From Chromosomes to Genomes

karyotype abnormalities etc

What is a gene?

• A gene is a sequence of DNA that codes for a molecule that has a function. These molecules are usually proteins.​

What is the structure of DNA?

• DNA is a double helix made of two antiparallel nucleotide strands. Each nucleotide contains a deoxyribose sugar, phosphate group and nitrogenous base.

What is a nucleotide?

• A nucleotide is the basic building block (monomer) of DNA. It consists of: deoxyribose sugar, phosphate group, and a nitrogenous base (A, T, C or G)

What is an allele?

• An allele is an alternative form of a gene found at the same locus on homologous chromosomes.

What is a genome?

• An organisms complete set of DNA (inc. 23 pairs of chromosomes & 46 chromosomes)

What is the Human Genome Project?

A project aimed to fully sequence the human genome

What are centrioles and spindle fibers role in cell division?

• Spindle fibers which are required for the correct separation of chromosomes

• Centrioles organise spindle fibers

What is the difference between haploid and diploid?

• Haploid (n) = Haploid cells (e.g. sperm and egg cells) have 1 set of chromosomes

• Diploid (2n) = Diploid cells (e.g. somatic (body) cells) have 2 sets of chromosomes

Chromosome vs Chromatid:

• Before DNA replication → 1 chromosome = 1 chromatid

• After DNA replication → 1 chromosome = 2 sister chromatids joined at the centromere.

What is non-disjunction?

• When homologous chromosomes or sister chromatids fail to disjoin or separate at anaphase I or II​

• This causes either two copies of a chromosome rather than the usual one, to be present in a gamete OR a copy of one chromosome to be missing.​

What are the stages of meiosis?

Interphase (G1), Interphase (S), Prophase 1, Metaphase 1, Anaphase 1, Telophase 1, Cytokinesis 1, Prophase 2, Metaphase 2, Anaphase 2, Telophase 2, Cytokinesis 2

What happens during interphase?

• G1 - The cell grows and preforms its normal metabolic functions, primary growth period before DNA is copied

• S - Cell replicated its genetic material, each chromosome duplicated into 2 identical sister chromatids

What happens in prophase 1?

• During prophase I, chromosomes condense, nuclear membrane disappears, homologous chromosomes pair to form tetrads, crossing over occurs at chiasmata, where chromosome segments are exchanged, increasing genetic variation.

What happens in metaphase 1?

• During metaphase I, tetrads line up at the metaphase plate (middle of the cell). Spindle fibres attach to the centromeres of homologous chromosomes from opposite poles

What happens during anaphase 1?

• Homologous chromosomes separate and are pulled toward opposite poles

What happens during telophase 1?

• Homologous chromosomes separate and move to opposite poles, leaving each pole with a haploid set of chromosomes. Each chromosome consists of linked sister chromatids

What happens during cytokinesis 1?

• Cytokinesis occurs, producing two haploid cells. Chromosomes do not replicate again before meiosis II (formation of 2 daughter cells not genetically identical).

What happens during prophase 2?

• During prophase II, a spindle apparatus forms and spindle fibres attach to the centromeres of chromosomes in each haploid cell, preparing for sister chromatid separation.

What happens during metaphase 2?

• During metaphase II, sister chromatids line up at the metaphase plate, with their centromeres facing opposite poles.

What happens during anaphase 2?

• The centromeres of sister chromatids separate and the now separate sisters travel toward opposite poles.
  

What happens during telophase 2?

• Separated sister chromatids arrive at opposite poles, and nuclei form around the chromatids

What happens during cytokinesis 2?

• Cytokinesis separates the cytoplasm, there are four haploid daughter cells (not genetically identical).

What is genetic variation?

Genetic variation is the differences in DNA or genetic information between individuals of the same species. It can be created during meiosis through:

• Crossing over

• Independent assortment

What is crossing over?

• Occurs during Prophase I of meiosis

• Homologous chromosomes pair up, and non-sister chromatids exchange sections of DNA, resulting in the exchange of alleles between homologous chromosomes.

• The point where chromosomes exchange DNA is called a chiasma

What is independent assortment?

• During Metaphase I, homologous chromosome pairs randomly align at the equator, resulting in different combinations of maternal and paternal chromosomes in the gametes.

What is sexual reproduction?

• Sexual reproduction depends on meiosis, which halves the chromosome number to produce gametes

• The offspring produced are genetically different from each other and from their parents

• This genetic variation occurs due to the mixing of genetic material during meiosis

What is asexual reproduction?

• Asexual reproduction involves mitosis, where one parent cell produces two daughter cells

• The daughter cells contain an exact copy of the parent’s genetic information

• Offspring are genetically identical to the parent (clones)

What is the point of meiosis 1?

Separate homologous chromosomes

What is the point of meiosis 2?

Separate sister chromatids

What is it called when you have one more/less chromosome?

1. 1 more: trisomy

2. 1 less: monosomy