parent cells produce haploid daughter cells.
-Homologous chromosomes associate in pairs on the equator before the 1st division.
-Crossing over of homologous chromosomes occurs.
-There is a total of 2divisions/4daughter cells produced.
Mitosis: -Diploid=Diploid Haploid=Haploid
-Homologous chromosomes do not pair together on the equator.
-No crossing over of chromosomes.
-1 division/2daughter cells produced
-Genetically identical cells produced.
Why is meiosis important?
Halving the number of chromosomes in meiosis to give haploid cells is important, so that when fertilisation occurs, the diploid number of chromosomes is restored in the cell and the chromosome number is not doubled.
Definition of a gene mutation?
A change in the DNA base sequence of chromosomes.
What are mutagenic agents?
Something that can cause an increase in the rate of gene mutation.
When do mutations occur?
They can arise spontaneously during DNA replication.
What is a substitution mutation?
One base is swapped with another different base.
What is the effect of a substitution mutation?
Sometimes none because the degenerate nature of the genetic code, some amino acids are coded for by more than one DNA triplet.
What is a deletion mutation?
One base is removed from the sequence.
What is the effect of a deletion mutation?
Always an effect - a deleted nucleotide results in an alteration of the base triplets from the mutation onwards. The reading frame has been shifted to the left by 1 letter and is known as a frame shift. All triplets from the point of mutation coding for the polypeptide could be altered, so all the amino acids from the point of mutation will hr altered.
What happens when a mutation causes a triplet to code for a stop codon?
This will cause the growing polypeptide chain to terminate prematurely, and would probably not be able to perform its proper function (depending on how early the termination occurs).
What happens if a mutation causes the DNA sequence to change?
-The sequence of bases in DJA and so the mRNA is altered
-The amino acid sequence coded for alters (primary structure)
-The hydrogen, disulphide and ionic bonds form in different places
-The protein folds into a different shape as the tertiary structure changes
-The protein is non-functional
-If it is an enzyme… different active site shape means that it is no longer complementary to the substrate - no enzyme substrate complexes can form.
What happens if a mutation causes the DNA sequence to change?
-The sequence of bases in DJA and so the mRNA is altered
-The amino acid sequence coded for alters (primary structure)
-The hydrogen, disulphide and ionic bonds form in different places
-The protein folds into a different shape as the tertiary structure changes
-The protein is non-functional
-If it is an enzyme… different active site shape means that it is no longer complementary to the substrate - no enzyme substrate complexes can form.
How many chromosomes are in a gamete cell?
23 chromosomes
What is chromosome non-disjunction?
A type of chromosome mutation, when chromosomes fail to separate properly during meiosis. This can take place during the 1st division (homologous chromosomes fail to separate) or 2nd division (sister chromatids fail to separate).
What do chromosome mutations lead to?
Inherited conditions because there are the wrong number of chromosomes in the resulting gametes, I.e. down-syndrome
