Human genome

What types of chromosomes make up a normal human karyotype?

A normal human karyotype consists of 23 pairs of chromosomes (46 in total), which are divided into autosomes and sex chromosomes.

• Autosomes — 22 pairs of chromosomes identical in both sexes. These include metacentric, submetacentric, and acrocentric forms.
• Sex chromosomes — 1 pair of chromosomes that differentiate males and females. The normal female karyotype is represented by XX chromosomes, and the normal male karyotype by XY.

Which nitrogenous bases form the genetic code in human DNA?

The genetic code in the human DNA molecule is formed by heterocyclic compounds divided into two classes.

• Adenine — a purine base with a two-ring structure.
• Guanine — a purine base with a two-ring structure.
• Cytosine — a pyrimidine base consisting of a single six-membered ring.
• Thymine — a pyrimidine base, which is methylated uracil.

Uracil is not part of DNA; it is found only in RNA.

What are the main forms of hereditary variation distinguished in classical genetics?

In classical genetics, hereditary (genotypic) variation is subdivided into two main forms depending on the mechanism of alteration of the genetic apparatus.

• Combinative variation — arises due to gene recombination during crossing over and the free combination of gametes in sexual reproduction.
• Mutational variation — arises as a result of mutations leading to changes in the structure of the genetic apparatus.

Both forms serve as the material base for evolutionary processes and natural selection.

What types of gene (point) mutations are distinguished by the nature of the nucleotide sequence change?

Several types of gene mutations are distinguished by the nature of the nucleotide sequence change.

• Missense mutations — alter the meaning of a codon, leading to the incorporation of a different amino acid into the protein.
• Nonsense mutations — lead to the appearance of a stop codon and premature termination of protein synthesis.
• Neutral mutations — the meaning of the code does not change due to the degeneracy of the genetic code (synonymous substitutions).
• Frameshift mutations — arise due to deletions (loss) or insertions of nucleotides.
• Splicing mutations — point substitutions at the exon-intron junction boundaries that disrupt splicing.
• Dynamic mutations — an increase in the number of trinucleotide repeats (expansion).

Which human diseases and traits are inherited in an X-linked recessive manner?

A number of diseases and traits are inherited in an X-linked recessive manner.

• Hemophilia A.
• Hemophilia B.
• Color blindness.
• Muscular dystrophy.
• X-linked recessive Charcot-Marie-Tooth disease.

What main elements and classes of sequences are included in the structure of the human genome besides protein-coding genes?

The structure of the human genome mainly consists of non-coding and repetitive sequences.

• Mobile genetic elements — retrotransposons and DNA transposons (e.g., LINE, SINE) capable of replicating and inserting into new locations.
• Simple repeats — short, repeatedly occurring nucleotide sequences.
• Duplicated segments — large genomic fragments present in multiple locations.
• Unique sequences — include regulatory elements and regions with unknown functions.
• Heterochromatin DNA refers to the regions most rich in repeats.