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Summary Medical Genetics for 3rd year
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1. Gene structure and function. DNA replication, transcription and translation.Organization of the human genome.
A gene is an entire nucleic acid sequence necessary for the synthesis of a functional RNA. There
are coding and noncoding genes.
Gene structure is the organization of specialized sequence elements within a gene. Genes are
made of DNA, where the particular DNA sequence determines the function of the gene.
The function of a gene is to code for a molecule that has a certain function. In comparison, an
allele is the expression of a gene:
For example:
The gene manifests that someone has hair and the allele manifests which color this hair may
have.
DNA replication:
DNA replication is the process in which DNA is simply copied. This process takes place in the
nucleus of the cell.
There are di erent components that are needed for replication to take place:
- helicase
- primase
- DNA polymerase
- topoisomerase
Transcription:
This is the process in which DNA is transcribed into RNA. this process also takes place in the cell
but he produced mRNA is moved into the cytoplasm through nucleopores to participate in the
translation process.
- RNA polymerase
- splicing, capping
Translation
Translation is the process in which proteins are made up of amino acids.
Certain components participate in this process such as:
- mRNA
- tRNA
- Aminoacids
- start -and endcodon
- APE
- Ribosome
Organization of the human genome
The human genome is the complete set of nucleic acids sequences for humans, encoded as DNA
within 23 chromosome pairs in cell nuclei and in a small NA molecule found in individual
mitochondria.
your genome: 6 billion nucleotides and 3-5 million SNPs, 60 new variants that are not inherited
from parents.
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, 2. Molecular mechanism of gene expression regulation.
- Exons: coding, Introns: non- coding
- critical gene product: housekeeping genes
Gene expression regulated at many levels: Activation of local structure of DNA, followed by
initiation, termination of transcription, processing of primary trasncript, transport in the cytoplasm,
translation of mature mRNA.
Transcriptional regulation: 1. Genetic (direct interaction of control factor with gene), 2.
Modulation with transcription machinery, 3. Epigenetic (non-sequence changes in DNA structure
in uencing transcription.
Genetic: 1. Inactivation of X-chromosome (Barr-bodies showing inactivation), 2. Introns/ exons. 3.
Imprinting (Some are inactivated depending on parental origin, meaning maternal or paternal). 4.
Genomic modi cation by rearranging DNA (immunoglobulin V D J rearrangement for more
combinations, 5. Methylation of DNA (hypomethylation - active)
Post-transcriptional regulation: RNA, stabilised by 5’ cap and poly-adenylate tail, small
interfering RNA for destruction, splicing/ alternative splicing.
Three prime untranslated regions of mRNA often contain regulatory sequences, altering post
transcriptionally gene expression.
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, Translational regulation: Less prevalent, but protein synthesis inhibitors include antibiotic
neomycin and toxin ricin.
Protein degradation: Once protein synthesis is complete, level of expression can be reduced by
protein degradation. Labeled by ubiquitin and proteasomes.
3. Change in chromatin structure as a mode of genomic modi cation. Genomic
modi cation- reorganization of the immunoglobulin genes. Epigenetics.
Another type of DNA modi cations that accompany the stable inheritance of the cell phenotype is
the extent of condensation and the binding of the protein which in uences the structure of the
chromatin.
Chromatin is the name we give to DNA + Histones (i.e. beads on a string structure). The general
organisation of chromatin is as follows:
1. DNA wraps around histones forming nucleosomes (aka euchromatin)
2. Multiple histones wrap into 30 nm bre
3. Higher-level DNA packaging of the 30 nm bre into the metaphase chromosome
(during mitosis/meiosis)
In a nondividing cell chromatin can be divided into two functional states, Euchromatin (loose) or
Heterochromatin (tightly packed). In Euchromatin the DNA can be accessed for transcription,
while it cannot in Heterochromatin. The structure of chromatin in di erent individuals can vary,
with di erent areas being loosely (euchromatin) or tightly (heterochromatin) packed, and this in
turn results in changes in gene expression. Such a mechanism adds an additional level of genetic
control beyond the primary mechanism that is the base pair sequence.
The variations of chromatin structure are the product of epigenetic chemical modi cation of the
the DNA molecule. Speci cally, the methylation and acetylation of the 5 position in cytosine .
DNA methylation sees the addition of methyl groups to the DNA molecule. This alters the activity
without altering the sequence. When the methyl group is located in a gene promoter, the methyl
group acts to suppress transcription . Both cytosine and adenine can be methylated.
Such changes fall under the category of Epigenetics , this is the study of heritable changes in
gene function that do not involve changes in the DNA sequence. In this case it is how
chromosome structure a ects gene activity and expression, although it can be used to describe
any heritable phenotypic change that does not involve genome modi cation.
As well as DNA methylation, the action of repressor proteins that attach to silencer regions of
DNA are another mechanism for epigenetic control, although this goes beyond the scope of this
question. It is however worth noting, the importance of epigenetic control is clear when you
consider the genetic structure of each cell in the human body is the same, it is only by virtue of
the epigenetic control of the DNA in each cell that allows cells to di erentiate.
Genomic modi cation - reorganization of the immunoglobulin genes Immunoglobulins
(antibodies) consist of a heavy and light chain . Both contain a constant (C) and variable (V) region
. In the forming of the Heavy and light chains genetic recombination of genes in the somatic
genome . In the heavy chain, the three genes are the Variable (V), Diversity (D) and Joining (J)
genes (known as VDJ recombination ), in the light chains, it is simply VJ recombination .
This reorganization is mediated by VDJ recombinases , two central examples are the RAG 1 and
2 enzymes , which are responsible for rst identifying the genes due to be recombined via
their ‘RSS motifs’ (think of these as identi cation labels), and cleaving the relevant part of the
chromatin.
V(D)J recombination allows for unique variability in the immunoglobulins in the body.
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, 4. Biology and genetics of mitochondria.
Mitochondria are double membrane-bound organelles found in eukaryotic organisms. They are
between 0.75 and 3 μm , and are responsible for supplying cellular energy , signalling, cellular
di erentiation and cell death. Mitochondria also have their own mitochondrial genome (similar to
bacterial genomes).
There are two types of genes that we must concern ourselves with regarding the genetics of
mitochondria,the rst is the Mitochondrial DNA, found within the Mitochondrial genome , and the
second is those genes found in the somatic genome that code for aspects of the mitochondria .
Mitochondrial DNA is circular , composed of about 16 kilobases , encoding for 37 genes (13 for
subunits of complexes I, III, IV and V and 22 for mitochondrial tRNA and 2 for rRNA. Mitochondrial
DNA lacks introns , unlike somatic DNA.
mtDNA is inherited solely via maternal inheritance . This is due to the fact that sperm
mitochondria are destroyed by the egg after fertilisation, and that most of the mitochondria of the
sperm are found in the tail, which never enters the egg.
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