diff --git a/GAD2.py b/GAD2.py
new file mode 100644
index 0000000..2d5dc1d
--- /dev/null
+++ b/GAD2.py
@@ -0,0 +1,209 @@
+import os
+import re
+from typing import TextIO, Optional, Union
+from abc import ABC, abstractmethod
+from Bio import SeqIO, SeqUtils
+
+class BiologicalSequence(ABC):
+    @abstractmethod
+    def __len__():
+        pass
+
+    def __getitem__():
+        pass
+
+    def __str__():
+        pass
+
+    def check_sequence():
+        pass
+
+class NucleicAcidSequence(BiologicalSequence):
+    """Proccising nucleic acid sequences.
+    This class is parental for DNASequence and RNASequence classes.
+    """
+    def __init__(self, seq) -> None:
+        self.seq = seq
+
+    def check_sequence(self):
+        """
+        Checkes the correctnessis of input string (DNA or RNA).
+        """
+        for nucleotide in self.seq:
+            if not nucleotide in type(self).complement_rule:
+                raise ValueError (f'Incorrect nucleotide in {self.seq}')
+        return True
+
+    def complement(self):
+        """
+        Create complement sequences of RNA or DNA acoording to the complementary base pairing rule. 
+        """
+        if self.check_sequence():
+            complement_sequence = ''
+            for nucleotide in self.seq:
+                if nucleotide in type(self).complement_rule:
+                    complement_sequence += type(self).complement_rule[nucleotide]
+            return type(self)(complement_sequence)
+
+    def gc_content(self):
+        return (sum(1 for _ in re.finditer(r'[GCgc]', self.seq)))/self.__len__()
+    
+    def __len__(self):
+        return len(self.seq)
+
+    def __getitem__(self, key):
+        return self.seq[key]
+
+    def __str__(self):
+        return self.seq
+
+class DNASequence(NucleicAcidSequence):
+    """
+    Procissing DNA sequences.
+    Argumemt: seq (str) - DNA sequence, letters case do not matter.
+    -Example nucl_seq = DNASequence('ATGCGC' OR 'ATgCgc' OR 'atgcgc).
+
+    Valid operations:
+    - transcribe - return transcibed sequence of DNA coding strand;
+    - complement - return complementary sequence according to complementary rule
+    - check sequence - is input sequence correct
+    - gc_content - return percent of GC in sequence
+    """
+    complement_rule = {'a': 't', 'A': 'T', 't': 'a', 'T': 'A',
+                       'g': 'c', 'G': 'C', 'c': 'g', 'C': 'G'}
+    
+    def __init__(self, seq: str) -> None:
+        super().__init__(seq = seq)
+
+    def transcribe(self):
+        """
+        Return transcribed sequence of DNA acoording to the complementary base pairing rule.  
+        Function can procced only DNA seqences.
+        """
+        transcribed_sequence = ''
+        transcribed_sequence = self.seq.replace('t', 'u').replace('T', 'U')
+        return type(self)(transcribed_sequence)
+
+class RNASequence(NucleicAcidSequence):
+    """
+    Procissing DNA sequences.
+    Argumemt: seq (str) - RNA sequence, letters case do not matter.
+    -Example nucl_seq = RNASequence('AUGCGC' OR 'AUgCgc' OR 'augcgc).
+
+    Valid operations:
+    - complement - return complementary sequence according to complementary rule
+    - check sequence - is input sequence correct
+    - gc_content - return percent of GC in sequence
+    """
+    complement_rule = {'a': 'u', 'A': 'U', 'u': 'a', 'U': 'A',
+                       'g': 'c', 'G': 'C', 'c': 'g', 'C': 'G'}
+    
+    def __init__(self, seq) -> None:
+        super().__init__(seq = seq)
+
+
+def check_gc(fastq_read: str, gc_params: tuple) -> bool:
+    """
+    Filters sequences in FASTQ file by GC percentage. 
+    
+    Input:
+    - fastq_read (str): FASTQ sequence read.
+    - gc_params (tuple): range for filtration, accepts upper or upper/lower border. Both borders are included.
+    
+    Output:
+    Returns boolean value is this read satisfied the criteria.
+    """
+    gc_result = SeqUtils.GC123(fastq_read)[0]
+    if gc_params[0] < gc_result < gc_params[1]:
+        return True
+
+
+def check_length(fastq_read: str, len_bound_params: tuple) -> bool:
+    """
+    Filters sequences in FASTQ file by sequence length.
+    
+    Input:
+    - fastq_read (str): FASTQ sequence read.
+    - len_bound_params (tuple): range for filtration, accepts upper or upper/lower border. Both borders are included.
+    
+    Output:
+    Returns boolean value is this read satisfied the criteria.
+    """
+    len_of_seq = len(fastq_read)
+    if len_bound_params[0] < len_of_seq < len_bound_params[1]:
+        return True
+
+
+def check_quality(fastq_quality: str, quality_params: int) -> bool:
+    """
+    Filters sequences in FASTQ file by mean quality score.
+    
+    Input:
+    - fastq_quality (str): FASTQ read quality
+    - quality_params (int): threshold value of reads quality in phred33 scale.
+    
+    Output:
+    Returns boolean value is this read satisfied the criteria.
+    """ 
+    mean_quality = sum(fastq_quality.letter_annotations["phred_quality"])/len(fastq_quality.seq)
+    if mean_quality >= quality_params:
+        return True
+
+
+def int_to_tuple(input_parameters) -> tuple:
+    """
+    Converts input parameters to tuple format.
+    If input is already a tuple, it will be return without changes.
+    If input parameter is int (only upper threshold is entered), function will return a tuple like (0, 'input').
+    
+    Input:
+    - input_parameters (tuple or int).
+    
+    Output:
+    Lower and upper threshold for functions in tuple format.
+    """
+    if isinstance(input_parameters, tuple):
+        return input_parameters
+    return (0, input_parameters)
+
+
+def run_fastq_filter(input_path: Optional[str] = None, output_filename: Optional[str] = None, gc_bounds: Union[int, tuple] = (0, 100), length_bounds: Union[int, tuple] = (0, 2**32), quality_threshold: int = 0) -> TextIO:
+    """
+    FASTQ Filtraror with BIOPYTHON utilities
+    Performs filter of input FASTQ file according to input parameters. 
+    Input will be filtered by: 
+        - GC content (gc_bounds);
+        - reads length (length_bounds);
+        - reads quality score (quality_threshold).
+        
+    Input:
+    - input_path (str): path to .fastq file; include 4 strings: 1 - read ID, 2 - sequence, 3 - comment, 4 - quality. Default - None.
+    - output_filename (str): name of output file, by default, it will be saved in the directory 'fastq_filtrator_resuls'. Default name will be name of input file.
+    - gc_bounds (tuple or int): GC content filter parameters, it accepts lower and upper (tuple), or only upper threshold value (int). Default value (0, 100).
+    - length_bounds (tuple or int): read length filter parameters, it accepts lower and upper (tuple), or only upper threshold value (int). Default value (0, 2**32).
+    - quality_threshold (int): upper quality threshold in phred33 scale. Reads with average quality below the threshold are discarded. Default value - 0. 
+    
+    Output:
+    Returns FASTQ only with filtered reads which satisfied all input/default conditions.
+    """
+
+    "Specify input path"
+    if not input_path.endswith('.fastq'):
+        raise ValueError('Incorrect input file extension, should be .fastq')   
+
+    "Specify output path"
+    output_path = 'fastq_filtrator_resuls'
+    if not os.path.exists(output_path):
+        os.makedirs(output_path)
+    if output_filename is None:
+        output_filename = os.path.basename(input_path)
+    "Passed the parameters"
+    gc_params = int_to_tuple(gc_bounds)
+    len_bound_params = int_to_tuple(length_bounds)    
+    "Filter and record results"
+    filtererd_fastq = open(os.path.join(output_path, output_filename), mode='w')
+    for seq_record in SeqIO.parse(input_path, "fastq"):
+        if check_gc(seq_record.seq, gc_params) and check_length(seq_record.seq, len_bound_params) and check_quality(seq_record, quality_threshold):
+                SeqIO.write(seq_record, filtererd_fastq, "fastq")  
+    filtererd_fastq.close()   
+    return filtererd_fastq