Genotype/phenotype

• The genotype of an organism is the genetic code in its cells. This genetic constitution of an individual influences – but is not solely responsible for – many of its traits. 
• The phenotype is the visible or expressed trait, such as hair color. The phenotype depends upon the genotype but can also be influenced by environmental factors.

Autosomes and sex chromosomes

• Autosomes are chromosomes that determine traits of the organism. Autosomes are the somatic chromosomes which control the body characters or somatic characters.
  

• Sex chromosomes are the allosomes which determine the sex of the organisms.

The normal human has 23 different types of chromosomes and 2 copies of each from each parent, adding up to a total of 46 chromosomes per cell. One of the 23 is a sex chromosome (X or Y), and the other 22 are autosomes.

Alleles and genes

A gene is a stretch of DNA or RNA that determines a certain trait. Genes mutate and can take two or more alternative forms- an allele is one of these forms of a gene. For example, the gene for eye color has several variations (or alleles) such as an allele for blue eye color or an allele for brown eyes.

An allele is found at a fixed spot on a chromosome. Chromosomes occur in pairs so organisms have two alleles for each gene — one allele in each chromosome in the pair. Since each chromosome in the pair comes from a different parent, organisms inherit one allele from each parent for each gene. The two alleles inherited from parents may be same (homozygous) or different (heterozygotes).

• To sum it all up: 
• an allele is a specific variation of a gene. We have two alleles of every gene, located on the same spot on different chromosomes. For example, the gene for eye color can either be the allele blue or the allele brown, but they are the same gene! 
• and a gene is a section of DNA that controls a certain trait and can be translated into a protein

Transcription in eukaryotes and prokaryotes

• In prokaryotes (bacteria), transcription occurs in the cytoplasm. Translation of the mRNA into proteins also occurs in the cytoplasm. In eukaryotes, transcription occurs in the cell's nucleus. mRNA then moves to the cytoplasm for translation.

• DNA in prokaryotes is much more accessible to RNA polymerase than DNA in eukaryotes.

• Eukaryotic DNA is wrapped around proteins called histones to form structures called nucleosomes.

• Eukaryotic DNA is packed to form chromatin. 

• While RNA polymerase interacts directly with prokaryotic DNA, other proteins mediate the interation between RNA polymerase and DNA in eukaryotes.

• mRNA produced as a result of transcription is not modified in prokaryotic cells. Eukaryotic cells modify mRNA by RNA splicing, 5' end capping, and addition of a polyA tail.  

Process of translation

Translation: the process of converting the mRNA codon sequences into an amino acid polypeptide chain. 

1. Initiation - A ribosome attaches to the mRNA and starts to code at the FMet codon (usualy AUG, sometimes GUG or UUG). 
2. Elongation - tRNA brings the corresponding amino acid to each codon as the ribosome moves down the mRNA strand. 
3. Termination - Reading of the final mRNA codon (aka the STOP codon), which ends the synthesis of the peptide chain and releases it.

Process of transcription in eukaryotic cells

Transcription and translation are spatially and temporally separated in eukaryotic cells- that is, transcription occurs in the nucleus to produce a pre-mRNA molecule. 

The pre-mRNA is typically processed to produce the mature mRNA, which exits the nucleus and is translated in the cytoplasm.

In a eukaryote, DNA never leaves the nucleus, so its information must be copied. This copying process is called transcription and the copy is mRNA. Transcription takes place in the cytoplasm (prokaryote) or in the nucleus (eukaryote). The transcription is performed by an enzyme called RNA polymerase. To make mRNA, RNA polymerase:

1. Binds to the DNA strand at a specific sequence of the gene called a promoter
2. Unwinds and unlinks the two strands of DNA
3. Uses one of the DNA strands as a guide or template
4. Matches new nucleotides with their complements on the DNA strand (G with C, A with U -- remember that RNA has uracil (U) instead of thymine (T))
5. Binds these new RNA nucleotides together to form a complementary copy of the DNA strand (mRNA)
6. Stops when it encounters a termination sequence of bases (stop codon)

DNA replication

The structure of DNA lends itself easily to DNA replication. Each side of the double helix runs in opposite, or anti-parallel directions. The beauty of this structure is that it can "unzip" down the middle and each side can serve as a pattern or template for the other side (called semi-conservative replication). However, DNA does not unzip entirely. It unzips in a small area (replication fork), which then moves down the entire length of the molecule. Cellular proofreading and error-checking mechanisms ensure near perfect fidelity for DNA replication.

Replication steps are as follows:

1. An enzyme called DNA gyrase makes a nick in the double helix and each side separates
2. An enzyme called helicase unwinds the double-stranded DNA
3. Several small proteins called single strand binding proteins (SSB) temporarily bind to each side and keep them separated
4. An enzyme complex called DNA polymerase "walks" down the DNA strands and adds new nucleotides to each strand. The nucleotides pair with the complementary nucleotides on the existing stand (A with T, G with C).
5. A subunit of the DNA polymerase proofreads the new DNA
6. An enzyme called DNA ligase seals up the fragments into one long continuous strand
7. The new copies automatically wind up again

Different types of cells replicated their DNA at different rates. Some cells constantly divide, like those in your hair and fingernails and bone marrow cells. Other cells go through several rounds of cell division and stop (including specialized cells, like those in your brain, muscle and heart). Finally, some cells stop dividing, but can be induced to divide to repair injury (such as skin cells and liver cells). In cells that do not constantly divide, the cues for DNA replication/cell division come in the form of chemicals. These chemicals can come from other parts of the body (hormones) or from the environment.