3. Voice of the Genome (2) Flashcards
Cell replication and epigenetics 3.6 - 3.15
Features of Ovum
Follicle Cells, Zona Pellucida, Cortical Granules, Lipid droplets, plasma membrane, haploid nucleus
Features of Sperm cell
Acrosome, haploid nucleus, head, middle piece, neck, mitochondrion, flagellum
How are eggs specialised for function?
cytoplasm contains nutrients and a store of energy for the dividing zygote after fertilisation. (lipid droplets)
Zona pellucida (protective coating) hardens after fertilisation to prevent the entry of other sperm cells (polyspermy)
Follicle cells: protective coating
haploid nucleus - full set of chromosomes restored.
How are sperm cells specialised for function?
A lot of mitochondria provide energy for the sperm cells to move. Acrosomes contains digestive enzymes which break down the zona pellucida and allow sperm to penetrate the egg
Process of fertilisation
- sperm head meets the zona pellucida and acrosome reaction occurs. enzymes from the acrosome digest the zona pellucida
- Sperm head plasma membrane fuses with the cell membrane of the egg allowing the sperm nucleus to enter the egg cell.
- Cortical reaction occurs - cortical granules in the ZP release enzymes that cause the zona pellucida to harden.
- The nuclei fuse and a full set of chromosomes is restored, forming a diploid zygote.
Location of genes on a chromosome
locus / loci
Linkage and chromosomes
The closer the loci of the genes on the chromosome, the more closely linked they are because they are less likely to be separated during recombination in meiosis
recombination in meiosis
exchange of genetic material between paternal and maternal chromosomes which leads to genetic variation and a combination of traits found in either parent.
Autosomal linkage
alleles on the same autosomal chromosome/ gene are more likely to be inherited together
autosomal chromosome
non-sex chromosome
Sex linkage/ why are men more likely to have disorders like haemophilia
Some genes are sex linked because they occur on the X chromosome.
Y chromosome is smaller than the X chromosome therefore they only need one copy for the allele to be expressed.
Process of meiosis 1
Nucleus of original parent is diploid and contains two sets of chromosomes. Chromosomes replicate and the homologous pairs of chromosomes are split up to produce two haploid nuclei.
Each chromosome still consists of two chromatids
Process of meiosis 2
chromosome separates to produce four haploid nuclei. Each chromosome consists of a single chromatid
What does meiosis result in
genetic variety -> results in production of gametes
Why is having genetically different offspring advantageous
natural selection - genetic variation = species can adapt to new environments due to variation which gives them a survival advantage.
How does meiosis ensure genetic variation
crossing over and independent assortment result in different combinations of alleles in gametes
crossing over
sections of DNA are swapped over between non sister chromatids at the chiasma resulting in the creation of a combination of new alleles, increasing genetic variation
when does crossing over occur
Meiosis 1 - homologous chromosomes pair up and are in close proximity to each other (a.k.a. bivalents) - non sister chromatids cross over and get entangled at the crossing point the chiasmata.
Section of chromatid breaks off and re-joins with the chromatid from the other chromosome
Where is crossing over more likely to occur?
Further down the chromosome away from the centromere.
independent assortment
various combinations of ways maternal and paternal chromosomes can be distributed between the two daughter cells
For two bivalents there are two possible orientations.
The pairs line up along the equator of the spindle independently of eachother
At the end of meiosis 2 each orientation gives two types of gamete - resulting in four types of gamete altogether
mitosis
process of cell division resulting in two genetically identical daughter cells
role of mitosis
identical daughter cells are used for growth, repair and asexual reproduction
Mitosis stages
PMAT - prophase metaphase anaphase telophase
Interphase
stage where the cell prepares for mitosis - growing and replicating chromosomes preparing for cell division
G1 stage
cell grows in size and synthesises mrna and proteins required for DNA synthesis. DNA replicates
S stage
the cell synthesises a complete copy of the dna in its nucleus
G2 stage
Cell prepares to divide, replicating organelles for a full set in each new cell
Prophase
chromosomes condense and nuclear envelope disintegrates
Anaphase
duplicated chromosomes separate and move towards the opposite poles of the cell by spindle fibres retracting
Metaphase
chromosomes align at the equator of the cell, spindle fibres attach to the chromatids
Telophase
chromosomes are now on opposite sides of the cell and nucleus reforms. forming two identical nuclei.
Cytokinesis
cytoplasm pinches in on itself to form two daughter cells
stem cell
undifferentiated cells
multipotency
cells which can differentiate into many different types of cell
totipotency
cells which can differentiate into every type of specialised cells including placental cells
pluripotency
cells that can differentiate into many types of specialised cells but not placental cells
uses of stem cell therapies
embryonic stem cells (umbilical cord), adult stem cells (bone marrow) are used to treat
Diabetes, multiple sclerosis, Parkinson’s, replace damaged tissues like nerve tissue in spinal cord injuries.
Ethics behind use of stem cells
For:
Stem cells ca save many lives and improve the quality of life for many people.
Against:
Embryos are killed in the process of embryonic stem cell extraction
risk of infection
potential to become cancerous
Differential gene expression step 1+2
1) stimulus acts on unspecialised cells
2) activator and repressor molecules bind to promoter regions on the DNA sequence, so some genes are switched on and become active, whereas other genes are switched off
3) active genes are transcribed to produce RNA
Differential gene expression step 4+ 5
4) mRNA is translated on ribosomes and used to produce proteins.
5) these proteins modify the cell by determining cell structures and controlling cell processes
Differential gene expression step 6
6) Changes to the cell by these proteins causes the cell to become specialised - these changes are permanent as they are difficult to reverse.
Lac Operon (lactose absent)
Lac repressor protein binds to the operator region.
The presence of this protein means that RNA polymerase is unable to bind to the promoter region.
Transcription of the structural gene does not take place and therefore no lactase enzyme is synthesised.
Can prokaryotes have an operon?
Yes - they can form an operon - a cluster of genes controlled by the same promoter and transcribed together.
example: lac operon in bacteria
Lac Operon (Lactose present)
Lactose binds to the binding site on the repressor protein - changing its shape, making it unable to bind to the operator site
RNA polymerase is able to bind to the promoter region and transcription takes place. all 3 structural genes (lac Z, Y, A) are translated
Why is the lac operon important
prevents waste of energy - molecules that are not needed are not transcribed and translated, saving energy.
Phenotype
observable characteristics of an individual resulting from the interaction between the genotype and the environment
Epigenetics
modifications that alter gene activity without changing the DNA sequence. can occur as a response to changes in environment
Operon
cluster of structural genes that are transcribed together
How do transcription factors control the expression of genes?
(1) gene expression can be controlled by altering the rate of transcription of genes.
(2) This is controlled by transcription factors - proteins that bind to DNA and activate/deactivate genes by increasing/ decreasing rate of transcription
activators
increase the rate of transcription
repressors
decrease the rate of transcription by preventing rna polymerase from binding and stopping transcription
Transcription factors in eukaryotes vs prokaryotes
Eukaryotes- transcription factors bind to specific dna sites near the start of their target genes.
Prokaryotes - control of gene expression involves transcription factors binding to operons
DNA Methylation
addition of a methyl group - can cause activation/ inactivation of chromatin. Represses a gene
The group binds to the CpG site (between cytosine and Guanine)
Increased methylation changes the DNA structure so that the proteins and enzymes needed for transcription can’t bind to the gene so the gene is not expressed.
DNA acetylation
addition of an acetyl (COCH3) group - activates chromatin and allows transcription
acetylated - chromatin is less condensed - proteins involved in transcription can bind to the DNA, activating genes
What happens when acetyl groups are removed?
acetyl groups removed from the histone = chromatin becomes highly condensed and the genes are repressed as transcription proteins can’t bind to them
Histone modification
DNA Acetylation - Histones are proteins that dna wraps around to form chromatin. How condensed chromatin is affects the accessibility of the DNA and whether it not the proteins and enzymes needed for transcription can bind to it.
How are epigenetic changes passed on following cell division
1) Following cell replication, epigenetic changes to its gene expression is passed on to daughter cells
2) epigenetic changes passed on = genes that were activated/ deactivate in the original cell will be activated/ deactivated in the daughter cells
3) if an epigenetic change occurred in response to a change in environment, daughter cells are equipped to deal with that change in environment
Monogenic inheritance
Characteristics influenced by one gene only.
Discontinuous variation + examples
variation caused by monogenic inheritance. Characteristics influenced by one gene only.
Example - eye colour
Continuous variation + examples
Variation caused by polygenic inheritance. (multiple genes at different loci)
and also environmental factors
Examples: Height - also influenced by nutrition
Lung cancer - presence of proto-oncogenes
Polygenic inheritance
Several genes at different loci are involved in determining a characteristic
what is fertilisation?
fusion of the nuclei from a male gamete and a female gamete
sister chromatids
identical copies formed by the DNA replication of a chromsome. both copies are joined by a common centromere
chromatids
one of the two identical halves of a chromosome that has been replicated in preparation for cell division
chromosome
package of DNA containing a part of genetic material of an organism. 23 pairs of chromosomes, 46 altogether in an adult human
homologous pair of chromosomes
Two chromosomes in a pair - one inherited from mother and the other one from the father
