Biology- Genetics (until protein synthesis)

What is DNA?

What are the base pairs?

DNA consists of nucleotides made up of a sugar, a phosphate group, and a nitrogenous base.

It is a double stranded molecule.

The four nitrogenous bases are adenine (A), thymine (T), cytosine (C), and guanine (G) that pair specifically (A with T and C with G).

Where is DNA found in:

Prokaryotic cells

Chloroplasts/Mitochondria

Eukaryotic cells

In prokaryotes: Single circular DNA molecule in the cytoplasm. They also contain plasmids. DNA is not associated with proteins.

In chloroplasts and mitochondria: Circular short DNA similar to prokaryotes. Again, not associated with proteins

In eukaryotes: Long linear DNA organized into chromosomes within the nucleus. Associated with proteins (histone) to form chromosomes.

How are chromosomes made?

Chromosomes are made by the coiling and condensing of DNA around histone proteins (creating solenoids), which helps organize and package the genetic material into chromosomes.

A lot of genetic material can be stored in the nucleus of each cell.

What happens to chromosomes during interphase?

The chromosomes are dispersed throughout the nucleus and are not visible as separate structures.

Facts about sister chromatids?

Each sister chromatid contains an genetically identical copy of DNA molecule, formed during DNA semi conservative replication before cell division.

The two sister chromatids are held together at the centromere.

Diploid organisms inherit full sets of…

chromosomes from both parents. As a consequence, diploid cells have homologous pairs of chromosomes.

Humans have a diploid number 2n=46 however other species have different diploid numbers.

What are homologous pairs of chromosomes?

Homologous pairs of chromosomes are pairs of chromosomes that contain the same genes, one inherited from each parent, and have similar structure and size. They carry the same genes at the same locus but mot necessarily the same allele. Not genetically identical.

What is a gene?

Base sequence that codes for:

The amino acid sequence of a polypeptide.

A functional RNA

A gene occupies a fixed position called a locus.

In eukaryotes most DNA molecules don’t code for amino acid sequences in polypeptides (only 2% in humans). There are non-coding multiple repeat sequences. There are coding regions called exons and non-coding sections called introns.

Definitions of:

Genome

Proteome

Allele

Gene Pool

Genome: The complete set of all genes in a cell, including the genes in the nucleus, mitochondria and chloroplasts

Proteome: The full range of all the proteins that a cell is able to produce using its genome

Allele: Some genes have two or more alternative forms, each allele has a different sequence of bases and consequently codes for a different polypeptide.

Gene Pool: All the different alleles of all the genes found within a population.

Protein Synthesis.

What is the genetic code?

Every 3 bases is a triplet and codes for a specific amino acid.

Four nucleotides so 4³=64 combinations of triplet codes for 20 amino acids

Some codes are known as degenerate as most amino acids are coded for by more than one triplet.

First triplet is always methionine (start) and three triplets which don’t code for amino acids as they are stop codons.

Universal - same in all organisms

Non-overlapping - each base is read only once each base is a part of only one triplet.

Types of RNA

mRNA- few thousand nucleotides long. Bases don’t bind to each other.

tRNA- about 75 nucleotides long. Folded into clover shape.

At one end, is amino acid bonding site with three bases exposed; anticodons.

61 types of tRNA (3 stop codons).

What is transcription?

• One gene unwinds (H-bonds broken)

• Complementary RNA nucleotides bind to exposed bases on template strand.

• Backbone is joined by RNA polymerase

• Both exons and introns are transcribed to make pre-mRNA

What is splicing?

Non-coding regions (introns) are cut out so the coding sections (exons) are edited together to make mRNA which is a copy of exons only.

Fun facts!

Prokaryotic DNA and mitochondria and Chloroplast DNA does not have introns so mRNA doesn’t need to be spliced.

What is translation?

• Ribosomes bind to first two codons on mRNA

• tRNA binds to complementary codon on mRNA

• tRNA is carrying specific amino acids for that codon

• Condensation reaction happens - peptide bond forms between amino acids

• First tRNA leaves ribosome and picks another amino acid up

• Moves along each codon triplet - repeats to make a chain/growing polypeptide

• When stop codon reached, ribosome disengages from mRNA.

• ATP needed for bonds forming.