DNA, Chromosomes, and Genes PDF

# DNA, Chromosomes and Genes ## DNA Composition - **DNA** is made up of a long chain of repeating units called **nucleotides**. - Each nucleotide consists of three parts: a **sugar (deoxyribose)**, a **phosphate group**, and a **nitrogenous base**. - There are four types of nitrogenous bases in D...

# DNA, Chromosomes and Genes ## DNA Composition - **DNA** is made up of a long chain of repeating units called **nucleotides**. - Each nucleotide consists of three parts: a **sugar (deoxyribose)**, a **phosphate group**, and a **nitrogenous base**. - There are four types of nitrogenous bases in DNA: **adenine (A)**, **thymine (T)**, **guanine (G)**, and **cytosine (C)**. **Diagram of DNA Structure:** - The diagram shows a double helix structure of DNA. - The two strands of DNA are antiparallel which means they run in opposite directions. - The two strands are held together by **hydrogen bonds** between the nitrogenous bases. - Adenine always pairs with Thymine (A-T) - Guanine always pairs with Cytosine (G-C) ## Chromosomes - Chromosomes are made up of DNA and protein. - The DNA is wound around **histone proteins**, forming **nucleosomes**. - The nucleosomes are further coiled and condensed to form **chromatin**. - During cell division, the chromatin condenses even further to form **chromosomes**, which can be seen under a microscope. **Diagram of Chromosome Structure:** - The diagram shows a chromosome, which can be observed during cell division. - The diagram highlights the different levels of condensation of DNA, from the initial DNA double helix to the final chromosome structure. - **DNA** is 2 nm wide. - **Nucleosomes** are 11 nm wide. - **Chromatin** is 30 nm wide. - **Chromatin loops** are 300 nm wide. - **Condensed chromatin loops** are 700 nm wide. - **Chromosome** is 1400 nm wide. ## Replication - DNA replication is the process of creating a copy of the DNA molecule. - It occurs during **interphase** of the cell cycle. - The process involves several steps that include unwinding the DNA helix, separating the two strands, and adding new nucleotides to each strand. **Diagram of DNA Replication:** - The diagram shows the process of DNA copying, which occurs within the nucleus of the cell. - The diagram shows how both strands of the DNA helix are copied and how the process occurs in a semi-conservative manner, which means that each new DNA molecule contains one old strand and one new strand. **Steps Involved in DNA Replication:** 1. **Helicases** unwind the parental double helix. 2. **Single-strand binding proteins** stabilize the unwound parental DNA. 3. **DNA polymerase** synthesizes the leading strand continuously in the 5' to 3' direction. 4. **Primase** synthesizes a short **RNA primer** on the lagging strand, which is then extended by **DNA polymerase** to form an **Okazaki fragment**. 5. **DNA ligase** joins the Okazaki fragments together to form a continuous lagging strand. ## From Gene to Protein - **Genes** are segments of DNA that code for specific proteins. - The process of converting genetic information from DNA into proteins is called **gene expression**. - Gene expression involves two steps: **transcription** and **translation**. **Diagram of Gene Expression:** - The diagram shows the process of gene expression, which takes place within both the nucleus and the cytoplasm of the cell. - **Transcription** occurs in the nucleus, where DNA is used as a template to make a messenger RNA (mRNA) molecule. - **Translation** occurs in the cytoplasm, where the mRNA molecule is used to guide the synthesis of a protein. **Steps Involved in Gene Expression:** 1. **Transcription:** The DNA helix unwinds and a molecule of mRNA is created using the DNA sequence as a template. 2. **RNA splicing:** Non-coding introns are removed from the mRNA molecule, and the remaining exons are spliced together. 3. **Export:** The mature mRNA molecule is transported out of the nucleus into the cytoplasm. 4. **Translation:** The mRNA molecule binds to a ribosome, and the sequence of codons in the mRNA is used to assemble a chain of amino acids, which eventually folds into a protein. **Diagram of Transcription and Translation:** - The diagram shows how the sequence of bases in a gene is transcribed into a sequence of bases in an mRNA molecule. - The mRNA molecule is then translated into a sequence of amino acids in a protein. - Each amino acid is coded for by a **codon**, which is a triplet of bases in the mRNA molecule.