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Summary Human Genetics, Reproduction and Prenatal Development
Overview of all cases, lectures and practicals
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Voorbeeld van de inhoud
Human Genetics, Reproduction and Prenatal DevelopmentBBS1005
, Case 01 BBS1005
DNA structure
DNA is the molecule that stores the hereditary information of an organism. Because it stores this
information the DNA molecule needs to fulfil several criteria:
1. It needs to be replicable
2. It needs to be stable
Main 6 features:
1. Phosphate sugar diester back bone.
2. Double stranded double helix
3. Anti-parallel stands → 5'-3' and 3'-5'
4. Nitrogenous bases
5. Minor-major groove → Asymmetrical orientation of the base pairs to do with the
hydrophobic nitrogenous bases.
6. Fixed distance between the base pairs (0.34 nm) and each helix turn is 3.4 nm.
Each nucleotide consists of:
1. 5-carbon sugar --> Deoxyribose which harworth projection (ribose with an -H instead of an -
OH at the 2' carbon).
2. Phosphate (on the 5' sugar carbon and the 3'carbon of the next sugar).
3. Nitrogenous base
1. Adenine (purine base - two nitrogen rings)
2. Guanine (purine base)
3. Thymine (pyrimidine base - one nitrogen ring)
4. Cyctosine (pyrimidine base)
4. Two different groups: Purine and Pyrimidine. A and G belong to the Purine group because
they have two rings in their structure. T and C belong to the pyrimidine group because there
is only one ring structure.
5. Nucleotides: Consist of a base, sugar and phosphate.
6. Nucleosides: Consist of a base and sugar.
The nucleotides are connected together by a phosphate diester on the OH-group on the 5'-carbon of
one sugar to the 3'-carbon in the next sugar.
1
,One DNA polymer strand connects to a second DNA polymer strand that is complementary to itself.
This means that the nucleic acid bases facing each other are part of the same complementary pair:
1. Adenine - Trymine (two hydrogen bonds)
2. Guanine - Cytosine (three hydrogen bonds)
The complementary structure of the DNA molecule makes it possible to replicate the molecules
based on the bases present in one strand, because they will always bond to their complementary
base. This complementary nature makes the DNA molecule suitable for storing and transferring
information. The structure of one strand can be determined based on the structure of the other
strand.
The information stored by the DNA codes for specific amino acids that are used to build the proteins
that each gene codes for. Because there are 20 amino-acids and 4 nucleotides there are
combinations of three nucleotide bases that code for an amino acid. There are multiple 3-base
combinations (codons) that represent the same amino acid as well as codons that indicate the
beginning and end of an amino-acid sequence. Only 1% of all the genes in our DNA are codes for the
production of proteins (the coding DNA), the function of the remaining part of the human genome is
as of yet unknown.
DNA supercoiling in chromosomes
When DNA is not in a dividing phase the DNA molecule is relatively loose and long and only coiled
around spooling proteins (histone spools) which reduce its length while keeping all areas of the DNA
available for RNA transcription for the production of proteins. When cells divide the DNA is
concentrated/coiled into chromosomes.
Storage of DNA
DNA is stored in the nucleus of each cell in a compact for called chromosomes. Each diploid human
cell contains 46 chromosomes in 23 pairs, 22 homologous and 1 non-homologous which determines
the sex of an individual (XX/XY). The haploid cells (gametes → egg and sperm cells) only contain 1 of
each chromosome thus 23 chromosomes.
Eukaryotic DNA Is Packaged into a Set of Chromosomes:
1. Each chromosome consists of a single, enormously long linear DNA molecule associated with
proteins that fold and pack the fine DNA thread into a more compact structure.
2. The complex of DNA and protein is called chromatin.
3. Each human cell contains two copies of each chromosome, one inherited from the mother
and one from the father. With the exception of the germ cells, and a few highly specialized
cell types that cannot multiply and lack DNA altogether.
4. The maternal and paternal chromosomes of a pair are called homologous chromosomes
(homologs). The only nonhomologous chromosome pairs are the sex chromosomes in males,
where a Y chromosome is inherited from the father and an X chromosome from the mother.
Thus, each human cell contains a total of 46 chromosomes, diploid (2n). 22 pairs common to
both males and females (autosomes), plus two so-called sex chromosomes (X and Y in males,
two Xs in females, allosomes)
5. Two chromatids are connected via the centromere, the ends of the chromosomes are called
telomeres.
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