The genome and its transmission

Question 1

Genome

Answer 1

The entirety of the DNA sequence (genes and non-coding segments) on one copy of each chromosome.

Question 2

The human genome

Answer 2

• 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

Question 3

Types of DNA sequence

Answer 3

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.

Question 4

Human genes

Answer 4

• 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

Question 5

Satellite DNA

Answer 5

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.

Question 6

Telomeres

Answer 6

Heterochromatin caps allow replication to occur to the tip of the chromosome and protect against damage. Long tracts of repeats can be unstable.

Question 7

Centromeres

Answer 7

Proteins involved in cell division bind to centromeric sequences, hence they are essential for segregation.

Question 8

Chromosomes

Answer 8

Single molecule of DNA, containing 100s of genes.

Question 9

Genes

Answer 9

Specific stretches of DNA whose sequences encode genetic information.

Question 10

Structure of chromatin

Answer 10

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.

Question 11

Types of chromatin

Answer 11

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.

Question 12

Mitosis

Answer 12

The division of diploid somatic cells into 2 genetically identical daughter cells. Important for growth, repair and replacing cells.

Question 13

Stages of mitosis

Answer 13

• prophase
• prometaphase
• metaphase
• anaphase
• telophase
• cytokinesis

Question 14

Prophase

Answer 14

• chromosomes condense
• spindle assemblies form outside nucleus
• nuclear membrane dissolves

Question 15

Prometaphase

Answer 15

• spindle microtubules bind to kinetochores on centromere region of chromosomes
• kinetochores face in opposite directions

Question 16

Metaphase

Answer 16

• 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

Question 17

Anaphase

Answer 17

• sister chromatids pulled to opposite poles
• polar microtubules elongate to push poles apart
• very fast

Question 18

Telophase

Answer 18

• nuclear membranes begin to reform
• chromosomes decondense
• nucleoli reappear; RNA synthesis begins
• initiation of plasma membrane cleavage

Question 19

Cytokinesis

Answer 19

• even separation of cytoplasm and plasma membrane

- formation of two identical daughter cells

Question 20

Meiosis

Answer 20

The division of diploid germline cells into genetically unique haploid gametes. Important for sexual reproduction.

Question 21

Generating genetic variation

Answer 21

• independent assortment of maternal and paternal homologous chromosomes at meiosis I
• crossing over creates unique recombinant chromosomes

Question 22

Stages of meiosis

Answer 22

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.

Question 23

Crossing over

Answer 23

DNA is exchanged at points of crossover (called chiasmata) between homologous chromosomes.

Question 24

Spermatogenesis

Answer 24

A spermatogonium cell in the testes can divide into more spermatogonia, or divide by meiosis into 4 spermatozoa.

Question 25

Oogenesis

Answer 25

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.