The genome and its transmission Flashcards
Genome
The entirety of the DNA sequence (genes and non-coding segments) on one copy of each chromosome.
The human genome
- approximately 3,200,000,000 base pairs long
- DNA sequences are found on chromosomes from 22 autosome pairs and both sex chromosomes (X and Y)
- mitochondrial DNA comprises an additional 16,600 bases and 37 genes
Types of DNA sequence
Protein-encoding exons (genes) make up 2-3% of the total DNA sequence. Repeating elements, such as LINEs, SINEs, simple sequence repeats and transposons make up ~50% of the genome. Other types of non-coding DNA include miscellaneous unique sequences and miscellaneous heterochromatin.
Human genes
- in humans, ~23,000 genes encode mRNA to make proteins
- vary in size and organisation (ie. introns and exons)
- unevenly distributed between and within chromosomes
- may encode several mRNA sequences, or functional non-protein coding RNA
- can overlap on different strands of DNA
- some are found within introns of other genes
Satellite DNA
Composed of arrays of short tandem repeats (1-2 bps =SSRs, 10-60 bps= VNTRs), mainly present in the densely packed heterochromatin of centromeres and telomeres. Serves no known purpose.
Telomeres
Heterochromatin caps allow replication to occur to the tip of the chromosome and protect against damage. Long tracts of repeats can be unstable.
Centromeres
Proteins involved in cell division bind to centromeric sequences, hence they are essential for segregation.
Chromosomes
Single molecule of DNA, containing 100s of genes.
Genes
Specific stretches of DNA whose sequences encode genetic information.
Structure of chromatin
DNA and histone proteins associate to form nucleosomes, which coil to form a 30nm chromatin fibre. This undergoes further supercoiling and packing to form chromosomes.
Types of chromatin
Euchromatin- extended, loosely packed state found throughout the nucleus which is accessible to transcription and replication machinery.
Heterochromatin- condensed, tightly coiled state found during cell division.
Mitosis
The division of diploid somatic cells into 2 genetically identical daughter cells. Important for growth, repair and replacing cells.
Stages of mitosis
- prophase
- prometaphase
- metaphase
- anaphase
- telophase
- cytokinesis
Prophase
- chromosomes condense
- spindle assemblies form outside nucleus
- nuclear membrane dissolves
Prometaphase
- spindle microtubules bind to kinetochores on centromere region of chromosomes
- kinetochores face in opposite directions
Metaphase
- spindle gathers chromosomes to align at equator, forming the mitotic plate
- tension on kinetochores at centromeres facing opposite directions
- regulated by the M checkpoint of the cell cycle; checking all chromosomes are attached to spindle fibres to prevent non-disjunction
Anaphase
- sister chromatids pulled to opposite poles
- polar microtubules elongate to push poles apart
- very fast
Telophase
- nuclear membranes begin to reform
- chromosomes decondense
- nucleoli reappear; RNA synthesis begins
- initiation of plasma membrane cleavage
Cytokinesis
- even separation of cytoplasm and plasma membrane
- formation of two identical daughter cells
Meiosis
The division of diploid germline cells into genetically unique haploid gametes. Important for sexual reproduction.
Generating genetic variation
- independent assortment of maternal and paternal homologous chromosomes at meiosis I
- crossing over creates unique recombinant chromosomes
Stages of meiosis
1) Each homologous chromosome replicates to give two pairs of chromatids, which form a bivalent (diploid–>tetraploid).
2) Meiosis I- homologous chromosomes separate.
3) Meiosis II- chromatids separate to give single chromosomes.
Crossing over
DNA is exchanged at points of crossover (called chiasmata) between homologous chromosomes.
Spermatogenesis
A spermatogonium cell in the testes can divide into more spermatogonia, or divide by meiosis into 4 spermatozoa.
Oogenesis
A stock of 2.6 million oocytes are already formed in the ovaries by 5 months gestation. Meiosis I is completed at ovulation each month, and meiosis II is only completed upon fertilisation. One oocyte divides into only one ovum and 2 small polar bodies, so all the cytoplasm goes to one daughter cell.
