Step by step describe Meiosis and its phases
The distinguishing features at each stage of Meiosis I
Prophase I: Homologous pairs of chromosomes are visible
Metaphase I: Homologous pairs are lined up side by side along the equator of spindle
Anaphase I: Whole chromosomes are being pulled to opposite poles with centromeres intact
Telophase I: There are 2 groups of condensed chromosomes around which nuclei membranes are forming
Cytokinesis: Cytoplasm is dividing and cell membrane is pinching inwards to form two cells
The distinguishing features at each stage of Meiosis II
Prophase II: Single whole chromosomes are visible
Metaphase II: Single whole chromosomes are lined up along the equator of the spindle in single file (at 90 degree angle to the old spindle)
Anaphase II: Centromeres divide and chromatids are being pulled to opposite poles
Telophase II: Nuclei are forming around the 4 groups of condensed chromosomes
Cytokinesis: Cytoplasm is dividing and four haploid cells are forming
Meiosis: Sources of Genetic Variation
Crossing over
Process:
During meiosis I homologous chromosomes pair up r
The non-sister chromatids can cross over and get entangled
These crossing points are called chiasmata
The entanglement places stress on the DNA molecules
As a result of this, a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome
result in a new combination of alleles on the two chromosomes
Independent assortment
Independent assortment is the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I
In prophase I homologous chromosomes pair up and in metaphase I they are pulled towards the equator of the spindle
Each pair can be arranged with either chromosome on top, this is completely random
The orientation of one homologous pair is independent (unaffected by the orientation of any other pair)
The homologous chromosomes are then separated and pulled apart to different poles
The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up
TYR gene & albinism
The amino acid tyrosine is converted to DOPA by the enzyme tyrosinase
DOPA is converted to dopaquinone again by the enzyme tyrosinase
Dopaquinone is converted to melanin
tyrosine → DOPA → dopaquinone → melanin
There is a recessive allele for the gene TYR that causes a lack of enzyme tyrosinase or the presence of inactive tyrosinase
Humans with albinism lack the pigment melanin in their skin, hair and eyes
HBB gene & sickle cell anaemia
Humans with sickle cell anaemia have abnormal haemoglobin in their red blood cells
β-globin is a polypeptide found in haemoglobin that is coded for by the gene HBB
There is an abnormal allele for the gene HBB which produces a slightly different amino acid sequence to the normal allele
The DNA base sequence GAG is replaced by GTG
This means that CTC is replaced by CAC on the complementary DNA template strand, meaning that GAG is replaced by GUG in the resulting mRNA
This change in amino acid sequence results in an abnormal β-globin polypeptide
The amino acid Glu is replaced with Val
They are unable to transport oxygen around the body
They stick to each other and clump together blocking capillaries
HBB is an autosoma recessive gene
A homozygous individual that has two abnormal alleles for the HBB gene produces only sickle cell haemoglobin
A heterozygous individual that has one normal allele and one abnormal allele for the HBB gene will produce some normal haemoglobin and some sickle cell haemoglobin
They are a carrier of the allele
F8 gene & haemophilia
The gene F8 codes for the Factor VIII protein
There are abnormal alleles of the F8 gene that result in:
Production of abnormal forms of factor VIII
Less production of normal factor VIII
No production of factor VIII
A lack of normal factor VIII prevents normal blood clotting and causes the condition haemophilia
The F8 gene is located on the X chromosome.
This means F8 is a sex-linked gene.
Haemophilia is a sex-linked condition
If males have an abnormal allele they will have the condition as they have only one copy of the gene
Females can be heterozygous for the F8 gene and not suffer from the condition but act as a carrier
HTT gene & Huntington’s disease
Huntington’s disease is a genetic condition that develops as a person ages
Usually a person with the disease will not show symptoms until they are 30 years old or older
An individual with the condition experiences neurological degeneration; they lose their ability to walk, talk and think
It has been found that individuals with Huntington’s disease have abnormal alleles of the HTT gene
The HTT gene codes for the protein huntingtin which is involved in neuronal development
People that have a large number (>40) of repeated CAG triplets present in the nucleotide sequence of their HTT gene suffer from the disease
The abnormal allele is dominant over the normal allele
Structural gene
A structural gene codes for a protein that has a function within a cell
For example, the F8 gene codes for the protein Factor VIII involved in blood clotting
Regulatory gene
A regulatory gene codes for a protein that helps to control the expression of another gene
Structural and regulatory genes that work together are usually found close together
Regulatory genes control structural genes and their levels of protein production
Regulatory genes sometimes have control over several structural genes at once
Inducible enzymes
Inducible enzymes are only synthesised when their substrate is present
The presence of the substrate induces the synthesis of of the enzyme by causing the transcription of the gene for the enzyme to start
repressible enzymes
Repressible enzymes are synthesised as normal until a repressor protein binds to an operator
The presence of the repressor protein represses the synthesis of the enzyme by causing the transcription of the gene for the enzyme to stop
lac operon function
The lac operon controls the production of the enzyme lactase (also called β-galactosidase) and two other structural proteins
Lac operon structure
The components of the lac operon are found in the following order:
Promoter for structural genes
Operator
Structural gene lacZ that codes for lactase
Structural gene lacY that codes for permease (allows lactose into the cell)
Structural gene lacA that codes for transacetylase
how are genes expressed
The lac repressor protein has two binding sites that allow it to bind to the operator in the lac operon and also to lactose (the effector molecule)
When it binds to the operator it prevents the transcription of the structural genes as RNA polymerase cannot attach to the promoter
When it binds to lactose the shape of the repressor protein distorts and it can no longer bind to the operator
absence of lactose
The regulatory gene is transcribed and translated to produce lac repressor protein
The lac repressor protein binds to the operator region upstream of lacZ
Due to the presence of the repressor protein RNA polymerase is unable to bind to the promoter region
Transcription of the structural genes does not take place
No lactase enzyme is synthesized
Atp and amino acids are conserved
presence of lactose
RNA polymerase binds to the promoter and expressed the regulatory gene.
This produces the repressor protein which gets inhabited by lactose sugar
changing the original shape the repressor protein cant bind with the promotor and the RNA polymerase bids to it and allows expression of structural gene
Lac Z Y A are expressed and
beta galactosidase, Lactose permease and transacetylase is produced and the lactose is broken down and used
How do gibberellins effect seed germination
Gibberelins diffuse to aleuron layer when water enters the seed
Gibberelins bind to the receptor
Destroy the DELLA protein
Pif binds to the promotor
Genes are expressed to produce synthesis amylase
Prophase I
Nuclear membrane and nucleolus disappears
Supper coiling to form sister chromatids (chiasma)
Centrioles more to opposite poles
Homologous chromosomes pair up =Synapsis
forming bivalents
cross over
Metaphase I
Bivalents line up along the equator
Random/independent assortment occurs
both can happen at the same time this can lead to 4 different alleles
Anaphase I
Spindle fibers shorten pulling the homologues pairs apart
separating the centromere to opposite poles
Metaphase II
Sister chromatids align in the equator
Anaphase II
Separation of sister chromatids
Telophase and cytokinesis
Cell division producing haploid cells (Reduction division)
Reduction division
Nuclear division that results in a reduction in chromosome number
Genetic variation can be risen from
Independent assortment
Crossin over
Random fertilization
